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IINTRODUCTIONNTRODUCTIONTOTOCDMACDMA
& I& ITSTS
PPOWEROWERCONTROLCONTROLANDAND HHANDOFFANDOFFPROCEDURESPROCEDURES
Acknowledgement
I wish to express my deep gratitude to Tata Teleservices Ltd., which gave me an
opportunity to undergo summer internship at their prestigious organization.
I am indebted to Mr. Shaheen Abass- General Manager-Network and Mr. Waseem
Khan-Manager, Network Operations for guiding me throughout this project, adding
constructive comments and suggestions, and always providing strong motivation without
which this project would not have been possible.
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CONTENTS
.................................................................................................................................................................................1
ACKNOWLEDGEMENT.....................................................................................................................................1
INTRODUCTION..................................................................................................................................................4
INTERNATIONALCOCKTAILPARTYANALOGY......................................................................................................6
INTRODUCTION TO SPREAD SPECTRUM...................................................................................................6
CONCEPTOFDIRECTSEQUENCESPREADSPECTRUM ...........................................................................................7
COMPARISON OF CDMA WITH OTHER COMMUNICATION SYSTEMS.............................................8
MULTIPLE USERS ARE ON ONE FREQUENCY CHANNEL SIMULTANEOUSLY. IN ANALOG
FDMA AND TDMA, MAXIMUM EFFORT IS MADE TO KEEP OTHER PEOPLE OFF THE SAME
CHANNEL. ............................................................................................................................................................8
IN CDMA, CHANNELS ARE DEFINED BY VARIOUS CORRELATIVE DIGITAL CODES IN
ADDITION TO FREQUENCY. ..........................................................................................................................8
IN ANALOG SYSTEMS, WHEN ALL AVAILABLE CHANNELS ARE IN USE, NO FURTHER
CALLS MAY BE ADDED. CAPACITY LIMIT IN CDMA IS SOFT AND CAN BE INCREASED WITH
SOME DEGRADATION OF THE ERROR RATE OR VOICE QUALITY, OR CAN BE INCREASED
IN A GIVEN CELL AT THE EXPENSE OF REDUCED CAPACITY IN SURROUNDING CELLS........8
CAPACITY = (CHANNEL BANDWIDTH / DATA RATE ) * 1 / (SIGNAL/NOISE) * 1/VAF * FR....8
WHERE VAF = VOICE ACTIVITY FACTOR, AND FR = FREQUENCY REUSE EFFICIENCY..........9
USING CHANNEL BANDWIDTH OF 1.228MHZ, DATA RATE OF 9.6KBPS, S/N=7, VAF=0.4, AND
FR=0.67 GIVES A CAPACITY OF 31 USERS. THIS IS THE MAXIMUM NUMBER OF USERS
POSSIBLE IN A GIVEN SECTOR IN A CELL UNDER THE GIVEN CONDITIONS...............................9
DIVERSITYTECHNIQUESIN CDMA......................................................................................................................9
CDMA MAKES USE OF SPATIAL, FREQUENCY AND TIME DIVERSITY TO ENHANCE
PERFORMANCE..................................................................................................................................................9
SPATIAL DIVERSITY.........................................................................................................................................9
CDMA USES DIVERSITY RECEPTION FOR BASE STATIONS BY USING MULTIPLE ANTENNAS
AT ONE RECEPTION SITE. SINCE THESE ANTENNAS ARE PLACED A NON-INTEGRAL
NUMBER OF WAVELENGTH APART, WHEN ONE ANTENNA IS EXPERIENCING A
MULTIPATH FADE IT IS LIKELY THAT THE OTHER ANTENNAS WILL NOT BE IN A FADING
CONDITION. THIS LEADS TO RECEIVER DESIGNS WHERE THE ANTENNA WITH THE BEST
SIGNAL IS SELECTED TO BE PROCESSED BY THE RECEIVER. .........................................................9
CDMA EXTENDS THE IDEA OF DIVERSITY RECEPTION WITH THE CONCEPT OF SOFT
HANDOFF, WHICH IS EXPLAINED IN DETAIL FURTHER ON...............................................................9
FREQUENCY DIVERSITY.................................................................................................................................9
FREQUENCY DIVERSITY IS INHERENT IN A SPREAD SPECTRUM SYSTEM WHERE A FADEOF THE ENTIRE SIGNAL IS LESS LIKELY THAN WITH NARROWBAND SYSTEMS. FADING IS
CAUSED BY DESTRUCTIVE INTERFERENCE OF DIRECT AND REFLECTED IMAGES OF A RF
SIGNAL. IN THE FREQUENCY DOMAIN, A FADE APPEARS AS A NOTCH FILTER THAT
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MOVES ACROSS A BAND. FOR EXAMPLE, A FADE 300KHZ WIDE WILL RESULT IN
COMPLETE LOSS OF A FDMA OR TDMA SIGNAL BUT ONLY REDUCES THE POWER IN A
PORTION OF A CDMA SIGNAL, WHICH EMPLOYS A CHANNEL WIDTH OF 1.23MHZ.................9
TIME DIVERSITY ...............................................................................................................................................9
RAKE RECEIVERS IN CDMA ARE USED TO FIND AND DEMODULATE MULTIPATH SIGNALS
THAT ARE TIME DELAYED FROM THE MAIN SIGNAL. ALSO, TRANSMITTED SIGNALS ARE
SPREAD IN TIME USING INTERLEAVING WHICH IMPROVES THE PERFORMANCE OFERROR CORRECTION BY SPREADING ERRORS OVER TIME. ........................................................ ....9
ADVANTAGES OF CDMA..................................................................................................................................9
CAPACITY LIMIT IN CDMA IS SOFT AND CAN BE INCREASED WITH SOME DEGRADATION
OF THE ERROR RATE OR VOICE QUALITY, OR CAN BE INCREASED IN A GIVEN CELL AT
THE EXPENSE OF REDUCED CAPACITY IN SURROUNDING CELLS................................................11
BASIC BLOCK DIAGRAM OF A DIGITAL COMMUNICATION SYSTEM...........................................11
CDMA VOCODERS............................................................................................................................................13
CODING PROCEDURES IN CDMA................................................................................................................14
1) Convolutional encoding.............................................................................................................................14
4) Long PN code scrambling.........................................................................................................................16
5) Walsh Code Modulation ...........................................................................................................................16
A simple example on Walsh coding................................................................................................................17
A simple example on Walsh decoding............................................................................................................17
Time misalignment of Walsh codes................................................................................................................17
6) Short PN code scrambling........................................................................................................................17
MODULATION ...................................................................................................................................................18
RAKE RECEIVER..............................................................................................................................................21
BASIC NETWORK ARCHITECTURE OF CDMA........................................................................................23
CDMA AIR INTERFACE...................................................................................................................................24
FORWARD LINK..................................................................................................................................................24
Dedicated Channels..................................................................................................................................... ..25
Common Channels.........................................................................................................................................271) Pilot Channel.........................................................................................................................................................272) Sync Channel.........................................................................................................................................................28
3) Paging Channel......................................................................................................................................................29
REVERSE LINK....................................................................................................................................................30
Common Channels - Reverse Access Channel...............................................................................................31R-ACH Timing..........................................................................................................................................................32
Dedicated Channels..................................................................................................................................... ..33
POWER CONTROL IN CDMA.........................................................................................................................36
THE NEEDFORPOWERCONTROL.......................................................................................................................36REVERSE LINKPOWERCONTROL.......................................................................................................................37
Reverse Link Open-Loop Power Control.......................................................................................................37
Reverse Link Closed-Loop Power Control........................................................................................ ......... ...41
FORWARDLINKPOWERCONTROL.......................................................................................................................46
Action by Mobile......................................................................................................................................... ...47
Action by Base Station........................................................................................................................ .......... .47
Forward Link Power Control with RS2.................................................................................................. .......48
SOFT HANDOFF IN CDMA SYSTEMS..........................................................................................................49
TYPESOF HANDOFF............................................................................................................................................491) Inter-sector or softer handoff.....................................................................................................................50
2) Inter-cell or soft handoff............................................................................................................................50
3) Soft-softer handoff.....................................................................................................................................51
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4) Hard handoff..............................................................................................................................................52
PILOT SETS..........................................................................................................................................................52
HANDOFF PARAMETERS......................................................................................................................................53HANDOFF MESSAGES..........................................................................................................................................54
SEARCH WINDOWS.............................................................................................................................................56HANDOFF PROCEDURES......................................................................................................................................57
SETUPAND ENDOF SOFT HANDOFF...................................................................................................................60
CONCLUSION.....................................................................................................................................................62
Introduction
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The cellular telephony industry has grown phenomenally since its inception in the
1980s. As a result of this extraordinary growth, the industry is faced with the problem of an
ever-increasing number of users sharing the same limited frequency bands. To expand the
user base, the industry must find methods to increase capacity without degrading the quality
of service.
One of the analog cellular systems in use is FDMA (frequency division multiple
access). This technology divides the frequency spectrum into 30khz channels; and uses
directive antennas and complex frequency reuse planning to maximize system capacity.
Each transmitter or receiver uses a separate frequency. A narrowband transmitter is used
along with a receiver that has a narrow-band filter so that it can demodulate the desired signal
and reject unwanted signals, such as interfering signals from adjacent radios. AMPS
(Advanced Mobile Phone Service) is based on this system.
To further increase capacity, a digital access method called TDMA (time division
multiple access) was implemented. This uses the same frequency channelization as FDMA
and adds a time sharing element. Each channel is shared in time by 8 users, to increase
system capacity. TDMA is used in the current GSM digital cellular systems.
In order to further increase capacity, we would need to make further smaller divisions
in frequency or time, which would result in greater interference and degraded quality of data
transmission. In order to provide a greater capacity without using these methods, a
technology called CDMA was implemented. CDMA (code division multiple access) is based
on the use of a modulation technique known as spread spectrum. Spread-spectrum techniques
have been employed in military communication and radar systems for about 50 years. In
direct contrast to a FDMA based system, where an attempt is made to minimize the
transmitted bandwidth, in CDMA all users are assigned correlative codes to distinguish them
from each other and they transmit signals simultaneously on the multiple access channel.
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TDMA
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Frequency divisions are still used, but in a much larger bandwidth (1.2288Mhz). CDMA also
uses sectored cells to increase capacity.
International cocktail party analogy
The correlative codes in CDMA allow each user to operate in the presence of
substantial interference. An analogy to this is a crowded international cocktail party. Many
people are talking at the same time, but you are only able to listen and understand one person
at a time, who is talking in the same language as you. This is because your brain can sort out
the voice and language characteristics and differentiate them from other talkers. As the party
grows larger, each person must talk louder, and the size of the talk zone grows smaller. Thus,
the number of conversations is limited by the overall noise interference in the room. CDMA
is similar to this cocktail party analogy, but the recognition is based on digital codes. The
interference is the sum of all other users on the same CDMA frequency, both from within and
outside the home cell and from delayed versions of these signals. It also includes the usual
thermal noise and atmospheric disturbances.
Other desirable attributes in CDMA include optimum subscriber station power
management, universal frequency reuse, soft handoff, and enabling the use of optimum
receiver structures for time-varying multipath fading channels. CDMA is allotted a frequencyband from 824-894Mhz. The uplink (base station to mobile) comprises 824- 845Mhz and the
downlink (mobile to base station) is composed of 869-894Mhz. There is a guard band in
between these two links. CDMA is a full duplex system since the two channels are used at the
same time.
Introduction to spread spectrum
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A major issue of concern in digital communications is the efficient use of bandwidth and
power. However, in some situations it is necessary to sacrifice this efficiency to meet certain
other design objectives. Spread spectrum was originally developed for military applications,
where resistance to jamming is of major concern. The definition of spread spectrum
modulation can be stated as:
A means of transmission in which the data sequence occupies a bandwidth in excess of
the minimum bandwidth necessary to send it.
The spectrum spreading is achieved before transmission through the use of a code that is
independent of the data sequence. The same code is used in the receiver (operating in
synchronism with the transmitter) to despread the received signal so that the original data
sequence may be recovered.
There are two techniques in spread spectrum; direct sequence and frequency hopping. The
complicated design of frequency hopping make it uneconomical for commercial use. Instead,
direct sequence technique that is cheaper and simpler is employed in commercial CDMA
systems.
Concept of direct sequence spread spectrum
In direct sequence, each information bit is chopped up into a large number of small
time increments called chips. This is achieved by multiplying the information bearing
narrowband signal b(t) by a PN code (pseudo random noise code) c(t), having a bandwidth
much higher than the data rate.
Processing gain is given by PG = 10 log (channel bandwidth / data rate)
Processing gain is a direct consequence of the direct sequence radio signal spreading
process. It refers to the increase in signal-to-noise ratio that results from this process, and is
required for successful data communications. Processing gain increases as the number of
chips per data bit increases, and this can be manipulated by the system designer to get the
desired effect.
As a result, the product modulated signal m(t) will have a spectrum that is nearly the
same as the wideband PN signal. Thus the PN sequence performs the role of a spreading
code.
m(t) = c(t)b(t)
The received signal r(t) consists of the transmitted signal m(t) plus an additive interference
denoted by i(t).
Hence, r(t) = m(t) + i(t) = c(t)b(t) + i(t)
To recover the original signal b(t), the received signal r(t) is applied to a demodulator that
consists of a multiplier followed by an integrator, and a decision device. The multiplier is
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supplied by a locally generated PN sequence, which is an exact replica and is synchronized
with that used in the transmitter. The multiplier output in the receiver is therefore given by
z(t) = c(t)r(t) = c2(t)b(t) + c(t)i(t)
Since the PN signal alternates between levels 1 and 1, c 2(t) =1 for all t.
Thus, z(t) = b(t) + c(t)i(t)
We can see from the above equation that the data signal b(t) is reproduced at the receiver,
except for the interference represented by the additive term c(t)i(t). Multiplication of the
interference by the wideband c(t) will result in the product c(t)i(t) being also wideband. The
narrowband data component b(t) can thus be retrieved by filtering out the interference term
using a low pass filter.
The diagram below illustrates the basic concept of spread spectrum in relation to CDMA
Comparison of CDMA with other communication systems
Multiple users are on one frequency channel simultaneously. In analog FDMA and
TDMA, maximum effort is made to keep other people off the same channel.
In CDMA, channels are defined by various correlative digital codes in addition to
frequency.
In analog systems, when all available channels are in use, no further calls may be added.
Capacity limit in CDMA is soft and can be increased with some degradation of the error
rate or voice quality, or can be increased in a given cell at the expense of reduced capacity
in surrounding cells.
Capacity = (channel bandwidth / data rate ) * 1 / (signal/noise) * 1/VAF * Fr
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Where VAF = voice activity factor, and Fr = frequency reuse efficiency
Using channel bandwidth of 1.228Mhz, data rate of 9.6kbps, S/N=7, VAF=0.4, and
Fr=0.67 gives a capacity of 31 users. This is the maximum number of users possible in a
given sector in a cell under the given conditions.
Diversity techniques in CDMA
CDMA makes use of spatial, frequency and time diversity to enhance performance.
Spatial Diversity
CDMA uses diversity reception for base stations by using multiple antennas at one reception
site. Since these antennas are placed a non-integral number of wavelength apart, when one
antenna is experiencing a multipath fade it is likely that the other antennas will not be in a
fading condition. This leads to receiver designs where the antenna with the best signal is
selected to be processed by the receiver.
CDMA extends the idea of diversity reception with the concept of soft handoff, which is
explained in detail further on.
Frequency Diversity
Frequency diversity is inherent in a spread spectrum system where a fade of the entire signal
is less likely than with narrowband systems. Fading is caused by destructive interference of
direct and reflected images of a RF signal. In the frequency domain, a fade appears as a notch
filter that moves across a band. For example, a fade 300khz wide will result in complete loss
of a FDMA or TDMA signal but only reduces the power in a portion of a CDMA signal,
which employs a channel width of 1.23Mhz.
Time Diversity
Rake receivers in CDMA are used to find and demodulate multipath signals that are time
delayed from the main signal. Also, transmitted signals are spread in time using interleaving
which improves the performance of error correction by spreading errors over time.
Advantages of CDMA
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Improved privacy and cloning protection
Since the communication band is spread, it can be transmitted at a low power without
being detrimentally affected by background noise. Since it is broadcast at such low
power, it is blended into the noise; hence an observer would overlook it, making it secure.Without knowing the codes used in spreading the data, it is impossible to decipher the
transmission. Also, because the codes are so long (and quick) simply viewing the code
would be next to impossible to solve the code, hence interception is very hard. Only a
receiver that has knowledge about the code of the intended transmitter, is capable of
selecting the desired signal. It is also extremely difficult to intercept the ESN number
(electronic serial number) of the mobile and hence cloning of mobiles is prevented.
Good anti-jam and interference rejection performance
Low output power
As the signal is spread over a large frequency-band, the power spectral density is getting
very small, so other communications systems do not suffer from this kind of
communications. Also, lower RF power is required at cell site.
Greater battery life of mobile
Output power of mobile is maintained to a minimum through power control, which results
in lower power consumption and greater battery life.
Reduction of multi-path effects and loss of information due to fading
Simplified frequency planningThe frequency reuse factor represents the number of cells separating two cells
transmitting and receiving over the same frequency. In FDMA and TDMA, frequency
reuse factor is usually around 7. Complex frequency reuse planning has to be done in
these systems to maximize capacity while eliminating interference. In CDMA frequency
reuse factor is 1 because every cell uses the same frequency. This greatly simplifies
frequency planning and also increases the capacity of the system.
Improved coverage characteristics
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The number of cell sites required for CDMA is often 50% less than other systems. This
results in a lower cost of sites, facilities and hardware requirement.
Increase in system capacity
Capacity limit in CDMA is soft and can be increased with some degradation of the errorrate or voice quality, or can be increased in a given cell at the expense of reduced capacity
in surrounding cells.
Excellent voice quality
Soft and softer handoffs improve call quality and reduce the percentage of dropped calls.
There is no degradation in voice quality when moving from one cell or sector to another.
Packetized structure supports simultaneous voice and data
Variable rate speech coding
Basic Block Diagram of a digital communication system
Any communication system consists of a transmitter and receiver and an intermediate
channel over which data is transmitted between these two.
Transmitter Structure:
The first step is to convert a continuous analog signal to a discrete digital bit stream.
This is called digitization. The next step is to add voice coding for data compression by
means of a vocoder. This is followed by channel coding which encodes the data in such a
way as to minimize the effects of noise and interference in the communications channel. The
next step in the transmitter is filtering. Filtering is essential for good bandwidth efficiency.
Without filtering, signals would have very fast transitions between states and therefore very
wide frequency spectra much wider than is needed for the purpose of sending information.11
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A single filter is shown for simplicity, but in reality there are two filters; one each for the I
and Q channels. This creates a compact and spectrally efficient signal that can be placed on a
carrier. The output from the channel coder is then fed into the modulator. The rest of the
transmitter looks similar to a typical RF transmitter or microwave transmitter/receiver pair.
The signal is converted up to a higher intermediate frequency (IF), and then further
upconverted to a higher radio frequency (RF). Any undesirable signals that were produced by
the upconversion are then filtered out.
Receiver Structure:
The receiver is similar to the transmitter but in reverse. It is more complex to design.
The incoming (RF) signal is first downconverted to (IF) and demodulated. The ability to
demodulate the signal is hampered by factors including atmospheric noise, competing
signals, and multipath or fading. Generally, demodulation involves the following stages:
1. carrier frequency recovery (carrier lock)2. symbol clock recovery (symbol lock)
3. signal decomposition toIand Q components
4. determiningIand Q values for each symbol (slicing)
5. decoding and de-interleaving
6. expansion to original bit stream
7. digital-to-analog conversion
Both the symbol-clock frequency and phase (or timing) must be correct in the receiver
in order to demodulate the bits successfully and recover the transmitted information. Symbol
clocks are usually fixed in frequency and this frequency is accurately known by both the
transmitter and receiver. The difficulty is to get them both aligned in phase or timing.
In the transmitter, it is known where the RF carrier and digital data clock are because
they are being generated inside the transmitter itself. In the receiver there is not this luxury.
The receiver can approximate where the carrier is but has no phase or timing symbol clock
information. A difficult task in receiver design is to create carrier and symbol-clock recovery
algorithms. That task can be made easier by the channel coding performed in the transmitter.
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Some of the basic blocks of the communication system in relation to CDMA are
explained next.
CDMA Vocoders
CDMA takes advantage of quiet times during speech to raise capacity. CDMA has three
vocoder standards for converting voice to digital form while providing a high degree of data
compression.
1) IS-96A : Variable rate( 8kbps max)
It is 4 to 8 times more efficient than PCM (Pulse code modulation) or
ADPCM(Adaptive delta PCM) and provides moderate quality voice
2) CDG : Variable rate(13kbps max)
It provides toll quality voice
3) EVRC: Variable rate( improved 8kbps)
It provides near toll quality voice
For the original vocoder, the channel is 9.6kbps when the user is talking and data rate
may drop to 4.8kbps, 2,4kbps or 1.2kbps depending on the speech activity of the user. This
set of data rates is known asRate Set 1(RS1). A decision as to the approximate rate is made
every 20msec. The CDG 14.4kbps vocoder is similar with the four data rates of 14.4, 7.2, 3.6,
and 1.8kbps. This set of data rates is known asRate Set 2(RS2). Normal telephone speech has
a voice activity factor of about 0.4.
When operating at lower data rates, the mobile station turns off its transmitter and
operates in TDMA mode. This results in randomization of transmission times of each mobile
and a reduction in average transmitted power. This lowers the level of interference for all
other users and increases the capacity of CDMA by nearly a factor of two.
In case of the base station, lower data rates do not result in pulsing of transmission.
Instead, the same bit pattern is repeated as many times as needed to return to the full rate. The
transmit power can then be adjusted down, since repetition of data results in addition of
processing gain. This allows more capacity since total interference is reduced due to lower
transmitted power.
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Coding procedures in CDMA
Coding in CDMA includes:
Convolutional coding
Symbol repetition
Interleaving Long code scrambling
Walsh code modulation
Short code scrambling
1) Convolutional encoding
In CDMA, the introduction of redundancy in digital data by convolutional encoding
provides powerful forward error correction capability to the system. This minimizes the
effect of channel noise and the number of errors between input and the decoded output.
In a convolutional code, the encoding operation may be viewed as the discrete-time
convolution of the input sequence with the impulse response of the encoder. It generates
redundant bits using modulo-2 convolutions The duration of the impulse response equals the
memory of the encoder, which operates on the incoming message sequence using a sliding
window equal in duration to its own memory. It accepts message bits as a serial sequence
and thereby generates a continuous sequence of encoded bits at a higher rate.
The encoder of a binary convolutional code with rate 1/n, measured in bits per
symbol, may be viewed as a finite- state machine that consists of an M-stage shift register
with prescribed connections to n modulo-2 adders, and a multiplexer that serializes the output
of the adders. An L-bit message sequence produces a coded output sequence of length n
(L+M) bits. The code rate is therefore given by
R = L / n (L + M) bits/symbol
Typically, we have L >> M. Hence the code rate simplifies to
R 1 / n bits/symbol
The constraint length of a convolutional code, expressed in terms of message bits, is defined
as the number of shifts over which a single message bit can influence the encoder output. In
an encoder with an M-stage shift register, the memory of the encoder equals M message bits,
and K = M + 1 shifts are required for a message bit to enter the shift register and finally come
out. Hence the constraint length of the encoder is K.
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The figure above shows a rate convolutional coder with K=9.
2)Symbol Repetition
The encoded data is repeated appropriate number of times to get the required data
rate. This redundancy also provides additional processing gain.
3)Interleaving
An interleaver is an input-output mapping device that permutes the ordering of a
sequence of symbols; i.e. takes the symbols at the input and produces identical symbols at the
output but in a different temporal order. Signal fading occasionally cause carrier level to go in
and out of receiver threshold, producing error bursts at the demodulator output. An effective
solution to minimizing the effect of noise bursts is to spread each message in time.
Interleaving is used in CDMA to improve the performance of error correction by spreading
errors over time. De-interleaving of data spreads the clumped errors over a greater period of
time, so that error correction can fix the resulting smaller, spread out errors.
A possible implementation of interleaving is shown below.
Input data stream
1 2 . . . y2
.
.
.
x
Input data stream is fed into shift registers arranged as a matrix of x rows and y
columns. The input data fills this matrix row-wise, data folding into the succeeding row as
each row is filled. The separation between adjacent elements in any column is therefore y bits
(interleaving depth). Now, coding is applied column-wise, but the coded word is transmitted
row-wise. At the decoder, the received bits are assembled into an identical shift register
matrix. The message word can be retrieved by decoding the assembled word column-wise
and reading the bits from the shift register matrix row-wise. During transmission, if
interference causes all bits in a single row of the interleaved data to become corrupted, such
an error corrupts only one bit of the coded word since coding was applied column-wise.
Single bit errors can be easily corrected, and thus all y bits of the affected row can be
corrected individually. We can thus reconstruct the entire corrupt row, which would
otherwise have been irretrievably lost.15
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4)Long PN code scrambling
The long code is a PN spreading sequence and is given by the polynomial p(x) = x 42
+ x 35 + x 33 + x 31 + x 27 + x 26 + x 25 + x 22 + x 21 + x 19 + x 18 + x 17 + x 16 + x 10 +
x 7 + x 6 + x 5 + x 3 + x 2 + x 1 + 1. It is of length 2 42 1 chips as it is generated by a 42-
length linear feedback shift register. It is primarily used for voice privacy, and not needed for
channelization. Each user of the mobile network may be assigned a unique temporal offset
for the long code with reference to system time. Since the pseudo-random pattern of the long
code has a period of 41.43 days, it is nearly impossible to blindly detect a user's temporal
offset. The offset is accomplished with the use of a long code mask, which is a 42-bit value
that is combined with the shift register state using a logical AND operation. The modulo-2
sum of the 42 bits which result from this AND operation provide a time-shifter version of the
long code sequence.
5) Walsh Code Modulation
Channelization by means of code multiplexing is accomplished using length-64 Walsh
codes, which are assigned to different channels. Walsh codes form an integral feature of
CDMA. They provide a means to uniquely identify each user. Walsh codes are generated by
the Hadamard matrix expansion.
W2n = Wn Wn
Wn Wn
The variable n must be a power of 2. The one by one matrix W(1) is defined as W(1)
= 0. All higher Walsh code matrixes are generated using by placing the entire set into the first
three matrix positions and then placing an inverse set into the lower right hand matrix
position. E1A/T1A-95-B standard CDMA uses Walsh code set 64, which consists of 64
unique codes, with each code having 64 bits.
For generalized time variant functions, the cross-correlation coefficient is given by
Zij = 1/T 0T fi(t) fj(t) dt
For digital codes, this formula simplifies to
Zij = (Nagreements Ndisagreements) / Ntotal_number_of_digits
Two functions are called orthogonal if and only if the cross-correlation coefficient
between them is zero. Walsh codes have an important and desirable feature of being
orthogonal to each other. All rows in the above expansion are mutually orthogonal.
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Correlators in CDMA mobile receivers give a cross-correlation coefficient = 1 when
receiving the desired code and zero for all other Walsh codes.
Note: For Walsh code addition to work, encoded data must use bipolar values so that 0 has a
value of 1.
A simple example on Walsh coding
Let user A have a Walsh code of 0 0 and user B has a code of 0 1. For voice data
bit 0, user As encoded output will be 1 -1, while that of B will be 1 1. For a voice
data bit of 1, encoded output of A will be 1 1 while that of B will be 1 1. Addition of
the encoded data patterns of the two users results in a data stream ranging from 2 to 2. Thus
the combined signal has a larger peak to average signal ratio (crest factor). So base station
has to have a wide dynamic to represent high peaks of modulated wave.
A simple example on Walsh decoding
To find the desired signal, multiply combined signal containing sum of Walsh codes
by inverse of the desired codes and integrate the result. (determine area under curve). For
example, continuing the previous example, user A multiplies summed waveform by its
inverse Walsh code 1 1. Integrating and dividing over the period gives the original voice
data bit (bipolar).
The two examples can be extended to 64 bits for the IS-95 system.
Time misalignment of Walsh codes
If the Walsh code for user A is delayed in Base station relative to Walsh code for user
B, the sum of the encoded waveforms has intermediate steps that do not change state on a
clock boundary. Thus the decoded signal is not the original data but a fractional value close
to the correct state. Thus, if the various channels are not properly aligned, the orthogonality
of the channel is degraded, resulting in interference and a corresponding reduction in
capacity.
6) Short PN code scrambling
If all the cells used the same 64 Walsh codes without another layer of scrambling, the
resulting interference would severely limit system capacity. Since all the cells use the same
frequency and the same set of Walsh codes, the only other means to allow cells to reuse the
same Walsh codes is by using time offsets. Different base stations are identified on the
downlink based on unique time offsets utilized in the spreading process. Therefore, all base
stations must be tightly coupled to a common time reference. In practice, this is accomplished
through the use of the Global Positioning System (GPS), a satellite broadcast system that
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provides information on Greenwich Mean Time and can be used to extract location
information about the receiver. This common time reference is known as system time.
Since everything in CDMA is synchronized to system time, a unique identifier can be
provided to each cell by using a time offset in a short sequence. Two short code PN
sequences are used since IS-95 employs quadrature spreading. These two codes are the in-
phase sequence PI (x) = x15 + x13 + x 9 + x 8 + x 7 + x 5 + 1 and the quadrature sequence PQ (x)
= x 15 + x 12 + x 11 + x10 + x6 + x 5 + x 4 + x 3 + 1. These two sequences are generated length-
15 shift register sequences; although they are nominally (2 15 1)=32767 chips, a binary '0' is
inserted in each sequence after a string of fourteen consecutive 0's appears in either sequence
to make the final length of the spreading sequence an even 32768 chips. The short sequence
runs at 1.2288Mbps rate. PN offsets are separated by multiples of sixty-four 1.2288Mbps
clock chips. This allows for 32678/64 = 512 unique time offsets for cell identification.
Good pseudo- random patterns like short codes are designed to have near perfect
auto-correlation when time aligned and have very weak auto-correlation at all other time
offsets. This property makes finding the short code at a given PN offset easy. Short codes
appear as white noise interference to receivers looking for different PN offset short codes.
Even with this added noise, auto-correlation at zero offset is strong. At other offsets, net auto-
correlation is not zero, but still relatively weak compared to zero offset.
Modulation
In CDMA, the same baseband sequence is duplicated on both channels, then spread
with different PN sequences on the I and Q channels as explained above. This technique
allows independent despreading and amplitude measurement of both channels. Next, the chip
sequences are passed through identical baseband filters to produce the baseband I/Q
modulating signals. The baseband sequence is finally modulated on both the inphase (I) and
quadrature (Q) channels.
The modulation format is Filtered QPSK (quadrature phase shift keying) in the base
station, and filtered offset QPSK (OQPSK) in the mobile station.
QPSK is a multilevel modulation technique that uses two bits per symbol. The
information carried by the transmitted signal is contained in the phase. In particular, the phase
of the carrier takes one of four equally spaced values, such as /4, 3/4, 5/4 and 7/4. The
signal shifts between phase states that are separated by 90. Each possible value of phase
corresponds to a unique dibit. So, we may use the above set of phase values to represent the
Gray-encoded set of dibits: 10, 00, 01, and 11, where only a single bit is changed from one
dibit to the next. Output waveform is sum of +/- sine and +/- cosine waves.18
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A coherent QPSK system is in fact equivalent to two BPSK systems working in
parallel and using two carriers in phase quadrature. The in-phase and quadrature channel
outputs may be obtained by performing two BPSK on the odd and even numbered bits of the
input sequence. It gives two times the bandwidth efficiency of BPSK (binary phase shift
keying). However, receiver complexity is increased.
The figure below illustrates the implementation of a BPSK scheme where the phases
are separated by 180.
The receiver consists of two stages of demodulation. In the first stage, the received
signal and a locally generated carrier are applied to a product modulator followed by a low
pass filter whose bandwidth is equal to that of the original signal. This stage of the
demodulation process reverses the phase shift keying applied to the transmitting signal. The
second stage of the demodulation performs spectrum despreading by multiplying the low pass
filter output by a locally generated replica of the PN code, followed by integration over a bit
interval. The locally generated PN sequence in the receiver must be synchronized to the PN
sequence used to spread the transmitted signal in the transmitter. Finally, a decision is made
as to which bit was transmitted.
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The 180 and 90 phase shifts in carrier phase possible in QPSK can result in changes
in the carrier amplitude, thereby causing additional symbol errors on detection.
The extent of amplitude fluctuations exhibited by QPSK signals can be reduced by
using offset QPSK (OQPSK). In this scheme, the bit stream responsible for generating the
quadrature component is delayed by half a symbol interval with respect to the in-phase
component. Thus the phase transitions likely in OQPSK are confined to +/- 90 which
reduces the peak-to-average ratio (PAR) in the signal the mobile must transmit. Reducing
PAR reduces the dynamic range one must design a mobile transmitter over, which generally
results in simpler design. However, the probability of symbol error is exactly same as that of
QPSK.
However, the base station sums together many channels and transmits them on top of
each other. The random nature of adding many signals together does not always avoid the
origin and hence QPSK is used instead of OQPSK, which is used by the mobile station.
Each channel is applied to an individual modulator. The various signals are added
together to form a composite multi-user signal. In practice, the signals might be combined
algebraically at baseband before being input to the modulator.
RAKE Receiver
Buildings and other obstacles in built-up areas scatter the transmitted signal.
Furthermore, because of interaction between several incoming waves, resultant signal at the
antenna is subject to rapid and deep fading. Diversity reception techniques are used to reduce
the effects of fading and improve the reliability of communication without increasing either
the transmitter power or the channel bandwidth. Instead of trying to overpower or correct
multipath problems, CDMA takes advantage of the multipath to improve reception quality in
fading conditions.
Multipath can be approximated as a linear combination of differently delayed echoes.The RAKE receiver tries to counter the effect of multipath by using a correlation method to
detect the echo signals individually and then adding them algebraically. Using this technique,
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intersymbol interference due to multipath is dealt with by reinserting different delays into the
detected echoes so that they perform a constructive rather than destructive role.
The generic receiver architecture commonly used in CDMA systems is known as a
RAKE receiver. The receiver consists of a number of correlators connected in parallel and
operating in a synchronous fashion. The receiver is called a RAKE since the bank of
correlators has an appearance similar to the fingers of a garden rake. Each correlator is fed
with:
a) A delayed version of the received signal
b) A replica of the PN sequence used as a spreading code to generate the spread spectrum
modulated signal at the transmitter. Note that synchronization between this reference
signal at the transmitter and receiver is important.
Let the nominal bandwidth of the PN sequence be W = 1/ T c, where Tc is the chip
duration. We need to make W sufficiently large to identify the significant echoes in the
received signal; i.e. the bandwidth should be wider than the coherent bandwidth of the
channel. To ensure constructive addition of the correlator outputs, phase and gain adjusters
are provided. An appropriate delay is introduced into each correlator output so that the phase
angles of the correlator outputs are in agreement with each other. The relative amplitudes and
phases of the multipath components are found by correlating the received waveform with
delayed versions of the signal or vice versa. The correlator outputs are weighted so that
correlator response to strong paths in the multipath environment have their contributions
accentuated, while the correlators not synchronizing with any significant path are
correspondingly suppressed. Multipath components with relative delays of less than 1/ W
cannot be resolved.
The outputs of the M correlators are denoted as Z1, Z2,.., and ZM. The weights of the
outputs are denoted by a1, a2, ., and aM respectively. The weighted coefficients are
computed in accordance with the maximal ratio combining principle. This states that the
signal to noise ratio of a weighted sum, where each element of the sum consists of a signal
plus additive noise of fixed power, is maximized when the amplitude weighting is performed
in proportion to the pertinent signal strength. The composite signal y(t) is given by
M
Y(t) = ak. ZkK=1
The weighting coefficients, ak, are normalized to the output signal power of the
correlator in such a way that the coefficients sum to unity.
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ak = Zk2
M
Zk2
K=1
The block diagram of a rake receiver is shown above.
Provided we use enough correlators in the receiver to span a region of delays sufficiently
wide to encompass all the significant echoes that will occur in the multipath environment, the
output behaves as if there was a single propagation path between the transmitter and receiver
rather than a series of multipath paths spread in time.
In CDMA, the forward link uses a three-finger RAKE receiver while the reverse link uses
four fingers. The detection and measurement of multipath parameters are performed by a
searcher receiver, which is programmed to compare incoming signals with portions of I and
Q channel PN codes. Multipath arrivals and neighboring base station signals manifest
themselves as correlation peaks that occur at different times. The time of each peak, relative
to the first arrival, provides a measurement of the paths delay.
Basic network architecture of CDMA
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Mobile station (MS) : The MS terminates the radio path on the user side and enables the
user to gain access to services from the network.
Base Station (BS) : The BS terminates the radio path and connects it to the Mobile
Switching Center (MSC). The BS is often segmented into the BTS and the BSC.
Base Transceiver system (BTS) : The BTS consists of one or more transceivers
placed on a single location and terminates the radio path on the network side.
Base Station Controller (BSC) : The BSC is the control and management system
for one or more BTSs. The BSC exchanges messages with both the BTS and the
MSC.
Mobile Switching Center (MSC) : The MSC is an automatic system that interfaces the
user traffic from the wireless network with the wireline network, ex. PSTN (Public
Switched Telephone Network), or other wireless networks.
CDMA Air Interface
The CDMA air interface consists of the forward link (base station to mobile) and the reverse
link (mobile to base station).
Forward Link
The forward link refers to the link from the base station to the mobile station. The IS-
95 forward link is designed in such a way to take advantage of the inherent ability of CDMA
systems to use a frequency reuse factor of 1 and to achieve coherent reception at mobile
receivers by means of a pilot signal. The types of channels used can be grouped into common
channels and dedicatedchannels. Common channels are broadcastto all the users in the cell
served by the base station. Dedicated channels are meant to be heard by only one user.
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Dedicated Channels
Dedicated channels deliver user traffic and user-specific signaling. There are two
types of dedicated channels that are used in IS-95: the forward fundamental channel and the
forward supplemental code channel.
The forward fundamental channel was simply called the forward traffic channel in
IS-95-A, as it was the only channel capable of delivering dedicated traffic. In IS-95-B, the
forward supplemental code channel was introduced as a means of improving data rates to
individual users. Voice always goes over a fundamental channel and can never go over a
supplemental code channel. However, data may travel over both types of channels.
The fundamental channel is variable rate. This is to take advantage of periods of time
where the voice activity is low and therefore the voice codec ( coder/decoder) rate may be
reduced. The Rate Set 1 (RS1) and Rate Set 2(RS2) for the different vocoders were explained
above.
The supplemental code channel is not variable-rate, yet can take either 9.6 or 14.4kbps
forms. This channel is primarily used for providing higher data rates to individual users
through the use of code channel aggregation, where an individual user is assigned several
supplemental code channels (up to 7) to increase data throughput.
The modulation streams for RS1 and RS2 are shown in the figures below.
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Note that each frame is appended with a frame quality indicator, which is a cyclic
redundancy check (CRC) that can be used by the receiver for error detection. In addition,
each frame is appended with 8 "tail bits", which are binary 0's. The purpose of these bits is to
flush the convolutional encoder and return it to the all-0 state at the end of each frame. This is
helpful in the decoding process, as each frame can be decoded individually at the receiver.
One mixed mode bit is also provided at the beginning of each frame in RS1, which indicates
whether the frame is pure channel data or if it contains at least some signaling. In RS2, the
beginning of each frame also includes 1 bit that is reserved in the forward link but used by the
mobile in the reverse link to indicate a frame erasure( the CRC check did not pass). This
assists the base station in performing forward link power control efficiently.
The next three processes involved are convolutional encoding, symbol repetition and
interleaving. In the forward traffic channel, a combination of symbol repetition and
puncturing is used to keep the input to the block interleaver always at 384 symbols at a time.
50 frames/sec gives a data rate of 19.2kbps. However, for lower data rates, the corresponding
channel gain reduces as the data rate reduces. For instance, if the block interleaver output for
full-rate symbols is transmitted with powerEs, the transmit power for a half-rate frame is
Es/2, quarter-rate isEs/4, and eighth-rate isEs/8.
Following interleaving, the long PN code is XORed with the data. In the forward
link, the long code is not used to spread the signal bandwidth. Thus the long code is
decimated down to a lower rate after the users unique long code mask is applied. This is
accomplished by using every 64th bit out of the long code data stream, which reduces the data
rate to 19.2kbps from 1.2288Mbps. The data rate of the long code now matches that of the
encoded voice data it is XORed with.
Power control puncturing
After long code scrambling, power control puncturing takes place. The power control
subchannel is punctured into the transmitted frame on the fundamental channel only. These
are one-bit power control commands punctured at an 800 Hz rate. The puncturing location is
randomized based on the long code state. For the previous power control interval (known as a
power control group, its duration is 1.25 ms). Since the power control bits replace the
encoded voice data, holes (missing data) are introduced into the date stream from the
receivers point of view. The Viterbi decoder in the receiver restores the data by using some
of the available processing gain in the system.
55 Walsh codes (8 to 31, and 33 to 63) are available for use as traffic channels. The
actual number that can be used is determined by the total interference levels experienced in
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any given cell. Nominal full loading would typically be around 30 traffic channels in use for
equally loaded cells. The Walsh code generator runs at a data rate of 1.2288Mbps, while the
encoded voice data runs at 19.2kbps, i.e. a ratio of 64. So, when the two data streams are
XORed together, the entire 64 bits of Walsh code are sent in inverted or non-inverted format
depending on the polarity of the voice data bit. This makes it relatively easy for a CDMA
mobile to find and decode its assigned Walsh code.
After the data undergoes Walsh code modulation and spreading, it is split into I and Q
channels and scrambled with the corresponding short codes at the same rate.
The final signals are low pass filtered to reduce occupied bandwidth, and converted into
analog signals. The resulting analog I and Q signals from all the channels are summed
together and then sent to the I/Q modulator for modulation onto an RF carrier.
Common Channels
The three types of common channels used in IS-95 are the Pilot, Sync and Paging channels.
Each has a unique Walsh code associated with it, and serves a particular purpose in the
forward link.
1) Pilot Channel
The mobile uses the pilot channel for the following purposes:a. Multipath channel amplitude estimation for coherent detection
b. Timing recovery for synchronization to network time reference (GPS-based)
c. Frequency offset correction for the mobile receiver
d. Pilot strength measurements for soft and hard handoff decisions
There are also several other possible uses for the pilot at the mobile receiver, such as
interference correction and inter-frequency handoff measurements. The pilot channel must be
a known sequence to be useful at the mobile station. The pilot channel undergoes orthogonal
modulation with Walsh code 0, which is the first row of the Walsh-64 matrix and is the all
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binary-0's code. Since the channel data coming into the Walsh modulator is also all 0s,
orthogonal modulation of a binary 0 with a binary 0 is a binary 0. Thus this actual operation
does not have to be carried out in the spreading process. Instead, a stream of binary 0's is
transmitted at the chipping rate for the pilot channel. The data is then split into I and Q
channels and scrambled with the short PN codes. The pilot channel must transmit at a
sufficiently high power such that mobiles at the cell boundaries can still receive it. As a
result, the pilot must occupy a significant amount of base station transmitter power (typically
20% of the total power).
2) Sync Channel
The sync channel is primarily used by the mobile to acquire a timing reference. The
mobile station, when it acquires the pilot channel, knows the PN timing of that particular base
station. However, the mobile does not know how the timing of this base station relates to
other base stations in the network. An IS-95 system requires base stations to transmit at fixed
time offsets from GPS-based time. This synchronization to system time ensures that one base
station's signal does not interfere with another, as the partial correlation properties of the PN
sequences used will allow the mobile to despread the desired base station and suppress other
base station signals. The Sync Channel Message appears on the sync channel to let the mobile
know timing parameters such as the PN timing offset of the base station relative to system
time. The bit rate of this message is 1.2kbps. This message is then undergoes convolutional
encoding, symbol repetition and block interleaving. The block interleaver depth is a function
of the sync channel frame duration, which is 26 2/3 ms.
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In the Sync channel, the actual messaging data at 1.2kbps is first passed through a
half-rate convolutional encoder which doubles the data rate to 2.4kbps. 2X-symbol repetition
doubles the rate to 4.8kbps. The data is then interleaved (which does not cause a change in
the data rate) and passed on to the Walsh modulator. The sync channel undergoes orthogonal
modulation with Walsh code 32. Since the data rate at this point is only 4.8kbps, the Walsh
code is repeated 4 times for each data bit. Following Walsh code modulation, the data at
1.2288Mbps is scrambled against short codes as described above. The overall coding process
provides 30dB of processing gain, which helps the mobile to receive the critical timing
information of the sync channel error free.
3) Paging Channel
Up to 7 paging channels, each with their own unique Walsh code, may be used by the
IS-95 base station. The first paging channel is assigned Walsh code 1. When more channels
are required, codes 2 to 7 are used. This channel provides system parameters, voice pages,
short message services, and any other broadcast messaging to users in the cell. The paging
channel can take two bit rates, 4.8kbps(half rate) or 9.6kbps(full rate). The rate is given in the
Sync Channel Message. Normally, half rate is used since it provides an extra 3dB of
processing gain. This paging channel bits are then passed through a half rate convolutional
encoder, repeated (if half rate is used) and block interleaved. The block interleaver depth is a
function of the paging channel frame duration, which is 20 ms.
The paging channel interleaver output bits are then scrambled against a special long
code. "Scrambling" entails a modulo-2 addition of the input bit and a bit from a
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predetermined sequence. In this case, the predetermined sequence is the long code generator
sequence with a mask unique to the particular paging channel being used. This sequence is
decimated from the nominal 1.2288 MHz rate to the necessary 19.2 kHz rate for scrambling
by simply taking every 64'th bit from the masked long code generator output. This is
followed by the Walsh code modulation and short code spreading.
So, once all of the various channels have undergone short code spreading, they are
low pass filtered to reduce occupied bandwidth, and converted into analog signals. The
resulting analog I and Q signals from all the channels are summed together and then sent to
the I/Q modulator for modulation onto an RF carrier.
Reverse Link
The reverse link refers to the link from the mobile station to the base station. The IS-
95 reverse link channels may also be grouped into common and dedicated channels. The
common channels in the IS-95 reverse link are meant primarily for tasks such as call
origination, registration and authentication, page responses, and delivery of SMS.
Unlike the forward link, the reverse link cannot support a pilot channel for
synchronous demodulation, which results in a lower capacity than the forward link. Also,
Walsh codes cannot be used for channelization. The varying time delays from each mobile to
the base station destroy the orthogonality of the Walsh codes. So, since the reverse link
doesnt benefit from non-interfering channels, capacity of the reverse link is reduced when
compared to the forward link. Channelization of users in the reverse link is accomplished by
the use of long code masks. Since the long code is 42 bits in length, this allows 2 42 or 4.3
billion unique channel assignments. Recall that each mobile must acquire a system time
reference based on the pilot signal it receives from the base station and the associated sync
channel information. Therefore, each mobile can utilize a unique long code mask assuming
that the mobile's long code generator is synchronized with the long code generator being used
by the base station.
As a result, the mobile may transmit with a unique long code mask known only to the
base station. In reality, due to propagation delays and imperfect timing references at the
mobile stations, the base station must also examine other timing offsets near what the mask
value indicates when acquiring an individual user. The high-speed searcher circuits in the
base station allow it to quickly search over a wide range of long codes to lock on to a
particular users signal. However, this process is still far less complex than if the base station
had to blindly acquire the mobile's timing offset.
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Common Channels - Reverse Access Channel
The reverse access channel (R-ACH) is the reverse link common channel in IS-95.
The R-ACH messaging is at 4.8kbps. It is convolutionally encoded (1/3 rate encoder),
repeated (2X), and interleaved over 576 symbols. Note that the next step is 64-ary
orthogonal modulation. This step entails grouping each set of 6 consecutive bits output from
the interleaver into a row address to a memory that contains the 64 by 64 Walsh matrix. Once
a row is selected, all 64 bits that make up the row entry are output at a rate of 307.2kbps.
Note that here the Walsh codes are not being used for channelization but are used to
randomize the encoded voice data with a modulation format that is easy to recover. Since the
mobile is not transmitting a pilot signal on the reverse link (unlike the forward link), coherent
detection is not possible. As a result, the base station receiver may correlate the received
signal with all of the 64 possible Walsh codes and determine a peak correlation to determine
which row was sent. This operation does not require an estimate of the channel amplitude,
but receiver performance is worse than if a pilot signal was used.
After orthogonal modulation, the sequence is spread to 1.2288 MHz by XORing the
encoded data with the long code. The long code generator state should be synchronized with
the base station long code generator based on the information the mobile has received from
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the sync channel. The signal may now be quadrature spread; however, note the 1/2-chip delay
in the Q-branch of the quadrature spreader.
This results in offset-QPSKmodulation. Offset QPSK modulation reduces the peak-
to-average ratio (PAR) in the signal the mobile must transmit. Reducing PAR reduces the
dynamic range one must design a mobile transmitter over, which generally results in simpler
design.
R-ACH Timing
R-ACH timing is critical, as several mobiles may try to send R-ACH messages
simultaneously. As a result, R-ACH messaging is sent in the form of access probes. The
mobile sends an access probe aligned with system time and waits for a response from the
base station on the forward paging channel. If it does not get a response before a timer
expires, it sends another probe at a power greater than the previous probe. The power
difference between the probes is a fixed step size measured in decibel units.
In order to reduce the probability that mobiles send probes simultaneously (i.e. a
"collision" occurs), the access probe timing is aligned with system time and a random
backoff. Based on a set sequence, the mobile transmits an access probe aligned with 20 ms
increments of system time but backed off by a time offset based on the results of the
algorithm. Since each mobile's algorithm is based on input parameters unique to the mobile,
the chances of collision are reduced.
16 probes are allowed in a sequence before the mobile must "give up" and start the
process again at the original power levels.
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Dedicated Channels
As in the forward link, reverse fundamental and supplemental channels are still
applicable. The reverse fundamental channel must be able to deliver variable rate data at Rate
Sets 1 and 2 for voice services, while the supplemental channels deliver data at full rate. The
basic transmission sequences for RS1 and RS2 are depicted in the figures below.
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Rate Set 1
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Rate Set 2
The convolutional encoder used in the reverse link is rate 1/3 for Rate Set 1, which is
different from the forward link that uses rate 1/2. A more powerful encoder than the forward
link is used to compensate for the reduced performance due to the lack of a pilot channel.
Note also the presence of the data burst randomizerfor both Rate Sets. In the forward link
for half-rate, quarter-rate and eighth-rate frames, symbol repetition was used with power
reduction for each transmitted symbol. Although symbol repetition is depicted for the reverse
link at the input to the interleaver, in fact only one symbol repetition is actually transmitted.
The data burst randomizer actually turns off ("gating off" the transmitter) the transmitter
during periods where repetitions are transmitted so as to ensure that only one symbol
repetition is ever actually sent. The pattern with which symbols are eliminated from the
transmission sequence is pseudorandom, determined by the state of the long code generator at
each power control group. As a result, the base station receiver must be able to detect these
on-off transitions, and the mobile must ignore power control commands sent by the base
station in response to a gated-off period.
Note also that the sinusoids used to modulate the spread signal to the carrier
frequency have an associated phase offset . This phase offset is unique to each supplemental
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channel transmitted by the mobile, and can be determined by an index based on the additional
number of supplemental channels used.
Power Control in CDMA
The Need for Power Control
CDMA is an interference-limited systemsince all mobiles transmit at the same
frequency, internal interference generated within the system plays a critical role in
determining system capacity and voice quality. The transmit power from each mobile must be
controlled to limit interference. However, the power level should be adequate for satisfactory
voice quality.
As the mobile moves around, the RF environment changes continuously due to fast
and slow fading, shadowing, external interference, and other factors. The objective of power
control is to limit transmitted power on the forward and reverse links while maintaining link
quality under all conditions. Due to non-coherent detection at the base station, interference on
the reverse link is more critical than it would be on the forward link. Reverse link power
control is therefore essential for a CDMA system.
Power control is also needed in CDMA systems to resolve the near-far problem. Tominimize the near-far problem, the goal in a CDMA system is to assure that all mobiles
achieve the same received power levels at the base station. The target value for the received
power level must be the minimum level possible that allows the link to meet user-defined
performance objectives (BER (bit error rate), FER (frame error rate), capacity, dropped-call
rate, and coverage). In order to implement such a strategy, the mobiles closer to the base
station must transmit less power than those far away.
Voice quality is related to FER which is largely correlated to E b /I t. The FER also
depends on vehicle speed, local propagation conditions, and distribution of other co-channel
mobiles. Since the FER is a direct measure of signal quality, the voice quality performance in
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a CDMA system is measured in terms of FERs rather than E b /I t . The recommended
performance bounds are
A typical recommended range for FER0.2% to 3% (optimum power level is achieved
when FER 1%)
A maximum length of burst error3 to 4 frames (optimum value of burst error2 frames)
Reverse Link Power Control
The reverse link power control affects the access and reverse traffic
channels. It is used for establishing the link while originating a call and
reacting to large path-loss fluctuations. The reverse link power control
includes the open-loop power control (also known as autonomous power
control) and the closed-loop power control. The closed-loop power control
involves the inner-loop power control and the outer-loop power control.
Reverse Link Open-Loop Power Control
The open-loop power control is based on the principle that a mobile closer to the base
station needs to transmit less power as compared to a mobile that is farther away from the
base station or is in fade. The mobile adjusts its transmit power based on total power received
in the 1.23-MHz band (i.e., power in pilot, paging, sync, and traffic channels). This includes
power received from all base stations on the forward link channels. If the received power is
high, the mobile reduces its transmit power. On the other hand, if the power received is low,
the mobile increases its transmit power.
In open-loop power control the base station is not involved. The mobile determines
the initial power transmitted on the access channel and traffic channel through open-loop
power control. A large dynamic range of 80 dB is allowed to provide an ability to guard
against deep fades.
The paging channel provides the Access Parameters message, which contains the
parameters to be used by the mobile when transmitting to the base station on an access
channel. The access parameters are:
The access channel number
The nominal power offset (NOM_PWR)
The initial power offset step size
The incremental power step size
The number of access probes per access probe sequence
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The time-out window between access probes
The randomization time between access probe sequences
Based on the information received on the pilot, sync, and paging channels, the mobile
attempts to access the system via one of several available access channels. During the access
state, the mobile has not yet been assigned a forward link traffic channel (which contains the
power control bits). Since the reverse link closed-loop power control is not active, the mobile
initiates, on its own, any power adjustment required for a suitable operation.
The prime goal in CDMA systems is to transmit just enough power to meet the
required performance objectives. If more power is transmitted than necessary, the mobile
becomes a jammer to other mobiles. Therefore, the mobile tries to get the base station
attention first by transmitting at very low power. The key rule is that the mobile transmits in
inverse proportion to what it receives.
When receiving a strong pilot from the base station, the mobile transmits a weak
signal back to the base station. A strong signal at the mobile implies a small propagation loss
on the forward link. Assuming the same path loss on the reverse link, only a low transmit
power is required from the mobile in order to compensate for the path loss. When receiving a
weak pilot from the base station, the mobile transmits back a strong signal. A weak received
signal at the mobile indicates a high propagation loss on the forward link. Conversely, a high
transmit power level is required from the mobile.
The mobile transmits the first access probe at a mean power level defined by
T x = - R xK+(NOM-PWR - 16 *NOM-PWR-EXT)+ INIT-PWR (dBm)
where T x = mean output transmit power (dBm),
R x = mean input receive power (dBm),
NOM-PWR = nominal power (dB),
NOM-PWR-EXT= nominal power for extended handoff (dB),
INIT-PWR = initial adjustment (dB),
K= 73 for cellular (Band Class 0), and
K= 76 for PCS (Band Class 1).
If INIT-PWR were 0, then NOM-PWR 16 NOM-PWR-EXT would be the
correction that should provide the correct received power at the base station. The values for
NOM-PWR, NOM-PWR-EXT, INIT-PWR, and the step size of a single access probe
correction PWR-STEPare system parameters specified in the Access Parameters message.
These are obtained by the mobile station prior to transmitting. The total range of the NOM-
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PWR 16 NOM-PWR-EXTcorrection is 24 to 7 dB. While operating in Band Class 0,
NOM-PWR-EXTis set to 0, making the total range of correction from 8 to 7 dB. The range
of theINIT-PWRparameter is 16 to 15 dB, with a nominal value of 0 dB. The range of the
PWR-STEPparameter is 0 to 7 dB. The accuracy of the adjustment to the mean output power
due to NOM-PWR, NOM-PWR-EXT, INIT-PWR, or a single access probe correction of
PWR-STEPshould be 0.5dB or 20%, whichever is greater.
The major flaw with this criterion is that reverse link propagation statistics are
estimated based on forward link propagation statistics. But, since the two links are not
correlated, a significant error may result from this procedure. However, these errors will be
corrected once the closed-loop power control mechanism becomes active as the mobile seizes
a forward traffic channel and begins to process power control bits.
After the Acknowledgment time window (T a) has expired, the mobile waits for an
additional random time (RT) and increases its transmit power by a step size. The mobile tries
again. The process is repeated until the mobile gets a response from the base station.
However, there is a maximum number of probes per probe sequence and a maximum number
of probe sequences per access attempt. The entire process to send one message and receive an
acknowledgment for the message is called an access attempt. Each transmission in the access
attempt is referred to as an access probe. The mobile transmits the same message in each
access probe in an access attempt. Each access probe contains an access channel preamble
and an access channel capsule. Within an access attempt, access probes are grouped into
access probe sequences. Each access probe sequence consists of up to 16 access probes, all
transmitted on the same access channel.
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There are two reasons that could prevent the mobile from getting an acknowledgment
after the transmission of a probe.
1. The transmit power level might be insufficient. In this case, the incremental step power
strategy helps to resolve the problem.
2. There might be a collision due to the random contention of the access channel by several
mobiles. In this case, the random waiting time minimizes the probability of future collisions.
The process is shown by the access probe ladder above.
For every access probe sequence, a back-off delay is generated pseudo-randomly.
Timing between access probes of an access probe sequence is also generated pseudo-
randomly. After transmitting each access probe, the mobile waits for Ta. If an
acknowledgment is received, the access attempt ends. If no acknowledgment is received, the
next access probe is transmitted after an additional random time.
If the mobile does not receive an acknowledgment within an access attempt, the
attempt is considered as a failure and the mobile tries to access the system at another time. If
the mobile receives an acknowledgment from the base station, it proceeds with the
registration and traffic channel assignment procedures. The initial transmission on the reverse
traffic channel shall be at a mean output power defined by
T x = - R x K +(NOM-PWR - 16 * NOM-PWR-EXT)+Sum of Access Probe Corrections
where the access probe correction is the sum of all the appropriate incremental power steps
prior to receiving an acknowledgment at the mobile.
The mobile station supports a total combined range of initial offset parameters, closed
NOM-PWR, and access probe corrections of at least 32dB for mobile stations operating in
Band Class 0 and 40dB for mobile stations operating in Band Class 1.
The sources of error in the open-loop power control are Assumption of reciprocity on the forward and reverse links
Use of total received power including power from other base stations
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Slow response time ~ 30 ms to counter fast fading due to multipath
Reverse Link Closed-Loop Power Control
Fading sources in multipath require a much faster power control than the open-loop
power control. The additional power adjustments required to compensate for fading losses are
handled by the reverse link closed-loop power control mechanism, which has a response time
of 1.25 ms for 1-dB steps and a dynamic range of 48 dB (covered in 3 frames). The quicker
response time gives the closed-loop power control mechanism the ability to override the
open-loop power control mechanism in practical applications. Together, two independent
power control mechanisms cover a dynamic range of at least 80 dB. The closed-loop power
control provides correction to the open-loop power control. Once on the traffic channel, the
mobile and base stations engage in closed-loop power control.
The reverse link closed-loop power control mechanism consists of two partsinner-
loop power control and outer-loop power control. The inner-loop power control keeps the
mobile as close to its target (E b /I t) set point as possible, whereas the outer-loop power
control adjusts the base station target (E b /I t) set point for a given mobile.
A power control subchannel continuously transmits on the forward traffic channel. This
subchannel runs at 800 power control bits per second. Therefore, a power control bit (0 or 1)
is transmitted every 1.25 ms. A 0 bit indicates to the mobile that it should increase its mean
output power level, whereas 1 indicates to the mobile to decrease its mean output power
level.
A 20-ms frame is organized into 16 time intervals of equal duration. These time
intervals, each of 1.25 ms, are called Power Control Groups (PCGs). Thus, a frame has 16
PCGs.
Prior to transmission, the reverse traffic channel interleaver output data stream is
gated with a time filter. The time filter allows transmission of some symbols and deletion of
others. The duty cycle of the transmission gate varies with the transmit data rate, i.e., variable
rate vocoder output, which, in turn, depends on the voice activity. The table below indicatesthe number of PCGs that are sent at different frame rates.
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The Data Burst Randomizer (DBR) determines the assignment of the gated-on and
gated-off groups. At the base station, the reverse link receiver estimates the received signal
strength by measuringE b /I tduring each power group (1.25 ms).
If the signal strength exceeds a target value, a power-down power control bit 1 is sent.
Otherwise a power-up control bit 0 is transmitted to the mobile via the power control
subchannel on the forward link.
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The transmission of a power control bit occurs on the forward traffic channel in the
second PCG following the corresponding reverse link PCG in which the signal strength was
estimated. For example, if the signal strength is estimated on PCG #2 of a reverse link frame,
then the corresponding power control bit must be sent on PCG #4 of the forward link frame.
Once the mobile receives and processes the forward link channel, it extracts the power
control bits from the forward traffic channel. The power control bits then allow the mobile to
fine-tune its transmit power on the reverse link. Based on the power control bit received fromthe base station, the mobile either increases or decreases transmit power on the reverse traffic
channel as needed to approach the target value of (E b /I t)nom or set point that controls the
long-term FER. Each power bit produces a 1-dB change in mobile power, i.e., it attempts to
bring the measuredE b /I tvalue 1 dB closer to its target value. Note that it might not succeed
because I t is also always changing. Therefore, further adjustments may be required to
achieve the desiredE b /I t. The base station, through the mobile, can directly change only E
b , notI t, and the objective is the ratio ofE b toI t, not any particular value forE b orI t.
The base station measuresE b /I t16 times in each 20-ms frame. If the measured E
b /I tis greater than the current target value ofE b /I t, the base station informs the mobile to
decrease its power by 1 dB. Otherwise, the base station orders the mobile to increase its
power by 1 dB.
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The relationship between E b /I tand the corresponding FER is nonlinear and varies
with vehicle speed and RF environment. Performance deteriorates with increasing vehicle
speed. The best performance corresponds to a stationary vehicle where additive white
Gaussian noise dominates. Thus, a single value ofE b /I tis not satisfactory for all conditions.
The use of a single, fixed value forE b /I tcould reduce channel capacity by 30% or more by
transmitting excessive, unneeded power.
The value of the variable a in the figure above is kept very small, so it may take 35
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