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
Add check bits
Information
A/D
mux
Information Bits
FEC
Code Symbols
Spreading
Code
Generator
Chips
Chips
Spreader
PSK
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.
Input
Correlator 2
Correlator 3
Searcher
Combiner
Correlator 1
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 ]
W3 = [ 0 1 1 0 ]
W2 = [ 0 0 1 1 ]
ORTHOGONAL SPREADING AT THE BASE STATION
Input Data
1 0 ....
Each data bit(symbol) is exclusive-or'd with a 64 bit
Walsh Function (Walsh Function 20)
Each data bit(symbol)
is exclusive- or'd with a 64 bit
Walsh Function (Walsh Function 20)
0000111100001111111100001111000000001111000011111111000011110000
0000111100001111111100001111000000001111.......
Walsh Function 20
Walsh Function 20
Pattern transmitted by the Base Station
Pattern transmitted by the Base Station
1111000011110000000011110000111111110000111100000000111100001111
0000111100001111111100001111000000001111........
DESPREADING OF THE RECEIVED PATTERN AT THE MOBILE STATION
Received Pattern at the Mobile Station
Received Pattern at the Mobile Station
1111000011110000000011110000111111110000111100000000111100001111
0000111100001111111100001111000000001111........
Each 64 bit(symbol) block of the received pattern is exclusiveor'd with Walsh Function 20
0000111100001111111100001111000000001111000011111111000011110000
0000111100001111111100001111000000001111........
1111111111111111111111111111111111111111111111111111111111111111
0000000000000000000000000000000000000000........
Output Data 1 0 .......
Orthogonal Spreading/Despreading
DESPREADING OF THE RECEIVED PATTERN AT THE MOBILE STATION WITH
INCORRECT WALSH FUNCTION
Received Pattern at the Mobile Station
1111000011110000000011110000111111110000111100000000111100001111
Received Pattern at the Mobile Station
0000111100001111111100001111000000001111........
Each 64 bit(symbol) block of the received pattern is exclusiveor'd with Walsh Function 40 which is not the same Walsh
function used for orthogonal spreading at the base station
0000111100001111111100001111000000001111000011111111000011110000
0000111100001111111100001111000000001111........
Walsh Function 40
Walsh Function 40
1111000000001111000011111111000000001111111100001111000000001111
0000111111110000111100000000111111110000.....
Inconclusive output - Equal number of 1s and 0s in the despread pattern
Orthogonal despreading with incorrect Walsh code
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.
Pilot
Sync
Forward
Paging
Variable-Bit-Rate
User Information
Power Control
Traffic
Signaling Messages
Access
Reverse
Variable-Bit-Rate
User Information
Traffic
Signaling Messages
CDMA 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 Transmitted by the Base Station)
Walsh Code
Pilot
Channel
Sync
Channel
Paging
Channel1
Paging
Channel7
Trafic
Channel1
Trafic
Channel55
W0
W32
W1
W7
W8
W63
Structure of Forward CDMA channel
I PN
Walsh
Funtion 0
+
1.2288
Mcps
Pilot
Channel
(All 0's)
+
+
Q PN
Walsh Function 32
R = 1/2, K = 9
Sync Channel
1200 bps
Convolutional
Encoder and
Repetition
Modulation
Symbol
4800 sps
Block
Interleaver
Modulation
Symbol
I PN
1.2288
Mcps
+
+
4800 sps
+
Q PN
I PN
Wp
Paging Channel
9600 bps
4800 bps
Convolutional
Encoder and
Repetition
Paging Channel
Address Mask
Modulation
Symbol
Long Code
PN Generator
Block
Interleaving
1.2288
Mcps
Modulation
Symbol
19.2
Ksps
Decimator
Pilot, sync, and paging channel generation
+
+
1.2288
Mcps
+
+
19.2
Ksps
Q PN
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.
PCM Voice
Vocoder
Convolutional
Encoding
Reduces bit rate needed to represent speech. Operates in a variable
mode of full, 1/2, 1/4 & 1/8 rates. Rate set 1 vocoder full-rate output is at
9.6 kbps and rate set 2 full-rate output is at 14.4 kbps.
Provides error detection/correction. Two symbols are output for each
incoming bit.
Symbol
Repetition
Repetition of input symbols from the encoder. Repetition is done to
maintain a constant input to the block interleaver. Full-rate symbols are not
repeated and sent at full power, half-rate repeated once & sent at half
power and so on. For rate set 1 the output is maintained at 19.2 ksps
(independent of vocoding rate) and for rate set 2 the output is 28.9 ksps.
Puncturing
Used only for for vocoder operating in rate set 2 mode. Deletes 2 out of
every 6 inputs for an output of 19.2 ksps. This results in an identical input
rate to the block interleaver of 19.2 ksps irrespective of the rate set of the
vocoder.
Block
Interleaving
Combats the effects of Rayleigh fading by ensuring that sequential data is
not lost.
Data Scrambling
Power Control
Subchannel
Provides security by scrambling the input data with a long code mask
permuted with the users ESN.
Provides a very fast power control subchannel (800 times per second).
The input data is puntured 800 times per second and a power up/down
command is sent to the mobile station. Each command can increase or
secrease a mobile stations power by 1 dB.
Orthogonal
Spreading
Provides identity and orthogonality to the forward channels by spreading
them with a unique Walsh code. Each input symbol is exclusive-or'd with a
64-bit Walsh code resulting in a data rate of 1.288 Mcps (megachips per
second).
Quadrature
Spreading
Provides unique base-station identity. The spreading sequence is 32768
chips and repeats every 26.66 ms. The same sequence is used by all base
stations but is phase-offset in each. There are 512 possible offsets.
Ensures that the mobile station is locked on to the right base station.
Baseband
Filtering
Converts the signals to the cellular frequency range (800 MHz) or the PCS
frequency (1900 MHz).
To RF section
Functions involved in creating a downlink traffic channel
Rate Set 1 Downlink Traffic Channel Generation
I PN
Power
Control
Bit
Wn
R=1/2, K=9
9600 bps
4800 bps
2400 bps
1200 bps
From Vocoder
Convolutional
Encoder and
Repetition
Modulation
Symbol
Block
Interleaver
19.2 Ksps
19.2
Ksps
Long Code
Mask (ESN)
Long
Code PN
Generator
1.2288
Mcps
M
U
X
Modulation
Symbol
800 Hz
Q PN
1.2288 Mcps
Decimator
Decimator
Rate Set 2 Downlink Traffic Channel Generation
I PN
Power
Control
Bit
Wn
R=1/2, K=9
14400 bps
7200 bps
3600 bps
1800 bps
From Vocoder
Convolutional
Encoder and
Repetition
Symbol
Punturing
Block
Interleaver
1.2288
Mcps
M
U
X
Modulation
Symbol
19.2 Ksps
19.2
Ksps
Q PN
800 Hz
User Address
Mask (ESN)
Long
Code PN
Generator
1.2288 Mcps
Decimator
Rate set 1 and 2 downlink traffic channel generation
Decimator
192 bits (20 ms)
9600 bps
Frame
172
12
8
Information Bits
Full Rate
F
T
80
8
8
Information Bits
1/2 Rate
F
T
96 bits (20 ms)
4800 bps
Frame
48 bits (20 ms)
2400 bps
Frame
40
8
Information Bits
1/4 Rate
T
24 bits (20 ms)
1200 bps
Frame
16
8
Information Bits
1/8 Rate
T
F is a Frame Quality Indicator (CRC) field. It is calculated on information bits
only. Not calculated for 2400 bps or 1200 bps frames. T is an encoder tail bit.
Set to zeroes on all frames.
Downlink/uplink traffic channel frame structure for rate set 1.
288 bits (20 ms)
14400 bps
Frame
1
267
12
8
R/E
Information Bits
Full Rate
F
T
144 bits (20 ms)
7200 bps
Frame
1
125
10
8
R/E
Information Bits
1/2 Rate
F
T
72 bits (20 ms)
3600 bps
Frame
1
55
8
8
R/E
Information Bits
1/4 Rate
F
T
36 bits (20 ms)
1800 bps
Frame
1
21
6
8
R/E
Information Bits
1/8 Rate
F
T
R is reserved bit used in the downlink, E is used in the reverse link to indicate bad frame
reception by MS or BS. F is a Frame Quality Indicator (CRC) field. It is calculated on the
information bits only. Calculated for all frames. T is an encoder tail bit. Set to zeroes on all
frames.
Downlink/uplink traffic channel frame structure for rate set 2.
Number of Bits per Frame (20 ms) in Full Rate (9,600 b/s) CDMA Traffic
Channels.
Transmission Mode
Blank-and-Burst
Dim-and-Burst
Speech Only
0
16
16
16
171
Control Message
168
152
152
152
0
Content Indicator
4
4
4
4
1
Parity Check
12
12
12
12
12
Coder tail bits
8
8
8
8
8
192
192
192
192
192
Speech
Information bits
Contents of 20 ms Frames, Forward Channels
Data Rate R b/s
1,200
2,400
4,800
9,600
Information rate RI b/s
800
2,000
4,000
8,600
Information bits per frame (IBPF)
16
40
80
172
Parity bits per frame (PBPF)
0
0
8
12
Data bits per frame (IBPF + PBPF + 8)
24
48
96
192
Coded bits per frame (CBPF)
48
96
192
384
Repetitions
8
4
2
1
384
384
384
384
Total bits per frame (BPF)
172 bits
9600 bps Primary Traffic only
1
171
MM
Primary Traffic
172 bits
9600 bps dim-and-burst with
rate 1/2 primary and signaling
traffic
1
1
2
80
88
MM
TT
TM
Primary Traffic
Secondary/ Signaling
Traffic
172 bits
9600 bps dim-and-burst with
rate 1/4 primary and signaling
traffic
1
1
2
MM
TT
TM
40
Primary Traffic
128
Secondary/ Signaling
Traffic
172 bits
9600 bps dim-and-burst with
rate 1/8 primary and signaling
traffic
1
1
2
MM
TT
TM
16
Primary Traffic
152
Secondary/ Signaling
Traffic
172 bits
9600 bps blank-and-burst with
signaling traffic only
1
1
2
168
MM
TT
TM
Secondary/ Signaling Traffic
MM indicates if mixed mode traffic is being used, it is set to 0 if only primary traffic is being sent, 1
otherwise.
TT is a traffic type field which indicates if secondary or signaling traffic is being sent.
TM is a traffic mode bit which indicates the mode of operation.
Frame structure for downlink 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.
Begin
set initial
power
send
probe
yes
ACK
received
before timeout
?
no
yes
yes
Access
Succeeds
Access
Fails
max attempts ?
max probes ?
no
new attempt
Access Protocol
no
new probe:
raise power,
wait random time
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.
PCM Voice
Vocoder
Reduces bit rate needed to represent speech. Operates in a variable
mode of full, 1/2, 1/4 & 1/8 rates. Rate set 1 vocoder full-rate output is at
9.6 kbps and rate set 2 full-rate output is at 14.4 kbps.
Convolutional
Encoding
Provides error detection/correction. Two symbols are output for each
incoming bit for rate set 1 and two symbols are output for each incoming
bit for rate set 2 resulting in an output of 28.8 ksps in both cases..
Symbol
Repetition
Repetition of input symbols from the encoder. Repetition is done to
maintain a constant input to the block interleaver. Full-rate symbols are not
repeated and sent at full power, half-rate repeated once & sent at half
power and so on. For rate set 1 the output is maintained at 19.2 ksps
(independent of vocoding rate) and for rate set 2 the output is 28.9 ksps.
Block
Interleaving
Combats the effects of Rayleigh fading by ensuring that sequential data is
not lost.
Orthogonal Modulation
Blocks of 6 input symbols are replaced by a corresponding 64-chip Walsh
code
Data Burst
Randomizer
Provides variable-rate transmission. Symbols which are repeated are
deleted (i.e. not transmitted). The transmitted duty cycle varies with the
vocoder data rate and the transmissions are randomized.
Direct Sequence
Spreading
Provides spreading of the code. In the reverse link data is spread using
the user's long code mask based on the ESN.
Quadrature
Spreading
The channel is spread with the pilot PN sequence with a zero offset. This
ensures that the base station is locked on to transmissions from its cell.
Baseband
Filtering
Converts the signals to the cellular frequency range (800 MHz) or the PCS
frequency (1900 MHz).
To RF section
Generation of the uplink traffic channel.
Access Channel and Rate Set 1 Uplink Traffic Channel Generation
R=1/3, K=9
Convolutional
Encoder &
Repetition
28.8
ksps
Block
Interleaver
Orthogonal
Modulation
307.2
kcps
I PN(No Offset)
1.2288
Mcps
Data Burst
Randomizer
1/2 PN
Chip
Delay
D
Traffic at
9600bps
4800bps
2400bps
1200bps
or
Access Channel
at 4800bps
1.2288
Mcps
Long Code Mask permuted
with user ESN for traffic
channel or Access
Channel long code mask
Q PN(No Offset)
Long
Code PN
Generator
Data Burst Randomizer is not used on the access channel
Rate Set 2 Uplink Traffic Channel Generation
R=1/2, K=9
Convolutional
Encoder &
Repetition
Block
Interleaver
28.8
ksps
Orthogonal
Modulation
307.2
kcps
I PN(No Offset)
1.2288
Mcps
Data Burst
Randomizer
1/2 PN
Chip
Delay
D
14400bps
7200bps
3600bps
1800bps
Long Code Mask permuted
with user ESN.
Long
Code PN
Generator
1.2288
Mcps
Access channel, rate sets 1 and 2 uplink traffic channel generation
Q PN(No Offset)
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.
Other
Signals
Terminal
Base Station
Receive
Signal
Transmit
Signal
Demultiplex
Multiplex
Other
Signals
0 or 1
0 or 1
Calculate
Evaluate
P closed
Radio
Signal
Receive
Signal
Amplifier
P open + P closed
Closed-loop power control
Other
Signals
Terminal
Base Station
Receive
Signal
Transmit
Signal
Measure
P receive
Calculate
P open
Radio
Signal
Receive
Signal
Amplifier
P open + P closed
Closed-loop power control
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).
PSTN
MSC
B
S
C
Cell
Site
B
Cell
Site
A
Soft handoff in CDMA
Switch
Old Base
Terminal
New Base
conversation
neighbor pilot > threshold:
add neighbor to candidate set
PILOT STRENGTH MEASUREMENT
select channel at new
base
new active set
traffic channel info
HANDOFF DIRECTION
program correlators
HANDOFF COMPLETE
conversation
select signal for
source decoder
active signal
< threshold
PILOT STRENGTH MEASUREMENT
HANDOFF DIRECTION
HANDOFF COMPLETE
HANDOFF DIRECTION
HANDOFF COMPLETE
release correlators
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 IS95. 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.