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INFINITE SOFTCOMM INFINITE SOFTCOMM SOLUTIONSSOLUTIONS
701,UDYOG VIHAR, PHASE -V701,UDYOG VIHAR, PHASE -VGURGAON, INDIAGURGAON, INDIA
TEL NO. 0124- 4005655, 4362150-52TEL NO. 0124- 4005655, 4362150-52FAX NO. 0124-4362151FAX NO. 0124-4362151
How Did We Reach This Point?How Did We Reach This Point?
TelegraphyTelegraphy
TelephonyTelephony
Radio MilestonesRadio Milestones
“Hello”
“Shalom” “Guten Tag”
“Time Division” “Frequency Division!”
“CHAOS”
“Buenos Dias”
“Bonjour”
The SYMPHONY!The SYMPHONY!
GSM Vs. CDMAGSM Vs. CDMA
7 cell re-use patternf7
f7
f2
f2
f6
f6
f1
f5f3
f4
f1
f5f3
f4
Frequency Reuse- GSM
f1
f1
f1
f1
f1
f1
f1
f1
f1
f1
f1
f1
f1
f1
f1
f1
FREQUENCY REUSE IN CDMAFREQUENCY REUSE IN CDMA
f1f1
GSM Vs CDMA
FREQUENCY REUSE IN CDMA & TDMA
TYPICAL TDMA SYSTEMEACH CELL USES DIFFERENT FREQUENCY
THE PATTERN IS REPEATED FOR THE NEXT SET OF CELL SITES
TYPICAL CDMA SYSTEMEACH CELL USES SAME FREQUENCY
F 1 F 1
F 1
F 1
F 1
F 1
F 1
F 1
F 2
F 5
F 4F 6
F 7 F 3
CODE DIVISION MULTIPLE ACCESS (CDMA)
CDMA - Code Division Multiple AccessCDMA - Code Division Multiple Access
CDMA is a "spread spectrum" technology– Spreads the information contained in a
particular signal of interest over a much greater bandwidth than the original signal.
Goal of spread spectrum – Interference mitigation
Spread spectrum helps mitigate the harmful effects of interference• Deliberate - Military Jammer (Use first by
DOD)• Inadvertent - Co-channel users
Shanon – Hartley TheoremShanon – Hartley Theorem To consider the technology used in CDMA, consider the Shannon-Hartley theorem,
which is given as follows: C = W log2 (1 + S/N)Where, C = capacity W = Bandwidth S/N = Signal to Noise RatioAccording to the theorem, an increase in the bandwidth causes a decrease in the signal
to noise ratio, thus requiring a trade-off between the two parameters. However, CDMA operates by maximizing the bandwidth, which brings about a corresponding reduction in the SNR This reduction is countered by the utilization of an efficient error correction code, which ensures optimum performance even in low SNR conditions.
Spread Spectrum Principles• Shannon’s work relates capacity to both bandwidth and signal tonoise ratio. It allows some simple conclusions to be drawn. In a bandwidth limited system the only way to increase capacity (ordata rate) is to do one or both of the following:• Increase the Signal Power• Decrease the Noise Power
Forward & Reverse LinksForward & Reverse Links
824MHz
869MHz
849MHz
894MHz
Forward linkReverse link
Uplink Downlink
Physical ChannelPhysical Channel
The 3 dB bandwidth of a channel is the frequency range where the signal at the edges is 3 dB
lower than the peak value at the center frequency, fc
CDMA Technology BenefitsCDMA Technology Benefits Capacity increase Improved call quality Simplified system planning
– use of the same frequency in every sector of every cell Enhanced privacy Improved coverage characteristics
– possibility of fewer cell sites– better performance in fading/interference prone
environment Increased talk time for portables Bandwidth on demand
Capacity in CDMACapacity in CDMA
Capacity of the system depends on interference (Total power received in the Receiving Antenna) in the system
Power Control Techniques are employed to reduce the total interference in the system.
By controlling power, the capacity can be dynamically adjusted (Capacity in CDMA systems is soft)
CDMA is altering the face of TelecommCDMA is altering the face of Telecomm Dramatically improving the telephone traffic (Erlang) capacity Dramatically improving the voice quality and eliminating the
audible effects of Multipath fading Reducing the incidence of dropped calls due to handoff
failures Providing reliable transport mechanism for data
communications, such as facsimile and internet traffic Reducing the number of sites needed to support any given
amount of traffic Simplifying site selection Reducing deployment and operating costs because fewer
cell sites are needed Reducing average transmitted power Reducing interference to other electronic devices Reducing potential health risks
All CDMA users occupy the same frequency at the same time! Frequency and time are not used as discriminators
CDMA operates by using CODING to discriminate between users.
CDMA interference comes mainly from nearby users
Each user is a small voice in a roaring crowd --but with a uniquely recoverable code
CDMA: Using A New Dimension
Frequency-Hopping Each user’s narrowband signal hops
among discrete frequencies, and the receiver follows in sequence
Spectrum (FHSS) CDMA is NOT currently used in wireless systems, although used by the military
Direct Sequence• Narrowband input from a user is coded (“spread”) by a user-unique broadband code, then transmitted.• Broadband signal is received; receiver knows, applies user’s code, recovers users’ data
TYPES of CDMA Technology
At Originating Site: Input A: User’s Data @
19,200bps Input B: Walsh Code #23 @
1.2288 Mcps Output: Spread Signal
Spectrum
At Receiver Site: Input A: Received spread
spectrum signal Input B: Walsh Code #23 @
1.2288 Mcps Output: User’s Data
@19,200bps just as originally sent
DSSS Spreading: Time-Domain View
Spreading from a Frequency-Domain View
CDMA uses larger bandwidth but uses resulting processing gain to increase capacity
Spreading: What we do, we can undo
Sender combines data with a fast spreading sequence, transmits spread data stream.
Receiver intercepts the stream, uses same spreading sequence to extract original data.
“Shipping and Receiving” via CDMA
Whether in shipping and receiving, or in CDMA, packaging is extremely important!
Cargo is placed inside “nested” containers for protection and to allow addressing
The shipper packs in a certain order, and the receiver unpacks in the reverse order
CDMA “containers” are spreading codes
CDMA’s Nested Spreading Sequences
CDMA combines three different spreading sequences to create unique, robust channels.
The sequences are easy to generate on both sending and receiving ends of each link.
U1 = 0110010101001000
C1 ( 100110….10110010)*
=U1C1 ( 100110………………………0000)
U1C1 ( 100110………………………00000)
U1 = 0110010101001000
C1 ( 100110….10110010)*
=
UnCn
U4C4
U3C3
U2C2
UnCn*C1 = 0, UnCn*Cn = Un
U4C4*C1 = 0, U4C4*C4 = U4
U3C3*C1 = 0, U3C3*C3 = U3
U2C2*C1 = 0, U2*C2*C2 = U2
C1*C1 = 1, C2*C2 = 1…. Cn*Cn = 1 BUT C1*C2 = 0…C1*Cn = 0
DSSS Spreading/ DespreadingDSSS Spreading/ Despreading
End to end overviewEnd to end overview
Types of CodeTypes of Code
Forward & Reverse LinkForward & Reverse Link
The Three CDMA Spreading TechniquesThe Three CDMA Spreading Techniques
Orthogonal SequencesOrthogonal SequencesOrthogonal SequencesOrthogonal Sequences• Definition:Orthogonal functions have zero correlation. Two binary sequences are orthogonal if the process of “XORing” them results in an equal number of 1’s and 0’s. Example:Example:
00000000((XOR) 01010101
------------01010101
0 0
0 1- Repeat: right & below
- Invert: diagonally
0 0
0 1
0 0
0 1
0 0
0 1
1 1
1 0
Walsh Codes 64 “Magic” Sequences, each 64 chips long Each Walsh Code is precisely Orthogonal with respect
to all other Walsh Codes
• it’s simple to generate the codes, or
• they’re small enough to use from ROM
Short codeShort code
Usage of codeUsage of code
Need for SynchronisationNeed for Synchronisation
Functions of the CDMA Forward Channels
WALSH CODE WALSH CODE contd. contd.
WALSH CODES ARE USED TO SEPARATE INDIVIDUAL USERS WHILE THEY SIMULTANEOUSLY OCCUPY THE SAME RF BANDWIDTH
THE SEQUENCE ARE ORTHOGONAL TO EACH OTHER AND ARE GENERATED USING THE HADAMARD MATRIX
WALSH-0 IS NOT USED TO TRANSMIT ANY BASEBAND DATA
PN CODEPN CODE
PN SEQUENCES
USED TO SPREAD THE BANDWIDTH OF THE MODULATED SIGNAL TO LARGER TRANSMISSION BANDWIDTHS
DISTINGUISH BETWEEN DIFFERENT USER SIGNALS
MULTIPLICATION BY A SHORT PN SEQUENCE IS DONE TO PROVIDE ANOTHER LAYER OF ISOLATION ON THE FORWARD LINK
WE CAN HAVE A MAXIMUM OF 512 DIFFERENT PN SEQUENCES EACH WITH A SEPARATION OF 64 CHIPS FROM EACH OTHER
SecuritySecurity
CHANNELSCHANNELS
CDMA IS-2000 HIGH LEVEL ARCHITECTURE
• FORWARD CHANNELS
PILOT CHANNEL (1)
SYNC CHANNEL (32)
FORWARD TRAFFIC
PAGING CHANNELS ( 1-7)
• REVERSE CHANNELS
REVERSE TRAFFIC CHANNELS
ACCESS CHANNELS
SummarySummary
Forward Reverse
Short PN Different offset- Each Sector
Zero offset-Used by MS
Walsh code 64 Channel 64 Ary modulation
Long code Use to scramble TCH
Identify TCH for each MS
Information about the long code is broadcast to the mobile station by the Sync Channel (or Control Channel) to help the mobile lock onto the base station, and helps provide separation from other base stations.
One of the codes used in conjunction with the Walsh Code is the PN (pseudo-random noise) short code. The PN short code on the forward link is used to provide the base station with a unique identification that the mobile station uses to identify the serving base station.
The user signal (or control channel) is multiplied by the Walsh code. The Walsh code provides each user or channel with a unique identifier and, in DS spreading, may spread the frame across the bandwidth.
Main process:– y(t) = b(t) c(t)• Scrambling:– If b(t) and c(t) have the same rate then y(t) has the same rate,and the spectrum of the signal is unchanged b(t) is said to be encrypted or scrambled• Spreading:– If c(t) has a higher rate than b(t), y(t) has the faster rate and itscorrespondingly wider spectrum .In addition to being scrambled, b(t) is said to have had its spectrum spread• CDMA codes are used to perform scrambling andspreading.
Rayleigh Fading In addition to delay spread, the same multipath environment causes severe local variations in signal strength as thesemultipath signals are added constructively and destructively at the receiving antenna. This type of variation is called Rayleigh fadingThis can cause large blocks of information to be lost.If the set of reflected signals have one dominant component, such as a line-of-sight signal, the fading is more appropriately modeled using the Rician model. Note that if the mobile speed is zero, there is no fading, except if signals are reflected frommoving objects.For slow fading, CDMA uses power control to adjust the transmitted power in order to overcome the fades. Power control is too slow for the fast Rayleigh fading; instead, FEC encoding and bit interleaving are used.
Channel Contd.Channel Contd.
Forward channelForward channel
PILOT CHANNELPILOT CHANNEL
• PILOT SIGNALS ARE TRANSMITTED BY EACH CELL SITE TO ASSIST MOBILE RADIO IN ACQUIRING AND TRACKING THE CELL SITE DOWNLINK SIGNAL
• PILOT CHANNEL IS ASSIGNED CODE CHANNEL NUMBER ZERO
• THE SIGNAL STRENGTH = Ec/Io• Ec/Io IS THE ENERGY PER CHIP PER INTERFERENCE
DENSITY MEASURED ON THE PILOT CHANNEL• Ec/Io EFFECTIVELY DETERMINES THE FORWARD
COVERAGE AREA OF A CELL OR A SECTOR
SYNC CHANNELSYNC CHANNEL
• SYNC CHANNEL IS GIVEN THE CODE CHANNEL NUMBER 32; FIXED DATA RATE 1200 KBPS
• ALLOWS RECIEVER TO OBTAIN FRAME SYNCHRONIZATION ON SIGNAL
• MESSAGES SENT ON SYNCH CHANNEL ARESYSTEM TIMECHARACTERISTICS OF THE SYSTEM
FORWARD TRAFFIC CHANNELSFORWARD TRAFFIC CHANNELS
• PAGING CHANNELS ARE GIVEN THE CODE CHANNEL NUMBER 1 THRU 7
• FORWARD TRAFFIC CHANNELS GROUPED INTO RATE SET 1( 9.6, 4.8, 2.4 or 1.2 KBPS) AND RATE SET 2 (14.4, 7.2, 3.6 or 1.8 KBPS)
• RATE SET 1 IS REQUIRED FOR IS-95 WHEREAS RATE SET 2 IS OPTIONAL
• SPEECH IS ENCODED WITH VARIABLE RATE VOCODER TO GENERATE FORWARD TRAFFIC CHANNEL DATA DEPENDING ON VOICE ACTIVITY
Reverse ChannelReverse Channel
REVERSE TRAFFIC CHANNELSREVERSE TRAFFIC CHANNELS
• IDENTIFIED BY LONG USER CODE OFFSET
• DATA TRANSMITTED ON REVERSE CHANNEL IS CONVOLUTIONALLY ENCODED, BLOCK INTERLEAVED, MODULATED BY MEANS OF 64-ary ORTHOGONAL MODULATION, AND DIRECT SEQUENCE SPREAD PRIOR TO TRANSMISSION
• DATA RATE IS 9.6, 4.8, 2.4 OR 1.2 KBPS
ACCESS CHANNELSACCESS CHANNELS
• ENABLES THE MOBILE TO COMMUNICATE NONTRAFFIC INFORMATION
• DATA RATE IS FIXED AT 4.8 KBPS
• IDENTIFIED BY A DISTINCT ACCESS CHANNEL LONG-CODE SEQUENCE OFFSET
• A PAGING CHANNEL NUMBER IS ASSOCIATED WITH ACCESS CHANNEL
Forward LinkForward Link
Putting it All Together: CDMA Channels
How a BTS Builds the Forward Code Channels
Code Channels in the Reverse Direction
Functions of the CDMA Reverse Channels
Variable rate vocoderVariable rate vocoder
Vocoders compress speech, reduce bit rate, greatly increasing capacity
CDMA uses a superior Variable Rate Vocoder
• full rate during speech
• low rates in speech pauses
• increased capacity
• more natural sound Voice, signaling, and user secondary data may be
mixed in CDMA frames.
Variable Rate Vocoding & Multiplexing
In CDMA, most call processing events are driven by messages
Some CDMA channels exist for the sole purpose of carrying messages; they never carry user’s voice traffic
• Sync Channel (a forward channel)
• Paging Channel (a forward channel)
• Access Channel (a reverse channel)
Some CDMA channels exist just to carry user traffic
• Forward Traffic Channel
• Reverse Traffic Channel
• On these channels, most of the time is filled with traffic and messages are sent only when there is something to do
Messages in CDMA
Rake ReceiverRake Receiver
What’s In a Handset? How does it work?
The Rake Receiver Handset uses combined outputs of the three traffic correlates (“rake fingers”). Each finger can independently recover a particular PN offset and Walsh code. Fingers can be targeted on delayed Multipath Reflections, or on different
BTSs. Searcher continuously checks pilots.
The Pilot Searching ProcessThe Pilot Searching Process
Handoff SignalingHandoff Signaling
Near-far Problem Path Loss Fading Performance Objectives
Power Control Is Required ?
Power Control in CDMAPower Control in CDMA
Open Loop power control– Purely mobile unit function– Done during initial stage when mobile is
turned onClosed Loop power control
– Involves both base station and mobile unit
Forward Power Control
The BTS continually reduces the strength of each user’s forward base band chip stream
When a particular handset sees errors on the forward link, it requests more energy
The complainer’s chip stream gets a quick boost; afterward ,continues to diminish
Reverse Power Control
• Reverse Open Loop: Handset adjusts power up or down based on received BTS signal (AGC).
• Reverse Closed Loop: Is handset too strong? BTS tells up or down 1 dB 800 times/second.
• Reverse Outer Loop: BSC has FER trouble hearing handset? BSC adjusts BTS set point.
Types of HandoffTypes of Handoff
SOFT HANOFF
1. Soft handoff - 2 BTS are involved
2. Soft Soft - 3 BTS are involved
3. Softer - Two Sector of same BTS
HARD HANDOFF
- Between different frequency.
Hard & Soft handoverHard & Soft handover
CDMA Soft Handoff Mechanics CDMA soft handoff is driven by the handset Handset continuously checks available pilots Handset tells system pilots it currently sees System assigns sectors (up to 6 max.), tells handset Handset assigns its fingers accordingly
The Handset considers pilots in set ACTIVE SET: Pilots of sectors in use CANDIDATE SET: Pilots mobile
requested, but not yet set up NEIGHBOUR SET: Pilots told to mobile
by system, as nearby sectors to check REMAINING SET: Any pilots used by
system but not already in the other sets Handset sends Pilot Strength
Measurement Message to the system whenever:
• It notices a pilot in neighbor or remaining set exceeds T_ADD
• An active set pilot drops below T_DROP for T_TDROP time
• A candidate pilot exceeds an active by T_COMP
The Complete Rules of Soft Handoff
Softer Handoff Handset will ask for whatever pilots it wants If multiple sectors of one BTS simultaneously serve a
handset, this is called Softer Handoff Handset can’t tell the difference, but softer handoff
occurs in BTS in a single channel element Handset can even use combination soft-softer handoff
on multiple BTS & sectors
Soft H/O ADV. & DIS. ADV.Soft H/O ADV. & DIS. ADV.
Reverse linkReverse link
Forward linkForward link
Processing GainProcessing Gain
Spreading gain or processing gain is achieved when noise components, or noise-like
components, remain spread when the original signal (user 1 in the figure) is despread. The
original signal appears to have gained energy relative the noise. It can also be seen as if the noise
has been suppressed. By filtering out most of the wideband noise energy the original signal
can be extracted, provided sufficient bit energy over noise ratio, Eb/NT. It can be seen that the
“signal to noise” ratio after despreading will favor user 1 by a factor of G = BW/bw (or Fc/Fb or
Tb/Tc). G is then called spreading gain or processing gain. Processing gain can also be seen
as the number of chips per bit.
Processing GainProcessing Gain
Processing GainProcessing Gain
Energy per BitEnergy per Bit
•Eb/NT is traffic channel bit energy over noise.•Ec/I0 is pilot channel chip (bit) energy over interference.
Capacity EnhancementCapacity Enhancement
Capacity FactorCapacity Factor
Idle Mode Handoff
An idle mobile always demodulates the best available signal In idle mode, it isn’t possible to do soft handoff and listen to
multiple sectors or base stations at the same time -- the paging channel information stream is different on each sector, not synchronous -- just like ABC, NBC, CBS, and CNN TV news programs aren’t in word-sync for simultaneous viewing
Since a mobile can’t combine signals, the mobile must switch quickly, always enjoying the best available signal
The mobile’s pilot searcher is constantly checking neighbor pilots If the searcher notices a better signal, the mobile continues on the
current paging channel until the end of the current superframe, then instantly switches to the paging channel of the new signal
The system doesn’t know the mobile did this! (Does NBC’s Tom Brokaw know you just switched your TV to CNN?)
On the new paging channel, if the mobile learns that registration is required, it re-registers on the new sector
Signal processingSignal processing
•Speech encoding. This step is only used if speech information is transmitted. Data transmission omits this step.•Quality indicator•Forward Error Correction (FEC) encoding•Interleaving•Scrambling•Spreading•Digital modulation•RF modulation•Amplification of RF signal.
Speech encodingSpeech encodingIn order to transmit speech over a digital system, it must be digitized and encoded using a vocoder. Normal speech is received as an analog signal. The analog signal is converted into a digital signal using a process called Nyquist sampling, in which the analog input is typically sampled 8,000 times per second. The product of Nyquist sampling is a digital waveform called PCM (pulse code modulation).The PCM output is transferred to a vocoder (voice coder), which compresses the digitized voice signal into either Rate Set 1 (RS1) with an output of 8 kbps, or Rate Set 2 (RS2) with an output of 13 kbps, depending on the type of vocoder. In CDMA, variable rate vocoders are used.
Variable Rate VocoderThe variable rate vocoder employs a codec (coder/decoder) that compresses digitized speech from the analog-to-digital (A/D) converter and produces an output that complies with the data rate to be transmitted. During these “lulls” in theconversation, the vocoder can reduce its bandwidth requirements, before the FEC encoder, from full rate (9600 bps for EVRC) to 1/2 rate, 1/4 rate, or 1/8 rate (1200 bps for EVRC). Since the transmitter only transmits the lowest bit rate required, the required transmit power is minimized, and the channel interference is reduced.
FrameInformation bits are grouped into frames. A frame is the basic
timing interval in the system. Thelength of a frame depends on what channel on which it is
transmitted (e.g., Sync Channel,Traffic Channel), what type of information transmitted in the frame
(e.g., overhead messages,traffic information), and what air-interface standard is used
Error MeasurementsThere are a number of error measurements available in CDMA
transmission: Bit Error Rate(BER), Frame Error Rate (FER), and Packet Error Rate (PER).The frame quality indicator is a Cyclic Redundancy Code (CRC).
End of presentationEnd of presentation