CDMA Basics is-95 Forward & Reverse Channel

VNIT, Nagpur

CDMA Basics, IS – 95 System Forward and

Reverse Link

Visvesvaraya National Institute of Technology, Nagpur

Vishal R. SatputeAssistant Professor,

Department of Electronics engineering,VNIT, Nagpur.

Sunday, January 01, 2012

V. R. Satpute, Asst. Prof., VNIT, Nagpur 2

VNIT, NagpurCDMA

CDMA (Code Division Multiple Access) CDMA Basics, CDMA Forward Link and, CDMA Reverse Link.



VNIT, NagpurCDMA [1]

CDMA (Code Division Multiple Access) In CDMA, codes are assigned to each user because entire

channel is allotted for entire time duration.

These codes must be orthogonal means their dot product is zero.Ex. Two codes (chips) as given below are orthogonal to each other.

{-1, +1, -1, -1, +1, +1} and {+1, +1, -1, +1, -1, +1}

Dot product of these two codes come out to be zero.




How CDMA works? Suppose, we allot these two codes to two different users say user A

and B. Let, Ak = { -1, +1, -1, -1, +1, +1} i.e. key for user A. Let, Bk = {+1, +1, -1, +1, -1, +1} i.e. key for user B. Check for orthogonal code? Now assume that A and B wants to send a binary (Ad) logic 1 and (Bd)

logic 0 respectively. Hence, say logic 1 = +1 and logic 0 = -1. Then system spreads these logic levels using the keys given above i.e.

Ak and Bk. The transmitted signal for A and B will be: AS = Ad * Ak = +1 * { -1, +1, -1, -1, +1, +1} = {-1, +1, -1, -1, +1, +1} BS = Bd * Bk = -1 * {+1, +1, -1, +1, -1, +1} = {-1, -1, +1, -1, +1, -1} Both signals are then transmitted using same channel and at same

time.




• At the receiver side a new signal i.e. C which is received which is addition of two different signals AS and BS.

• C = AS + BS = {-2, 0, 0, -2, +2, 0}.• At the receiver side, again to detect the original bits, the same

key assigned to user A and B is applied in the following manner.

• To detect Ad:– Ad = C * Ak = {-2, 0, 0, -2, +2, 0} * {-1, +1, -1, -1, +1, +1} = 6 (>0)– Bd = C * Bk = {-2, 0, 0, -2, +2, 0} * {+1, +1, -1, +1, -1, +1} = -6 (<0)– Since received energy is Ad = 6 it can be treated as logic 1 and Bd = -6 it

can be treated as logic 0.




• If B transmits the information with a higher signal level say C = AS + 5 * BS, then at the receiver side, information received will be Bd = - 30 and Ad = +6.

• Since, Ad and Bd ratio is very large, it is difficult to detect Ad while Bd can be easily detected.

• To avoid these issues which occurs because of same band and same time, power control information is required to be given to every MS within the cell so that power level received from individual MS will be same at BTS.

• In CDMA system, over 1000 times per sec power level information is required to be sent which unnecessarily increase system complexity and consumes lot of energy.

• Code sequence in IS – 95 system is 242 – 1 bits long.



VNIT, NagpurIS – 95 Forward Link [1]

• IS = Interim Standard• IS – 95 allows each user within a cell to use same radio

channel and users in adjacent cells also the same radio channel because it is using direct sequence spread spectrum CDMA.

• The vocoder available in IS – 95 system uses voice activity detection and reduces the data rate to 1200 bps when there is silence period.

• In IS – 95, the forward and reverse links are separated by 45 MHz of spectrum and IS – 95 is also compatible with AMPS system.

• IS – 95 uses 824 – 849 MHz for reverse and 869 – 849 MHz for forward links respectively.




• In IS – 95, the forward and reverse links are different. • on the forward link the user data is encoded with rate ½

convolution encoder• While for reverse link it is rate 1/3 convolution encoder.• For both base station and the subscriber, RAKE receivers

are used.• In IS – 95, 64 – bit Walsh codes are used which are

orthogonal to each other.




Modulation Parameters for IS – 95 Forward Channel

Parameter Data rate (bits / sec)User data rate 9600 4800 2400 1200Coding Rate ½ ½ ½ ½

User data repetition period 1 2 4 8

Base-band coded data rate 19,200 19,200 19,200 19,200

PN chips/coded data bit 64 64 64 64

PN chips rate (Mcps) 1.2288 1.2288 1.2288 1.2288

PN chips/bit 128 256 512 1024




• Note here that: 19200 * 64 = 1.2288 Mcps




• Forward Link (BTS MS)

– Band allotted to operator = 50 MHz. – Forward Link = 869 to 894 MHz = 25MHz– Each channel is 1.25 MHz wide.– One channel means 64 different users.– Entire 1.25 MHz is given to all users.– One channel is further subdivided into 4 sub-channels– 1. Pilot:

• Timing• Phase Reference for Coherent Demodulation• Means for Signal Strength Comparison

– 2. Sync:• Broadcasts future state of the long code register (Data rate = 1200bps)

– 3. Paging:• Call Control Information (Data rate = 9.6, 4.8, and 2.4kbps)

– 4. Forward Traffic:• Voice Data (includes power control sub channel, Data rate = 9.6, 4.8, 2.4, and 1.2kbps)




• Data on forward traffic channel is grouped into 20msec frames.

• User data rate is adjusted and always the chip rate is 1.2288Mcps.(Mcps = Million Chips Per Second)

• Chips per bit may change depending upon bit rate.

• Bit rate = 9.6, 4.8, 2.4, and 1.2kbps (kbps = kilo bits per second)

RateBitMcpsbitperchipsofNo 2288.1.







• Convolutional Encoder and Repetition– Adds redundancy to data transmission for error robustness.– Rate, r = ½, where r = input bits / output bits,– Rate ½ means for every 1 input bit it gives 2 output bits.– Maintains an output data rate of 19.2kbps regardless of input rate.




• Convolutional Encoder and Repetition– To maintain a constant rate of 19.2kbps, repetition of data block is

done in the following manner.– Repetition Factor = 19.2kbps/ data rate– Ex. If data rate = 2.4kbps then repetition factor is = 4 i.e. original

data + 3 times repeated data.

Block Interleaving: – It adds the symbols into a frame of 20msec i.e. it puts the symbols

into a frame of 24 by 16 array.– Per second we have 1/20ms = 50 such frames.– Thus (24*16) per frame * 50 frames/sec = 19200 bits/sec– It adds transmission robustness by interleaving of data.




• Long PN Sequence:– The long PN code is assigned to every user in a

repeated manner with a period of 242 - 1 time

interval.– The long code in IS-95 is given as:

– Each PN sequence is generated by modulo-2 addition inner product of a 42-bit mask and a 42-bit state vector of sequence generator.

1)(

123567

1016171819212225262731333542

xxxxxxxxxxxxxxxxxxxxxp




Initial state of the generator is taken as:

Two types of masks are used in the long code generator. Public Mask: For ESN of device Private Mask: For Mobile Station Identification (MIN)

All CDMA calls are initiated using Public Mask and then private mask is given after authentication.

1 00000000000000000000000000000000000000000

41 continuous zeros




• Public code is specified follows:– M41 to M32 = 11000 11000 and ESN number is

permuted as follows:– ESN = {E31, E30,…………..E0}– Permuted ESN =

{E0,E31,E22,E13,E4,E26,E17,E8,E30,E21,E12,E3,E25,E16,E7,E29,E2

0,E11,E2,E24,E15,E6,E28,E19,E10,E1,E23,E14,E5,E27,E18,E9}– Hence, Public code is

11000 11000 {Permuted ESN}11000 11000 {E0,E31,E22,E13,E4,E26,E17,E8,E30,E21,E12,E3,E25,E16,E7,E29, E20,E11,E2,E24,E15,E6,E28,E19,E10,E1,E23,E14,E5,E27,E18,E9}




• Power Control Sub-Channel:– To minimize BER for each user, IS-95 strives to force each user to

provide the same power level at base station.– Power control are sent to each subscriber using ‘bit 0’ to tell user to

increase power level by 1dB and ‘bit 1’ to decrease of power by 1dB.– Since signal and interference continuously varies hence, power level

control information is necessary and transmitted to each subscriber at an interval of 1.25msec.

– In a frame of 20msec, 16 power control bits are sent.– In a 1.25msec duration, 24 data symbols are transmitted.– Power control bits are transmitted using puncturing method.– During a 1.25 msec period, twenty four data symbols are transmitted.– IS-95 transmits power control bits in sixteen different combinations.– Last 4-bits are used to locate power control bits information.




• Ex. Suppose bit positions {23,22,21,20} are {1011}• During transmission the pattern is:

1514131211109876543210

1011

32102322212019181716151413121110987654321023222120

1 Power control bit = 2 Data bits

1.25msec = 24 traffic data bits




• Orthogonal Covering:– Orthogonal covering is performed following the data scrambling on the

forward link.– Each traffic channel transmitted on the forward CDMA channel is spread

with a Walsh function at a fixed chip rate of 1.2288Mcps.– Walsh function comprises of 64 orthogonal codes and are generated

using Hadamard matrix.

NN

NNN HH

HHHH

HH

24

21

;

0110110010100000

1000

]0[




Orthogonal Covering: After orthogonal covering, symbols are spread in

Quadrature and In phase components. The In-phase and Quadrature equations are,

1)(

1)(345610111215

57891315

xxxxxxxxxPxxxxxxxP

Q

I




• I and Q Mapping in forward CDMA channel:– I and Q are mapped according to following rule.

-π/410-3π/4113π/401π/400

PhaseQI





V. R. Satpute, Asst. Prof., VNIT, Nagpur


• The pilot channel:– Provide a reference signal for all MSs that

provides the phase reference for coherent demodulation

– 4-6 dB stronger than all other channels– Used to lock onto other channels– Obtained using all zero Walsh code; i.e.,

contains no information except the RF carrier– Spread using the PN spreading code to identify

the BS. (512 different BS*64 offsets)– No power control in the pilot channel




The pilot channel (IS – 95)




• Used to acquire initial time synchronization

• Synch message includes system ID (SID), network ID (NID), the offset of the PN short code, the state of the PN-long code, and the paging channel data rate (4.8/9.6 Kbps)

• Uses W32 for spreading

• Operates at 1200 bps

The Sync channel (IS – 95)




The Sync channel (IS – 95)




• Used to page the MS in case of an incoming call, or to carry the control messages for call set up

• Uses W1-W7

• There is no power control

• Additionally scrambled by PN long code, which is generated by LFSR of length 42

• The rate 4.8 Kbps or 9.6Kbps

IS – 95 Paging Channel




IS – 95 Paging Channel



VNIT, NagpurIS – 95 Reverse Link [1]

• Reverse Link (MS BTS)– Band allotted to operator = 50 MHz.– Forward Link = 824 to 849 MHz = 25MHz– Each channel is 1.25 MHz wide.– One channel means 64 different users.– Entire 1.25 MHz is given to all users.– One channel is further subdivided into 2 sub-channels– 1. Access Channels: (Fixed data rate = 4800 bps)

– Used by mobile to initiate a communication with base station and to respond to paging channel message.

– 2. Reverse Traffic channel. (Variable data rate)– Reverse traffic channel may contain 32 access channels.– Data rate = 9.6, 4.8, 2.4, and 1.2kbps.




• Data on Reverse traffic channel is grouped into 20msec frames.

• User data rate is adjusted and always the chip rate is 1.2288Mcps.

• Per 6-bits of user one 64 – bit Walsh code is transmitted.

• User bit rate = 9.6, 4.8, 2.4, and 1.2kbps

RateBitMcpsbitperchipsofNo 2288.1.







• Convolutional Encoder and Repetition– Adds redundancy to data transmission for error

robustness.– Rate, r=1/3, where r = input bits / output bits; k = 9.– Generator vector is g0, g1, and g2 = 557 (octal), 663

(octal) and 771 (octal).– Maintains an output data rate of 28.8 kbps (9.6 kbps * 3 )

kbps regardless of input rate by repeating.– In reverse channel six encoded bits are replaced by a

64 – bit Walsh code.




• Convolutional Encoder and Repetition– To maintain a constant rate of 28.8kbps, repetition of

data block is done in the following manner.

• Block Interleaving: – It adds the symbols into a frame of 20msec i.e. it puts the

symbols into a frame of 32 by 18 array.– It adds transmission robustness by interleaving of data.– Code symbols are written into matrix by columns and

read out by rows.

RepetitionFactor=28.8kbpsdata rate




• Variable Data Rate Transmission:– Variable data rate are sent on reverse CDMA channel – Data rate varies from 9600 bps to 1200 bps – One frame on reverse channel = 20msec – When data rate = 9600 bps then all interleaver output bits are transmitted

that is 100% transmission – When data rate = 4800 bps then 50% transmission – When data rate = 2400 bps then 25% transmission– When data rate = 4800, 2400 and 1200 bps then some power control

groups are gated-ON while others are gated-OFF– During gated –OFF, mobile stations reduces its EIRP by 20dB with

respect to power of the previous gated-ON period.




Data burst randomizer generates a masking pattern of 0’s and 1’s that randomly masks the redundant data generated by the code repetition process.

A block of 14 bits taken from long code determines the masking pattern.

Last 14 bits of the 2nd to last power control group of previous frame is taken for masking pattern.

These bits are represented as: b0 b1 b2 b3 b4 b5 b6 b7 b8 b9 b10 b11 b12 b13 where b0 and b13 are

first and last bits respectively.




• If data rate is 9600 bps then transmission occurs on all sixteen power control groups.

• If data rate is 4800 bps then transmission occurs on following 8 power control groups.

• b0 2+b1 4+b2 6+b3 8+b4 10+b5 12+b6 14+b7

• If data rate is 2400 bps then, transmission occurs on only four power control groups.

Sr. No. If Then Else If Then1 b8 = 0 0 + b0 Else b8 = 1 2 + B1

2 b9 = 0 4 + B2 Else b9 = 1 6 + B3

3 b10 = 0 8 + B4 Else b10 = 1 10 + B5

4 b11 = 0 12 + B6 Else b11 = 1 14 + B7




• If data rate is 1200 bps then transmission occurs on only two power control groups.

– (If b8 = 1 and b12 = 0) then b0 or (If b8 = 1 and b12 = 1) then 2 + b1 OR● (If b9 = 0 and b12 = 1) then 4 + b2 or (If b9 = 1 and b12 = 1) then 6 + b3

– (If b10 = 0 and b13 = 0) then 8 + b4 or (If b10 = 1 and b13 = 0) then 10 + b5 OR (If b11 = 0 and b13 = 1) then 12 + b6 or (If b11= 1 and b13 = 1) then 14 + b7




• Direct Sequence Spreading:– Reverse traffic channel is spread by long code PN

sequence which operates at a rate of 1.2288Mcps.– Each Walsh chip is spread by a factor of 4 (1228.8/307.2)

• Quadrature Modulation:– On Reverse traffic channel, OQPSK (offset QPSK) is

used.– The delay element D has a delay of 406.901nsec delay

with respect to “I” channel pilot.– OQPSK has it's own advantages.



VNIT, NagpurReferences…

“Wireless Communication: Principles and Practices”, 2nd Edition, Theodre S. Rappaport, “Pearson Education”.

• “Mobile Communication”, 2nd edition, Jochen Schiller, “Pearson Education”.

• Wireless Network Evolution: 2G to 3G, Vijay K. Garg, “Pearson Education”.

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CDMA Basics is-95 Forward & Reverse Channel

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