Air Specifications of IS-95

1. Frequency allocation: The forward channel carries information from the Base Station to the mobile unit; the reverse channel carries information from the mobile unit to the base station. The transmission channels are shown; the reception of each channel follows the reverse sequence. The forward channels are between 869 and 894 MHz, while the reverse channels are between 824 and 849 MHz. Within these bands, four sub-bands are available for CDMA, of widths 1, 0.1, 9 and 10 MHz.. A PCS version of IS 95 has also been designed for international use in the 1800-2000 MHz band.

In most of the countries, the channel bandwidth is 1.25 MHz.

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Figure 19: Forward and reverse spectrum allotted for IS 95

2. Switching technique: IS 95 is a circuit switched technology giving voice and data services. The data services are delivered at a rate of 9.6 Kbps.

3. Multiple access technique: As the name suggests, IS 95 uses DSSS as multiple access technique with Frequency division duplexing. The data to be sent is multiplied with several high frequency codes namely, Walsh codes, long PN code and short PN code. Significance of these codes are explained later. The chip rate of all the three codes is 1.288 Mcps.

Forward Channel Specifications in IS-95

The forward link uses the same frequency spectrum as AMPS (869-894 Mhz). Each carrier is of 1.25 MHz. Modulation scheme used is QPSK. There are 4 types of logical channel: A pilot, a synchronization, 7 paging and 55 traffic channels. All these channels are first multiplied by a set of orthogonal codes called as Walsh codes.The IS-95 system can be thought of as having many layers of protection against interference. It allows many users to co-exist, with minimal mutual interference. The user information in the forward channel is fed for three stage coding namely, the Walsh coding process, The Long PN coding and the short PN coding. The purpose of all these codes are different from each other which will be discussed at a later stage.

  • Walsh Codes:

    Walsh codes are a matrix of 64 orthogonal set of codes. They are also called as Hadamard codes. A $2 \times 2$, $4 \times 4$ and a $8 \times 8$ Walsh code matrix is shown below.

$W_{2}=\left[\begin{array}{ll}{0} & {0} \\ {0} & {1}\end{array}\right]$, $W_{4}=\left[\begin{array}{llll}{0} & {0} & {0} & {0} \\ {0} & {1} & {0} & {1} \\ {0} & {0} & {1} & {1} \\ {0} & {1} & {1} & {0}\end{array}\right]$, $W_{8}=\left[\begin{array}{llllllll}{0} & {0} & {0} & {0} & {0} & {0} & {0} & {0} \\ {0} & {1} & {0} & {1} & {0} & {1} & {0} & {1} \\ {0} & {0} & {1} & {1} & {0} & {0} & {1} & {1} \\ {0} & {1} & {1} & {0} & {0} & {1} & {1} & {0} \\ {0} & {0} & {0} & {0} & {1} & {1} & {1} & {1} \\ {0} & {1} & {0} & {1} & {1} & {0} & {1} & {0} \\ {0} & {0} & {1} & {1} & {1} & {1} & {0} & {0} \\ {0} & {1} & {1} & {0} & {1} & {0} & {0} & {1}\end{array}\right]$

Each row in this matrix is a Walsh code. The first row is termed as Wo followed by W1 and so on. IS-95 uses 64 walsh codes . Hence the last row of this matrix is termed $W_{63}$.

Multiplying each element of $W_1$ to each element of $W_2$ and summating, we get the following:


Thus , the two codes are orthogonal to each other. In similar way we can prove that all the Walsh codes are orthogonal to each other. Before transmission, the desired information is multiplied with one of the Walsh codes and then processed further. These Walsh codes are specific for a specific type of information. For example: $W_0$ is only used to transmit pilot carrier.

a) Pilot Channels: The pilot channel allows a mobile station to acquire timing for the forward CDMA channel, provides a phase reference for coherent demodulation, and provides each mobile with a means for signal strength comparison between Base Stations for determining when to initiate handoff. It is also used to identify sectors or cells. It is usually kept 4-6 dB stronger than all other channels while transmission as it is a very important signal. The transmit power of a pilot channel is constant, i:e no power control is used. It is obtained using all zero Walsh code; i.e. contains no information except the RF carrier.

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Figure 20: Processing of Pilot channel

As shown in Figure 20, a stream of all zeroes is first multiplied with $W_0$ walsh code. The resultant stream is at a rate of 1.2288 Mcps. This stream is divided into even and odd parts and fed for QPSK modulation. Significance of PN codes is dealt later in the same section.

b) Synch Channels: Synch message includes system ID (SID), network ID (NID), the offset of the PN short code, and the paging channel data rate (4.8/9.6 Kbps). It is used to acquire initial time synchronization. Uses W32 for spreading. It operates at 1200 bits/second. The transmission of synch channel bits is done as follows:

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Figure 21: Processing of synch channel

The synch messages at 1.2 kilo symbols per second (ksps) is fed to a 1/2 rate of a convolution encoder which yields coded symbols at rate of 2.4 ksps. This is fed for symbol repetion to give a constant rate of 4.8 ksps. The synch message is block interleaved and multiplied with Walsh code 32. It then goes for short PN coding modulated and transmitted.

c) Paging channels: It transmits control information to the MS such as,

  1. Page message to indicate incoming call
  2. System information and instructions
  3. Handoff thresholds
  4. Maximum number of unsuccessful access attempts
  5. List of surrounding cells PN Offsets
  6. Channel assignment messages
  7. Acknowledgments to access requests

It operates at either 4.8 kbps or 9.6 kbps

The 19.2 kbps stream is block interleaved. Block size is 20 ms (384 bits) but the information is essentially a stream.The data is scrambled by multiplying it with a 19.2 kbps stream generated by decimating a long code generator output.

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Figure 22: Processing of paging channel

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Figure 23: Types of paging channel messages in IS 95

All the types of paging channels used in IS 95 and their functions are given in Table 5.

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Figure 24: Forward channel structure of IS 95

As shown in Figure 24, Walsh code 0 is used to send the frequency synchronization information i:e pilot. The synch message is multiplied with $W_{32}$ before transmission. These two are the most important channels for the MS. The paging channels use $W_1$ to $W_7$. The remaining walsh codes are used for sending traffic information. Effectively 55 users can be accommodated in one physical channel of 1.25 Mhz. However the service provider does not require all 7 Walsh codes to transmit paging messages. Even those walsh ( $W_1$ to $W_7$ ) can be allotted to new users for traffic. Thus, number of users can be increased to 61.

d) Forward Traffic Channel: The forward traffic channel processing in CDMAOne is as shown in Figure 25.

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Figure 25: Forward channel transmission sequence

  • Convolution Encoder: IS 95 uses a half rate convolution encoder with constraint length 9 ,doubling the nominal rate from 9.6 kpbs to 19.2kbps, 4.8 kbps data to 9.6 kbps, etc.
  • Repetition Circuit: Repeats coded symbols a number of times to obtain the the desired constant rate of 19.2 kbps irrespective of the input rate. So lower rate encoded data is increased from 9.6, 4.8 or 2.4 kbps to 19.2 kbps. So, an input data at a rate of 2.4 Kbps which has become 4.8 kbps after passing through encoder, has to be repeated 4 times to obtain the data rate of 19.2 kbps.This is done in order to keep a constant baseband symbol rate.
  • Block Interleaver: IS 95 uses Block interleaving. In this method data to be transmitted is written in rows of a matrix. While transmitting, it is read columnwise. This seperates the successive data and spreads important bits over time as proof against deep fades or noise bursts. The matrix used is of order $24 \times 16$ and a delay is introduced after transmission of every column.
  • Long PN code generation: Each user in IS 95 is uniquely identified with a long PN code. This is of length 2^{42} − 1 chips . 42 bit shift register is used to generate the long code. The long PN sequence is uniquely assigned to each user is a periodic long code with period chips. The long code is specified by the following characteristic polynomial.

    $p(x)=x^{42}+x^{35}+x^{33}+x^{31}+x^{27}+x^{26}+x^{25}+x^{22}+x^{21}+x^{19}+x^{18} +x^{17}+x^{16}+x^{10}+x^{7}+x^{6}+x^{5}+x^{3}+x^{2}+x^{1}+1$

  • Decimator: This long PN code is fed to a decimator which divides the code rate by 64 i.e. the decimator divides the incoming chip sequence into groups of 64 chips and outputs only the first chip out of every group.Hence the rate of this code at output of decimator is 19.2 Kcps( Kilo chips per second).

  • Scrambling: The decimator output is then modulo-2 added with the actual data to be sent. This process is called as scrambling. It is the first stage of coding. The data rate remains the same after scrambling procedure. By this process, we ensure that this data will only be decoded by the specific unique user on the receiver side.
  • Power control: Every 1.25 msec, or 24 data symbols, a power control bit is inserted, in order to instruct the mobile unit to raise or lower its power (to equalize the power received from every mobile unit in the cell). The location of the power control bit is determined from the PN sequence.
  • 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 1.2288 Mcps. As discussed earlier,the Walsh functions comprise of sixty-four binary sequences, each of length 64, which are completely orthogonal to each other and provide orthogonal channelization for all users on the forward link. The Walsh sequence repeats every 52.083 $\mu s$, which is equal to one coded data symbol. In other words, each data symbol is spread by 64 Walsh chips. These orthogonal codes are used to differentiate between the transmissions within a cell. Walsh coding process spreads the information signal in frequency domain.
  • Quadrature Spreading: The data are split into two bit streams, which are Modulo 2 added to two different but well defined “Pilot” pseudo-noise sequences generated from 15 bit shift registers. The code repeats 75 times every 2 seconds, or at 26.7 msec intervals. This is called short PN coding process. Every BTS has its own unique short PN code. The BTS multiplies it’s short PN code with data to be transmitted so that ther is no interference between transmissions of various BTSs in vicinity.
  • Quadrature modulation: The binary I and Q outputs are mapped onto four phases of a quadrature modulator, i.e. QPSK modulation is done and then transmitted. The baseband quadrature data are modulated onto the forward cellular radio band, 869 to 894 MHz.The IS-95 channel occupies 1.25 MHz within this band, the rest of which is occupied by other cellular services such as AMPS.

    Reverse Channel Specifications in IS-95

  • These are fundamentally different from the forward channels.

  • The reverse channel bandwidth is 824-869 MHz range.
  • Uses OQPSK for power efficiency
  • No spreading of the data using orthogonal codes
  • Same orthogonal codes are used for WAVEFORM Encoding
  • Two types of logical channels: The Access channels and the reverse Traffic channels.

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Figure 26: Reverse channel transmission sequence

  • Speech Encoder: The speech encoder used in IS 95 is quadrature code excited linear predictive coder ( QCELP). It produces nominal 9600 bps data stream, dynamically reduced to 4800, 2400 or 1200 bps during pauses and gaps in speech; quiet periods correspond to 1200 bps data.
  • Convolution encoder : It encodes the data from one stream to three,tripling the data rate from 9.6 kpbs to 28.8 kbps,4.8 kbps data to 14.4 kbps etc. Reverse channel uses 1/3 rate convolution encoder with constraint length 9. This is required because the reverse channel signal is more prone to fading as MS is transmitting at a lower power level. Hence channel coding bits should be more.
  • Repetition Circuit: Repeats coded symbols, so lower rate encoded data is increased from 9.6, 4.8 or 2.4 kbps to 19.2 kbps. After repetition, the symbol rate out of the coder is fixed at 28,800 bps.
  • Block Interleaver: Reads data into the columns of a 32 $\times$ 18 array,and out of the rows; introduces a 20 msec delay, but spreads important bits over time as proof against deep fades or noise bursts.
  • Orthogonal Mapping: The 28.8 kbps data are split into sequential sets of six bits each, which are mapped to one of 64 Walsh functions. The data rate is therefore raised to 28.8 k $\times$ 64 chips/ 6 bits = 307.2 kpbs.
  • Burst Randomizing: The Walsh symbols are broken into groups of six, each group being 1.25 msec in duration.These are collected into frames of 16 power groups, or 1.25 msec $\times$ 16 = 20 msec. At 9600 bps, all 16 groups are transmitted; at 4800 bps, 8 randomly selected groups are transmitted; at 2400 bps, 4 groups; at 1200 bps, 2 groups. The transmitted groups are chosen randomly,according to a formula based on 14 bits of the PN sequence of the second last group in the previous frame.
  • Direct Sequence Spreading: The data is Modulo 2 added to every bit of a pseudo-noise (PN) sequence created from a 42 bit shift register. The PN sequence is generated at 1.2288 MHz, so each Walsh chip is spread by four long code PN chips. ( Long PN code process to identify who the transmitter is.)
  • Quadrature Spreading: The data is split into two bit streams, which are Modulo 2 added to two different but well defined “Pilot” pseudo-noise sequences generated from 15 bit shift registers. (Short PN code process to identify the receiving BTS)
  • Quadrature Modulation: The binary I and Q outputs are mapped onto four phases of a quadrature modulator, using offset quadrature phase shift keying (OQPSK). (The Q channel is shifted by half a chip ( 406.92 $\mu s$) for improved spectral shaping.)
  • RF Modulation: The baseband quadrature data are raised to the reverse cellular radio band, 824 to 849 MHz.The IS-95 channel occupies 1.25 MHz within this band, the rest of which is occupied by other cellular services such as AMPS.

Note that there is continuous transmission from a cell phone when a conversation is in progress. The lowest data rate is 1200 bps, with three 1.25 msec “power control” groups being transmitted in every 20 msec frame. A mobile phone is therefore not silent during conversations, and can be located by its telltale emissions.


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