code division multiple access (cdma)
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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
ConvolutionalEncoder and
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
ConvolutionalEncoder and
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
ConvolutionalEncoder and
Repetition
BlockInterleaver
LongCode PNGenerator
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
Information B its1/8 Rate
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
Information B its1/2 Rate
F T
1
R/E
55 8 8
72 b its (20 ms)
3600 bpsFrame
Information B its1/4 Rate
F T
1
R/E
21 6 8
36 b its (20 ms)
1800 bpsFrame
Information B its1/8 Rate
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 Primary T raffic Secondary/ S ignalingTraffic
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 dim -and-burst withrate 1/4 prim ary and signalingtraff ic
9600 bps dim -and-burst withrate 1/8 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
LongCode PNGenerator
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
LongCode PNGenerator
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.