code division multiple access (cdma)

Code Division Multiple Access(CDMA)

Prepared by:Anil Ramroop ID-0024144

Perapong Uttarapong ID-0026852

Code Division Multiple Access(CDMA)

• Multiple Access is a technique by which multiple users use the same physical resource.

• The most prevalent multiple access techniques are TDMA,FDMA and CDMA.

• CDMA is based on Spread Spectrum which evolved some 50 years back.

• Each traffic channel is multiplied by a unique high speed bit stream to spread the channel in the frequency domain.

• At the receiver end the spread signal is multiplied by the same high speed stream to retrieve the data.

• Out of the CDMA implementations cdmaOne is the one which is most widely deployed commercially.

• cdmaOne is based on the IS-95(1993) standard and is a trade mark of CDMA development group (CDG).

cdmaOne overview and Terminology

A/D mux

FEC

Spreading CodeGenerator

Spreader PSK

Information

Chips Chips

Code Symbols

Information Bits

Add check bits

CDMA Cellular Reuse

• Same Frequency is used in every cell – Interference becomes low power noise

• Spectral efficiency much higher than AMPS.– 20 times theoretically.

– 5~8 times in practice.

• CDMA Design Parameters ( Same as AMPS )– Forward Channel Frequency 869 – 894 Mhz

– Reverse Channel Frequency 824 – 849 Mhz

– Tx/Rx Frequency Spacing 45 Mhz

IS-95 CDMA

• Existing 12.5 Mhz assigned cellular bands are used to derive 10 different CDMA bands 1.25 Mhz per band.

• Frequency Reuse factor in CDMA = 1. • Channel Rate = 1.2288 Mcps (cycles per sec). • Multipath Fading exploited in CDMA.

– Rake receivers are used to combine the output of several received signals. – Fading does occur on the individual signals, but each signal is affected

differently and so using several of them to make a decision improves the probability of obtaining a correct decision – Multipath Diversity combining.

– At Mobile• Three correlators used to receive three different signals. With a fourth one used

as a roving finger which is used to detect new strong incoming signals. Process ensures that the Rake receiver always used the three strongest signals.

– At Base Station• All four correlators are used to receive signals Antenna Diversity.

The Rake receiver

• One of the main advantages of the CDMA system is its ability to resolve different multipath components.

• This is possible since CDMA is a wide band system.(??)

• In order to resolve multipath signals the subscriber unit/BTS should make use of multiple receivers operating at different phases. Each of these receivers are called fingers.

• The outputs of these fingers are added to form a strong output.

Correlator 1

Correlator 2

Correlator 3

SearcherC

ombi

ner

Input

Correlator 1

Correlator 2

Correlator 3

SearcherC

ombi

ner

Input

Output

The Coding and Modulation process in CDMA

• 64 bit Walsh Codes are used to provide 64 channels within each frequency band.• Walsh codes used for spreading in the forward link. • Walsh codes used to provide orthogonal modulation and not spreading to the full

1.2288 rate in the reverse link.

• Besides Walsh codes, 2 other codes are used in IS-95– Long PN code: generated from a 42 bit shift register having (242 – 1) = 4.398 x 1012

different codes. A mask is used to overlay the codes, the mask differs from channel to channel. The chip rate is 1.2288Mcps. These codes are used for:

• Data scrambling/encryption in the forward path• Data spreading and encryption in the reverse path

– Short PN code: generated from a pair of 15 bit shift registers having 215 –1 = 32767 codes. These codes are used for synchronization in the forward and reverse links and cell identification in the forward link

• Each cell uses one of 512 possible offsets.• Adjacent cells must use different offsets. • Chip Rate is 1.2288Mcps ( i.e., not used for spreading )

Direct Sequence CDMA

• Multiply data with a Pseudo-random noise sequence (PN)

Hadamard-Walsh Code

The four orthogonal sequences in this Walsh code set are taken from the rows of the matrix H4 ; that is,

W0 = [ 0 0 0 0 ] W1 = [ 0 1 0 1 ] W2 = [ 0 0 1 1 ]

W3 = [ 0 1 1 0 ]

0000111100001111111100001111000000001111000011111111000011110000

1111000011110000000011110000111111110000111100000000111100001111

1111000011110000000011110000111111110000111100000000111100001111

0000111100001111111100001111000000001111000011111111000011110000

1111111111111111111111111111111111111111111111111111111111111111

0000111100001111111100001111000000001111........

0000111100001111111100001111000000001111........

0000111100001111111100001111000000001111.......

0000111100001111111100001111000000001111........

0000000000000000000000000000000000000000........

O RT HO G O NAL SPREADING AT T HE BASE ST AT IO N

Input Data 1 0 ....

Each data bit(symbol) is exclusive-or'd with a 64 bitW alsh Function (W alsh Function 20)

Each data bit(symbol) is exclusive- or'd with a 64 bit W alsh Function (W alsh Function 20)

W alsh Function 20 W alsh Function 20

Pattern transmitted by the Base Station Pattern transmitted by the Base Station

DESPREADING O F T HE RECEIVED PAT T ERN AT T HE M O BILE ST AT IO N

Received Pattern at the M obile Station Received Pattern at the M obile Station

Each 64 b it(symbol) b lock of the received pattern is exclusive-or'd with W alsh Function 20

O utput Data 1 0 .......

O rthogonal Spreading/Despreading

1111000011110000000011110000111111110000111100000000111100001111

0000111100001111111100001111000000001111000011111111000011110000

1111000000001111000011111111000000001111111100001111000000001111

0000111100001111111100001111000000001111........

0000111100001111111100001111000000001111........

0000111111110000111100000000111111110000.....

D ESPREADIN G O F T HE R ECEIVED PAT T ER N AT T H E M O BILE ST AT IO N W IT HINC O RR ECT W ALSH FUN CT IO N

R eceived Pattern a t the M obile S ta tion R eceived Pattern a t the M obile S ta tion

Each 64 b it(symbol) b lock o f the rece ived pattern is exclusive-or'd w ith W alsh Function 40 which is not the same W alshfunction used for orthogonal spreading at the base sta tion

Inconclusive output - Equal number o f 1s and 0s in the despread pattern

O rthogonal despread ing w ith incorrect W alsh code

W alsh Function 40 W alsh Function 40

CDMA Channels

• Forward and Reverse Channels are separated by 45 Mhz.– Forward Channel comprises of the following channels:

• Pilot channel (always uses Walsh code W0 )

• Paging channel(s) ( use Walsh code W1 – W7 )

• Sync channel (always uses Walsh code W32 )

• Traffic channels ( use Walsh codes W8 – W31 and W33 – W63 )

– Reverse Channel comprises of the following channels:• Access channel

• Traffic channel

– Link Protocol can be summarized as follows: • Mobile acquires phase, timing and signal strength via the Pilot channel

• Mobile synchronizes to Base Station via the Sync. Channel

• Mobile gets system parameters via the paging channel.

• Mobile and BS communicates over the traffic channels during a connection.

• Mobile and BS communicate over the access and paging channels during system acquisition and paging.

Forward

Reverse

Access

Traffic

Variable-B it-RateUser Inform ation

Signaling Messages

Pilot

Sync

Paging

Traffic

Variable-B it-RateUser Inform ation

Power Control

S ignaling Messages

CDM A logical channels

Forward/Reverse Channel Spreading and Scrambling Process

• Forward channels are spread using one of 64 orthogonal Walsh functions. Note – Perfect separation between the channels in the absence of multipath interference. – To reduce interference between mobiles that use the same Walsh function

in neighboring cells, all signals in a particular cell are scrambled using the the short PN sequence for cell identification.

– For the paging and traffic channels, the long PN sequence is used to scramble the signal before spreading.

• Reverse channels are spread using the long PN sequence. – All 64 orthogonal Walsh functions are used to provide orthogonal

modulation. – The stream is then scrambled using the short PN sequence for cell

identification purposes.

CDMA Vocoder & Transmission Rates

• IS-95 supports different transmission rates. The vocoder (QCELP) outputs 9.6 Kbps when there is a full speech signal and 1.2 Kbps when a silent period is detected. (Note 1)

• Intermediate rates such as 4.8Kbps and 2.4 Kbps are progressively used to either increase or decrease rates based on the speech signal content.

• Rate decisions are made every 20msec interval ( the interval over which samples are collected and processed).

• In CDMA – A signal (rate set 1) is always sent for it takes too long for the receivers to ramp up again for reception.

• To accommodate all the different data rates using the same air interface, bits in the lower bit rate streams are repeated to bring the rate up to 9.6Kbps.

• However the bits are output at a correspondingly lower power. For example: the 1.2 Kbps bits are repeated 8 times to bring it up to 9.6 Kbps, but, the signal strength is reduced to 1/8 the power.

CDMA Vocoder & Transmission Rates

(Cont.)

• In 1995, Qualcomm introduced a higher rate coder (QCELP13) called Rate Set 2 that produces a 14.4 Kbps speech signal and 1.8 Kbps when a silent period is detected. The other intermediate rates are 7.2 Kbps and 3.6 Kbps.

• So as not to change the air interface and the transmitters and receivers (in particular the interleaver), the following were done:

– Reverse link rate set 2(RC2) signal is encoded at 1/2 rate as opposed to 1/3 rate used in rate set 1(RC1).

– Forward Link puncturing of the code is used to reduce it from ½ to ¾ (i.e., 2 symbols from every 6 encoded symbols are dropped).

• IS-95 also supports variable rate transmission on the reverse link as follows:– Instead of repeating the symbols and sending them at 9.6 or 14.4 Kbps, the repeated

symbols are randomly deleted from the frame (after interleaving). • Thus, mobiles transmitting at the same rate do not have all their bits arrive at the same

time at the BS which reduces interference.

• When this mode is used, the symbols are sent at full power as oppose to reduce power when using repetition.

Forward Logical Channels• Pilot Channel

– Transmitted at all times ( sequence of 0’s ).– Uses Walsh Code W0.

– Provides phase and timing reference to the mobile terminal.– Provides signal strength to the mobile for channel acquisition.– Re-used in every cell and sector with different short PN code offset.

• Sync Channel -- can be received by a mobile after it locks on to a pilot channel. Features of the Sync Channel:

– Operates at 1200 bps. – Has a frame length of 26.666 msec.– Uses Walsh code W32 and uses the same PN sequence & offset as the Pilot channel. – Provides timing information to the mobile for synchronization. – Provides pilot PN offset.– Provides system time ( needed for the short PN sequence generation ). – Provides system and network Ids. – Provides paging channel rates. – Provides BS protocol revision level. – CDMA channel number

Forward Logical Channels (Cont.)

• Paging Channel is used to page mobiles and transmit system information. – Bit rate of 9600 or 4800 bps.

– Frame Length 80msec – messages can occupy several slots (1-4).

– Use Walsh codes W1 – W7 ( System can use 1–7 paging channels depending on traffic load ).

– Transmit the system parameter message: registration information, BS class, BS longitude/latitude, power control thresholds, etc.

– Transmit the access parameter message: # of access channels, initial access power requirements, # of access attempts, authentication info., etc.

– Carry the channel assignment for a traffic channel to mobile.

Downlink CDMA Channel(1.23 MHz Transm itted by the Base Station)

Pilo tChannel

SyncChannel

PagingChannel1

PagingChannel7

TraficChannel1

TraficChannel55

W 0 W 32 W 1 W 7 W 8 W 63W alsh Code

Structure of Forward CDM A channel

ConvolutionalEncoder and

Repetition

BlockInterleaver


Repetition

BlockInterleaving

Long CodePN G enerator

Decimator

+

+

+

+

+

+

+ +

+

+

W alsh Function 32

Pilo tChannel(A ll 0 's)

Sync Channel1200 bps

Paging Channel9600 bps4800 bps

R = 1/2, K = 9

M odulationSymbol

4800 sps

M odulationSymbol

4800 sps

W alshFuntion 0

1.2288M cps

1.2288M cps

1.2288Mcps

W p

M odulationSymbol

19.2 Ksps

M odulationSymbol

Paging ChannelAddress M ask

1.2288M cps

19.2 Ksps

Q PN

I PN

Q PN

Q PN

I PN

I PN

P ilot, sync, and paging channel generation

Forward Logical Channels (Cont.)

• Forward Traffic Channels are used to carry user data and signaling data. Features are as follows:– Bit rates up to 9600bps (rate set 1) and up to 14.4Kbps (rate set

2).

– Frame length of 20ms (192 bits for rate set 1 and 288 bits for rate set 2)

– Use Walsh codes W8 – W31 and W33 – W63.

– Can be used in two modes: Blank & Burst or Dim & Burst• Blank & Burst is similar to NA-TDMA, signaling data replaces

speech data

• Dim & Burst multiplexes signaling data or a secondary data stream with speech data (speech data sent at 4.8, 2.4 or 1.2 Kbps for RC1 and 7.2, 3.6 or 1.8Kbps for RC2.

Vocoder

ConvolutionalEncoding

SymbolRepetition

Puncturing

BlockInterleaving

Data Scrambling

Power Contro lSubchannel

O rthogonalSpreading

Q uadratureSpreading

BasebandF iltering

Reduces bit rate needed to represent speech. Operates in a variablem ode of full, 1/2, 1/4 & 1/8 rates. Rate set 1 v ocoder full-rate output is at9.6 kbps and rate set 2 full-rate output is at 14.4 kbps.

Prov ides error detection/correction. Two sym bols are output for eachincom ing bit.

Repetition of input sym bols from the encoder. Repetition is done tom aintain a constant input to the block interleaver. Full-rate sym bols are notrepeated and sent at full power, half-rate repeated once & sent at halfpower and so on. For rate set 1 the output is m aintained at 19.2 ksps(independent of vocoding rate) and for rate set 2 the output is 28.9 ksps.

Used only for for v ocoder operating in rate set 2 m ode. Deletes 2 out ofevery 6 inputs for an output of 19.2 ksps. This results in an identical inputrate to the block interleaver of 19.2 ksps irrespectiv e of the rate set of thev ocoder.

Com bats the effects of Rayleigh fading by ensuring that sequential data isnot lost.

Prov ides security by scram bling the input data with a long code m askperm uted with the users ESN.

Prov ides a very fast power control subchannel (800 tim es per second).The input data is puntured 800 tim es per second and a power up/downcom m and is sent to the m obile station. Each com m and can increase orsecrease a m obile stations power by 1 dB.

Prov ides identity and orthogonality to the forward channels by spreadingthem with a unique W alsh code. Each input sym bol is exclusive-or'd with a64-bit W alsh code resulting in a data rate of 1.288 Mcps (m egachips persecond).

Prov ides unique base-station identity. The spreading sequence is 32768chips and repeats ev ery 26.66 m s. The sam e sequence is used by all basestations but is phase-offset in each. There are 512 possible offsets.Ensures that the m obile station is locked on to the right base station.

Converts the signals to the cellular frequency range (800 MHz) or the PCSfrequency (1900 MHz).

To RF section

PCM Voice

Functions involved in creating a dow nlink traffic ch annel


Repetition

BlockInterleaver

LongCode PNGenerator

Decimator Decimator

MUX

PowerControl

Bit W n

I PN

Q PN

ModulationSymbol

19.2 Ksps

ModulationSymbol

9600 bps4800 bps2400 bps1200 bps

From Vocoder

Long CodeMask (ESN)

1.2288 Mcps

1.2288 Mcps

19.2 Ksps

800 Hz


Repetition

BlockInterleaver


Decimator Decimator

MUX

PowerControl

Bit W n

I PN

Q PN

ModulationSymbol

19.2 Ksps

14400 bps7200 bps3600 bps1800 bps

From Vocoder

User AddressMask (ESN)

1.2288 Mcps

1.2288 Mcps

19.2 Ksps

800 Hz

Rate Set 1 Dow nlink T raffic Channel G eneration

Rate Set 2 Dow nlink T raffic Channel G eneration

SymbolPunturing

Rate set 1 and 2 dow nlink traffic channel generation

R=1/2, K=9

R=1/2, K=9

172 12 8

192 b its (20 ms)

9600 bpsFrame

80 8 8

96 b its (20 ms)

4800 bpsFrame

Information B itsFull Rate

In formation B its1/2 Rate

40 8

48 b its (20 ms)

2400 bpsFrame

Information B its1/4 Rate

F T

T

T

F

16 8

24 b its (20 ms)

1200 bpsFrame


T

F is a F rame Q uality Ind icator (CRC) fie ld . It is ca lcu la ted on in formation b itson ly. Not ca lcu la ted for 2400 bps or 1200 bps frames. T is an encoder ta il b it.Set to zeroes on a ll frames.

Downlink/uplink traffic channel fram e structure for rate set 1.

267 12 8

288 b its (20 ms)

14400 bpsFrame

Information B itsFull Rate

F T

R is reserved bit used in the downlink, E is used in the reverse link to indicate bad fram ereception by MS or BS. F is a Fram e Quality Indicator (CRC) field. It is calculated on theinform ation bits only. Calculated for all fram es. T is an encoder tail bit. Set to zeroes on allfram es.

Dow nlink/up link traffic channel fram e structure for rate set 2 .

1

R /E

125 10 8

144 b its (20 ms)

7200 bpsFrame


F T

1

R/E

55 8 8

72 b its (20 ms)

3600 bpsFrame


F T

1

R/E

21 6 8

36 b its (20 ms)

1800 bpsFrame


F T

1

R/E

Transm ission Mode

S peech

C ontro l Message

C onten t Ind ica to r

P arity C heck

C oder ta il b its

In fo rm ation b its

B lank-and-B urst D im -and-B urst S peech O n ly

16

152

4

12

8

192

16

152

4

12

8

192

16

152

4

12

8

192

0

168

4

12

8

192

171

0

1

12

8

192

Number of B its per F rame (20 ms) in Full Rate (9 ,600 b/s) CDM A TrafficChannels.

Data Rate R b/s

In formation ra te R I b /s

In formation b its per frame (IBPF)

Parity b its per frame (PBPF)

Data b its per frame (IBPF + PBPF + 8)

Coded b its per frame (CBPF)

Repetitions

Tota l b its per frame (BPF)

1,200

800

16

0

24

48

8

384

2,400

2,000

40

0

48

96

4

384

4,800

4,000

80

8

96

192

2

384

9,600

8,600

172

12

192

384

1

384

Contents of 20 ms F rames, Forward Channels

1 171

21 1 80 88

21 1 40 128

21 1 16 152

21 1 168

M M Primary T raffic

M M TT TM Primary T raffic Secondary/ S ignalingTraffic



M M TT TM Secondary/ S ignaling T raffic

172 b its

172 b its

172 b its

172 b its

172 b its

9600 bps Prim ary Traff ic only

9600 bps dim -and-burst withrate 1/2 prim ary and signalingtraff ic



9600 bps blank-and-burst withsignaling traff ic only

M M indicates if m ixed m ode traffic is being used, it is set to 0 if only primary traffic is being sent, 1otherw ise.TT is a traffic type field w hich indicates if secondary or signaling traffic is being sent.TM is a traffic m ode bit w hich indicates the m ode of operation.

Fram e structure fo r dow nlink traffic channel for rate set 1

Reverse Logical Channels• Access Channel: is a random access channel used by mobiles to send

information (not user data) to the BS.

– One or more access channels are paired with a paging channel (max. is 32 in total)

– Mobiles respond to paging messages on their corresponding access channels.

– Bit rate is 4800bps.

– Long PN code mask consists of:• Access channel number, BS identifier, corresponding paging channel number, PN_offset

(No PN offset is used for the quadrature spread).

– Mobiles compete for access as follows:• Mobile chooses an access channel at random from the set associated with the paging

channel.

• If two mobiles choose the same access channel and PN time alignment their transmissions will interfere with each other – Thus, the BS will not be able to distinguish between them.

• No channel sensing for collision avoidance.

• If a mobile does not get an ACK back before the timer expires it makes another attempt (at a higher power level) after a random wait. It repeats this process for a max. number of times, if it does not succeed, it waits a random time and then restarts the process all over again.

set in itia lpower

sendprobe

ACKreceived

before timeout?

max probes ?

max attempts ?new probe:

ra ise power,wait random time

new attemptAccessFails

AccessSucceeds

Begin

yes no

no

no

yes

yes

Access Pro toco l

Reverse Logical Channels

• Reverse Traffic Channel: used to carry user data (primary and secondary) and signaling data. A BS will support up to 61 channels.

– Data transfers at 4 different levels within a rate set supported. – Signaling information is multiplexed with the user data, where possible

(i.e. if variable data rates are supported). If not possible, then the signaling information takes over the channel briefly to transmit a message (blank and burst)

– Instead of signaling information, a secondary traffic stream can be multiplexed (i.e., voice is primary and data is secondary).

– Long PN mask is used to uniquely identify a mobile. Can be of two types:• Public consists of the mobile’s ESN.• Private derived from the encryption and authentication process.

– Orthogonal modulation consists of sending one of 64 possible Walsh functions for each group of 6 coded bits.

• Walsh Function number = C0 + 2C1 + 4C2 + 8C3 + 16C4 + 32C5 where the C’s represent the coded bits. Output rate is 28.8 x 64 / 6 = 307.2Kbps.

Vocoder

C onvolutiona lEncoding

SymbolR epetition

B lockIn terleaving

O rthogonal M odula tion

D ata BurstR andomizer

D irect SequenceSpreading

Q uadratureSpreading

BasebandF iltering

Reduces bit rate needed to represent speech. O perates in a variablem ode of full, 1/2, 1/4 & 1/8 rates. Rate set 1 v ocoder full-rate output is at9.6 kbps and rate set 2 full-rate output is at 14.4 kbps.

Prov ides error detection/correction. Two sym bols are output for eachincom ing bit for rate set 1 and two sym bols are output for each incom ingbit for rate set 2 resulting in an output of 28.8 ksps in both cases..

Repetition of input sym bols from the encoder. Repetition is done tom aintain a constant input to the block interleaver. Full-rate sym bols are notrepeated and sent at full power, half-rate repeated once & sent at halfpower and so on. For rate set 1 the output is m aintained at 19.2 ksps(independent of v ocoding rate) and for rate set 2 the output is 28.9 ksps.

Com bats the effects of Rayleigh fading by ensuring that sequential data isnot lost.

B locks of 6 input sym bols are replaced by a corresponding 64-chip W alshcode

Prov ides v ariable-rate transm ission. Sym bols which are repeated aredeleted (i.e. not transm itted). The transm itted duty cycle v aries with thev ocoder data rate and the transm issions are random ized.

Prov ides spreading of the code. In the reverse link data is spread usingthe user's long code m ask based on the ESN.

The channel is spread with the pilot PN sequence with a zero offset. Thisensures that the base station is locked on to transm issions from its cell.

Conv erts the signals to the cellular frequency range (800 MHz) or the PCSfrequency (1900 MHz).

To RF section

PC M Voice

G eneration o f the up link traffic channel.

ConvolutionalEncoder &Repetition

BlockInterleaver

OrthogonalModulation

Data BurstRandomizer


DTraffic a t9600bps4800bps2400bps1200bps

orAccess Channel

at 4800bps

Long Code M ask permutedwith user ESN for tra ffic

channel or AccessChannel long code mask

R=1/3, K=928.8ksps

307.2kcps

Q PN(No O ffset)

1.2288M cps

1.2288M cps

I PN(No O ffset)Access Channel and Rate Set 1 Up link T raffic Channel G eneration

1/2 PNChip

Delay

ConvolutionalEncoder &Repetition

BlockInterleaver

OrthogonalModulation

Data BurstRandomizer


D14400bps7200bps3600bps1800bps

Long Code M ask permutedwith user ESN.

R=1/2, K=928.8ksps

307.2kcps

Q PN(No O ffset)

1.2288M cps

1.2288M cps

I PN(No O ffset)Rate Set 2 Up link T raffic Channel G eneration

1/2 PNChip

Delay

Data Burst Randomizer is not used on the access channel

Access channel, rate sets 1 and 2 up link traffic channel generation

Power Control

• Power control is of paramount importance for a CDMA system. In order to have max. efficiency, the power received at the BS from all Mobiles must be nearly equal.

– Mobile’s power too low, then many bit errors will occur. – Mobile’s power too high, then the interference level increases.

• Power Control at Mobile– Closed Loop: power control information is sent to the mobile from the BS.

Puncturing is used, 2 data symbols are replaced by one power control symbol (double the power). This bit either indicates a transition up or down in power in 1db increments. The power bit is sent 16 times per 20ms (every 1.25ms) (Pclosed.)

– Open Loop: The mobile senses the strength of the pilot signal and can adjust its power based upon that. If the signal is very strong, the assumption can be made that the mobile is very close to BS and the power should be dropped. The mobile uses Ptarget sent in the access param. Msg. – (Popen).

• The transmitted power at the mobile (in units of dBm) is Ptran= Popen + Pclosed

• Power Control at BS– The BS decreases its power level gradually and wait to hear from the mobile what

the frame error rate (FER) is (power measurement report). If high, the BS then increases its power level.

ReceiveSignal

T ransmitS ignal

Demultip lex M ultip lex

Calcu late Evaluate

AmplifierReceiveSignal

O therS ignals

RadioSignal

O therS ignals

O therS ignals

Base StationTerminal

0 or 1

0 or 1

P closed

P open + P closed

Closed-loop pow er contro l

ReceiveSignal

T ransmitS ignal

M easure

Calcu late

AmplifierReceiveSignal

RadioS ignal

Base StationTerminal

P receive

P open

P open + P closed

Closed-loop pow er contro l

Handoffs • CDMA supports three types of handoffs

– Hard handoff ( Similar to the NA-TDMA (IS-136) )– Soft handoff

• Handoff between two different cells (between two different sites) operating on the same frequency.

– Softer handoff• Handoff between two different sectors of the same cellular site.

• Mobile assists in the handoff process, therefore it is referred to as Mobile Assisted Hand Off (MAHO).

– Mobile report signal measurements to the BS. The roving finger of the Rake receiver is used to measure the pilot signals of neighboring BSs (neighbor list messages sent to mobiles periodically).

– During call setup, a mobile is given a list of handoff thresholds and a list of likely new cells. The mobile keeps track of those cell that fall above the threshold and sends this information to the MSC whenever requested.

• Mobile and MSC classify the neighboring BSs to keep track of the handoff process.

– Based upon data received from the mobile the MSC constantly re-classifies the BSs with regard to the mobile:

• Active list: contains BSs currently used for communication at least one BS.• Candidate list: contains list of BSs that could be used for communication based upon current

signal strength measurements.

Handoffs (Con’t)

– Neighbor list: contains a list of BSs that could soon be promoted to candidate list.

– Remaining list: all other BSs that do not qualify.

• When the MSC moves a BS from the candidate list into the active list, it directs BS to serve the mobile. – MSC informs both the new BS and the mobile and assigns a forward

channel number (Walsh code) for communication.

• Soft handoffs consist of the mobile being served by two BSs. This means the following:– Mobile receives the signal from two BSs.

– Two BSs also receives the signal from the mobile.

• Soft handoffs also eliminate the ping pong effect (i.e., when traveling along the boundary of two cells) as the mobile is being served by two BSs and does not have to switch BSs until absolutely necessary.

Handoffs (Con’t)

• Mobile initiates the handoff– The mobile analyze the measurements and inform the MSC when a

handoff might be necessary. (If one BS’s signal strength becomes much higher that the other).

• Handoff process is controlled by the MSC.– When a handoff occurs all three correlators are switched over to the new

cell and used as a Rake receiver again.

– The connection to the current BS is cutoff and the new BS becomes the current BS.

• Summary of handoff process is as follows:– Mobile communicates with original/current BS.

– Mobile communicates with current cell BS and new cell BS.

– Mobile communicates with the new cell BS (which becomes the current cell).

M SC

PSTN

BSC

CellS ite

B

CellS ite

A

Soft handoff in CDM A

Sw itch

T erm inal New BaseO ld Base

conversation

neighbor p ilo t > threshold:add neighbor to candidate set

PILO T STRENG TH M EASUREM ENT

select channel a t newbase

new active set

HANDO FF D IRECTIO N

tra ffic channel in fo

program corre la tors

HANDO FF CO M PLETE

conversation

select signal forsource decoder

active signal< threshold

PILO T STRENG TH M EASUREM ENT

HANDO FF D IRECTIO N HANDO FF D IRECTIO N

HANDO FF CO M PLETE HANDO FF CO M PLETE

re lease corre la tors

conversation

Soft handoff procedure

Mobile Management

• Mobiles must register with a system if they want to receive or make calls.

• There are 5 different types of autonomous registration messages in IS-95. System msgs on the Sync channel indicates will ones are in effect. – Power up

– Power down (de-registration)

– Timer exceeds a threshold

– Distance between new and old BS exceeds a certain limit. • BS’s sends out GPS info. in system’s messages which includes distance

threshold.

– New zone (cells under one MSC are clustered in zones).

• There are 4 other types of registration that are not mobile initiated, i.e., BS asks for it – mobile changes some parameter and informs the BS implicitly in the page response.

Mobile Management (Cont.)

• When a mobile registers it also will indicate which slots it will listen to when the paging channel is in slotted mode. – It also provides other parameters such as protocol version and class type

that it is using so that the MSC knows how to communicates with it and what services to provide.

• Roaming: CDMA system consists of system Ids (SID) and network Ids (NID).– System has many networks within it so a mobile has to keep track of the

SID/NID pair of the area it is in (broadcast by the BSs).

– Each mobile has a list of home SIDs and NIDs. If it enters an area that has an NID that is not on the list, but the SID is classified as NID roaming.

– If the SID is not on the list it is SID roaming.

– Once the mobile knows it’s a roamer it will figure out what kind of services it will be able to access in this foreign (non home) environment.

code division multiple access (cdma)

Documents