Cdma Project Report.

7/30/2019 Cdma Project Report.

1/62

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.

1


2/62

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

2


3/62

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

3


4/62

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

4


5/62

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.

5

FDMA

TDMA


6/62

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

6


7/62

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

7


8/62

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

8


9/62

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

9


10/62

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

10


11/62

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


12/62

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.

12


13/62

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.

13


14/62

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.

14


15/62

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


16/62

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.

16


17/62

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

17


18/62

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 pseudorandom 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


19/62

19


20/62

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.

20


21/62

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,

21


22/62

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.

22


23/62

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

23


24/62

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.

24


25/62

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.

25


26/62

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

26


27/62

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

27


28/62

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.

28


29/62

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

29


30/62

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.

30


31/62

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

31


32/62

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.

32


33/62

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.

33


34/62

Rate Set 1

34


35/62

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

35


36/62

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

36


37/62

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

37


38/62

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-

38


39/62

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.

39


40/62

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

40


41/62

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.

41


42/62

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.

42


43/62

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.

43


44/62

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

Cdma Project Report.

Documents

Transcript of Cdma Project Report.