Wireless Communication

Chapter: Network Concepts and Standards
Section: Cellular Systems, IS95

The IS95 Reverse Channel

In the IS95 cellular CDMA system, the Reverse (uplink) Channel (mobile to base station) contains Access Channels and Reverse Traffic Channels. The Reverse Traffic Channel carries (or other user data) and network signaling data from mobile to the base station, while the Access Channel is used by the mobile to request the base station to set up a call and to respond to Paging Channel messages (call set-up by the base stations).

A Traffic Channel has a distinct user long code sequence. This allows separation of different user signals at the base station. Moreover, each Access Channel uses a distinct Access Channel long code sequence.

The maximum effective power (ERP) radiated by a mobile transmitter is 8 dBW, that is 6.3 Watts with respect to a half wave dipole. The mobile station adjusts its output power level in response to commands from the base station, over the Forward Traffic Channel. For each received power control bit the output power is modified by 1 dB. Special measures are required to combine power control commands received from different multipath components or from different base stations during soft handoff.

The reverse is95 channel uses convolutional encoding, block interleaving to combat burst errors, 64-ary orthogonal modulation, and spreading by an m-sequence.

Speech segments or data is grouped into frames of 20 milliseconds each. Reverse Traffic Channel frames consists of 192 bits. These 192 bits is composed of 172 information bits followed by 12 frame quality indicator bits and eight Encoder Tail Bits. The Reverse Traffic Channel runs at 9600, 4800, 2400 or 1200 bit/s, which can be selected on a frame-by-frame basis. While the channel burst transmission rate is fixed at 28,800 symbols per second, these different rates are created by varying the transmit duty cycle; for 9600 bit/s frames it is 100 percent, for 4800 bit/s frames it is 50 percent, and so forth.

Convolutional Encoding

The mobile station uses convolutional coding. Code symbols are generated by modulo-2 addition (EXOR-ing) of selected taps of a serially time-delayed data sequence. The length of the data sequence delay is equal to K-1, where K is the constraint length of the code.

Figure: The IS95 reverse channel convolutional encoder.

The convolutional code has rate 1/3 and has a constraint length of K=9. This is a rate 1/3 code generates three code symbols for each data bit input to the encoder. The generator functions for these three bits are

        g₀ equals 557 (octal), 
        g₁ equals 663 (octal), and 
        g₂ equals 711 (octal).

Rate 1/3 is a relatively powerful error correction coding that requires quite sophisticated circuitry at the base station receiver. It is used because at the CDMA base station multiple signals arrive with approximately the same power. Due to multipath delay spreads and synchronization errors, these user signals cause mutual interference. The stronger the error control code, the more signals can be tolerated on the channel, so the higher the user capacity of the system.

Symbol Repetition

Code symbols coming from the convolutional encoder are repeated before being interleaved. The repetition rate on the Reverse Traffic Channel depends on data rate to artificially create a bit rate of 9600 bit/s or 28,800 channel bit/s. That is,

At 4800 bit/s, each symbol is repeated one more time, i.e, each symbol occurs twice
At 2400 bit/s, each symbol occurs 4 times.
At the 1200 bit/s, each symbol occurs 8 times.

On the traffic channel, these repeated symbols are not truly transmitted multiple times to the base station. The repeated code symbols are interleaved and all but one of symbol repetitions is deleted prior to actual transmission. This also creates the variable transmission duty cycle. The access channel, on the other hand has a fixed data rate of 4800 bit/s. Here each symbol occurs twice.

Block Interleaving

The mobile station interleaves code symbols, on the Reverse Traffic Channel as well as on the Access Channel. The block interleaver uses 576 elements to span 20 ms. It forms an array with 32 rows and 18 columns.

Orthogonal Modulation

The reverse IS95 channel uses 64-ary orthogonal modulation, where one out of an alphabet of 64 possible modulation symbols is transmitted for each six code symbols. The modulation symbol is generated using Walsh Hadamard sequences. The transmission rate is fixed at 4800 modulation symbols per second. As six code symbols are modulated as one out of 64 modulation symbols, the channel bit rate is 6 times 4,800 or 28,800 bit/s. The time required to transmit a single modulation symbol is 208.333... microsec (1/4800 sec). One-sixty-fourth of the duration modulation symbol is referred to as a Walsh chip and is equal to 1/307200 second. Thus the Walsh chip rate is 307.2 kchips/s. The PN-sequence runs at 1.2288 Mchips/s, so each Walsh chip is spread by four m-chips.

Variable Data Rate Transmission

The output of the interleaver is switched on and off by a gate that allows transmission of certain interleaver output symbols and deletes of others. Only if the transmit data rate is 9600 bit/s, the transmission gate passes all interleaver symbols to the transmitter. When the transmit data rate is 4800 bit/s, the transmission gate allows one-half of the interleaver output symbols to be transmitted, and so forth. The splits each frame into 16 periods of equal length (i.e., of 1.25 ms). These are called power control groups. A power control groups is either transmitted in full or deleted completely.

The location of power control groups that are passed by the gate are (pseudo-) randomized within the frame. The data burst randomizer ensures that every code symbol input to the repetition process is transmitted exactly once. By switching the transmitter off during periods that no new data comes from the interleaver, the mobile station reduces the interference to other users on the same channel. When transmitting on the Access Channel, each symbol is sent twice, i.e., two identical copies of the code symbols are transmitted.

Data Burst Randomizing Algorithm

The data burst randomizer generates a masking pattern of zeros and ones to balance the number of zeros and ones. The masking pattern is determined by the data rate and by a block of 14 bits taken from the long code. These are the last 14 bits of the long code used for spreading in the previous to the last power control group of the previous frame.

Direct Sequence Spreading

The long code

The Reverse Traffic Channel and the Access Channel is spread by the long code, in the following way

For the Reverse Traffic Channel, the spreading code is the EXOR (modulo-2 addition) of the data burst randomizer output stream and the long code.
For the Access Channel, the spreading code is the modulo-2 addition of the 64-ary orthogonal modulator output stream and the long code.

This long code is has a period 2^42-1 chips. It is obtained from a Linear Feedback Shift Register (LFSR) with the characteristic polynomial:


p(x) = x⁴² + x³⁵ + x³³ + x³¹ + x²⁷ + x²⁶ + x²⁵ + x²² + x²¹ + x¹⁹ +
x¹⁸ + x¹⁷ + x¹⁶ + x¹⁰ + x⁷ + x⁶ + x⁵ + x³ + x² + x¹ + 1.

However, the long code is not simply the output of the LFSR, but all 42 bits in the LFSR are used to generate one output bit.. In fact, a chip of the long code is generated by the modulo-2 inner product of a 42-bit mask and the 42-bit state vector of the sequence generator. The mask used for the long code varies depending on the channel type on which the mobile station is transmitting. When transmitting on the Access Channel, the mask is

M41 through M33 are set to "110001111";
M32 through M28 are set to the Access Channel number chosen;
M27 through M25 are set to the code channel number for the associated Paging Channel (the range is 1 through 7),
M24 through M9 are set to the identifier for the current base station; and
M8 through M0 are set to the pilot tone value for the current CDMA Channel .

The mobile uses one of two long code masks unique to that mobile station: a public long code mask unique to the mobile station's ESN or a private long code mask. The public long code mask is

M41 through M32 is set to "1100011000", and

M31 through M0 is set to a permutation of the mobile station's ESN bits.
This permutation is specified as


ESN =	(E₃₁, E₃₀, E₂₉, E₂₈, E₂₇, E₂₆, E₂₅, . . . E₂, E₁, E₀)

Permuted ESN = 
	(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).

Quadrature Spreading (Short code)

After direct sequence spreading, the Reverse Traffic Channel and Access Channel are spread in quadrature. The sequences are the zero-offset I and Q pilot m-sequences used on the Forward CDMA Channel. These (short) sequences are periodic with period 2¹⁵ chips. They use the characteristic polynomials


P_I(x) = x¹⁵ + x¹³ + x⁹ + x⁸ + x⁷ + x⁵ + 1

for the in-phase (I) sequence and


P_Q(x) = x¹⁵ + x¹² + x¹¹ + x¹⁰ + x⁶ + x⁵ + x⁴ + x³ + 1

for the quadrature-phase (Q) sequence. The maximum length LFSR sequences, {i(n)} and {q(n)}, based on the above polynomials are of period 2¹⁵-1. These are generated from the following linear recursions:


i(n) = i(n-15) + i(n-10) + i(n-8) + i(n-7) + i(n-6) + i(n-2)

based on P_I(x) as the characteristic polynomial and


q(n) = q(n-15) + q(n-12) + q(n-11) + q(n-10) + q(n-9) + q(n-5) + q(n-4) + q(n-3)

based on P_Q(x) as the characteristic polynomial, Both i(n) and q(n) are binary-valued ("0" and "1" ) and the additions are modulo-2, i.e., they are in fact exclusive OR operations. In order to obtain the I and Q pilot m-sequences of period 2¹⁵ in stead of 2¹⁵-1, a "0" is inserted in the sequence after 14 consecutive "0"'s. It can easily be shown this occurs only once in each period of the m-sequence.

The pilot m-sequences repeat every 2¹⁵ output symbols and run at 1,228,800 symbols per seconds, i.e., they repeat every 26.6... . millisec. There are exactly 75 repetitions in every 2 seconds.

The data spread by the Q pilot m-sequence is delayed by half a m-chip time with respect to the data spread by the I pilot m-sequence.

Access Channel

An Access Channel transmission is a coded, interleaved, and modulated spread-spectrum signal. The Access Channel uses a random-access protocol. Access Channels are uniquely identified by their long codes.

The mobile station transmit information on the Access Channel at a fixed data rate of 4800 bit/s. The Reverse CDMA Channel may contain up to 32 Access Channels numbered 0 through 31 per supported Paging Channel. At least one Access Channel exists on the Reverse CDMA Channel for each Paging Channel on the corresponding Forward CDMA Channel.

Each Access Channel is associated with a single Paging Channel. An Access Channel frame contains 96 bits (20 ms frame at 4800 bit/s). Each Access Channel frame consists of 88 information bits and eight Encoder Tail Bits.

Reverse Traffic Channel Frame Quality Indicator

Each frame contains a frame quality indicator, except in the 2400 bit/s and 1200 bit/s modes. For both the 9600 bit/s and the 4800 bit/s rates, the frame quality indicator is a Cyclic Redundancy Check (CRC) calculated on all bits within the frame, except the frame quality indicator itself and tail bits. The 9600 bit/s transmission rate use a 12-bit frame quality indicator. The generator polynomial is


g(x) = x¹² + x¹¹ + x¹⁰ + x⁹ + x⁸ + x⁴ + x + 1.

At 4800 bit/s, an 8-bit frame quality indicator is computed, using the generator polynomial


g(x) = x⁸ + x⁷ + x⁴ + x³ + x + 1.

More about timing in IS95

In order to achieve reliable soft handoff in IS95 and to control interference among neighboring cells, it is relevant to keep all base stations synchronized. The Global Positioning System (GPS) time scale is used. It is synchronous with Universal Coordinated Time (UTC). The start of CDMA System Time is January 6, 1980 00:00:00 UTC, which coincides with the start of GPS time. Each point in the CDMA system has its own relation to the CDMA System Time. The timing is referenced to the antennas of the base station and to the RF connector of the mobile. The time at various points in the transmitter and the receiver is the absolute time referenced at the base station antenna. An offset of the one-way or round-trip delay is applied if appropriate.

The long spreading code and the pilot m-sequences for the Inphase and Quadrature channels are in their initial states at the start of System Time. The initial state of the long code is that state in which the output of the long code generator is the first "1" output following 41 consecutive "1" outputs, with the binary mask consisting of "1" in the MSB followed by 41 "1"s. This implies that the 42nd bit in the shift register equals "1" and that all other bits in the shift register are equal to "1". The initial state of the pilot m sequence, both I and Q, is that state in which the output of the pilot m sequence generator is the first "1" output following 15 consecutive "1" outputs. The alignment of the initial states of the long code and the pilot m-sequence does not occur again for more than 37 centuries.

Video

Phil Karn (Qualcomm) explains the basics of the IS95 spreading concept in a presentation at U.C. Berkeley. The spreading method contains a short code and a long code. The short code is mainly for acquisition. The long code is used to separate users.

link to Quicktime file