Channels in CDMA (IS-95)

The IS-95 CDMA system is unique in that its forward and reverse links have
different link structures. This is necessary to accommodate the requirements of
a land-mobile communication system. The forward link consists of four types
of logical channels: pilot, sync, paging, and traffic channels. There is one pilot
channel, one sync channel, up to seven paging channels, and several traffic
channels. Each of these forward-link channels is first spread orthogonally by its
Walsh function, then it is spread by a quadrature pair of short PN sequences.
All channels are added together to form the composite SS signal to be
transmitted on the forward link.

The reverse link consists of two types of logical channels: access and
traffic channels. Each of these reverse-link channels is spread orthogonally by a
unique long PN sequence; hence, each channel is identified using the distinct
long PN code. The reason that a pilot channel is not used on the reverse link is
that it is impractical for each mobile to broadcast its own pilot sequence.

Forward CDMA Channel

The IS-95 CDMA system uses a 64 by 64 Hadamard matrix to generate 64

Walsh functions that are orthogonal to each other, and each of the logic channels
on the forward link is identified by its assigned Walsh function.

Walsh Codes
Various physical channels may exist at any time on a radio interface. To
separate these channels at the receiver, they are spread with Walsh codes
at the transmitter. These codes are formed by the rows of an N _ N square
matrix, whose entries are either 0 or 1. Usually, N _ 2n where n is an
integer. They are orthogonal because if a 0 is mapped to -1 and a 1 to 1,
then the sum of the term-by-term products of any two rows of this matrix is
0. This matrix, also known as the Hadamard matrix.

IS-95 uses a set of 64 fixed-length Walsh codes to spread forward physical

channels. For example, Walsh code 0 is assigned to the pilot channel, code 32 to
the sync channel, codes 1—7 to paging channels, and the rest to the forward
traffic channels. In the reverse direction, they are used for orthogonal modulation
where every six symbols from the block interleaver output are modulated as one
of 64 Walsh codes.

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1. Pilot Channel

The pilot channel is identified by the Walsh function 0 (w0). The channel itself
contains no baseband information. The baseband sequence is a stream of 0s
that are spread by Walsh function 0, which is also a sequence of all 0s. The
resulting sequence (still all 0s) is then spread, or multiplied, by a pair of
quadrature PN sequences. Therefore, the pilot channel is effectively the PN
sequence itself. The PN sequence with a specified offset uniquely identifies the
particular sector that is transmitting the pilot signal. Note that both Walsh function
0 and the PN sequence are running at a rate of 1.2288 Mcps. After PN
spreading, baseband filters are used to shape the digital pulses. These filters
effectively low pass filter the digital pulse stream and control the baseband
spectrum of the signal. This way, the signal bandwidth may have a sharper roll-
off near the band edge. The pilot channel is transmitted continuously by the base
station sector. The pilot channel provides the mobile with timing and phase
reference. The mobile’s measurement of the signal-to-noise ratio (i.e., Ec /I0) of
the pilot channel also gives an indication of which is the strongest serving sector
of that mobile.

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2. Sync Channel

Unlike the pilot channel, the sync channel carries baseband information. The
information is contained in the sync channel message that notifies the mobileof
important information about system synchronization and parameters. Figure
shows that the baseband information is error protected and interleaved. It is then
spread by Walsh function 32 and further spread by the PN Sequence that is
identified with the serving sector. The baseband information is at a rate of 1.2
Kbps.

Scrambling Codes
In CDMA, each bit time is subdivided into m short intervals called chips.
Typically there are 64 or 128 chips per bit. Each station is assigned a
unique m-bit chip sequence. To transmit a 1 bit, a station sends its chip
sequence. To transmit a 0 bit, it sends the one's complement of its chip
sequence. No other patterns are permitted. Thus for m = 8, if station A is
assigned the chip sequence 00011011, it sends a 1 bit by sending 00011011
and a 0 bit by sending 11100100. Increasing the amount of information to
be sent from b bits/sec to mb chips/sec can only be done if the bandwidth
available is increased by a factor of m, making CDMA a form of spread
spectrum communication (assuming no changes in the modulation or
encoding techniques). In order to protect the signal, the chip sequence
code used is pseudo-random, it appears random, but is actually
deterministic, so that the receiver can reconstruct the code for
synchronous detection. This pseudo-random code is also called pseudo-
noise (PN).

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3. Paging Channel

Similar to the sync channel, the paging channel also carries baseband
information. But unlike the sync channel, the paging channel transmits at higher
rates; it can transmit at either 4.8 or 9.6 Kbps. The PRAT field in the sync
channel message informs the mobile of the data rate of the paging channel.
Once the mobile acquires timing and synchronization using the sync channel, the
mobile begins to monitor the paging channel. Although there can be up to seven
paging channels per sector, each mobile only monitors one paging channel.

4. Traffic Channel

The forward traffic channel is used to transmit user data and voice; signaling
messages are also sent over the traffic channel. The structure of the forward
traffic channel is similar to that of the paging channel. The only difference is that
the forward traffic channel contains multiplexed PCBs

The base station sends the power-control commands to the mobile using the
forward link. These power-control commands are in the form of power control bits
(PCBs). The amount of mobile power increase and power decrease per each
PCB is nominally

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Modulator

The output of the logical channels is fed into the modulator. The gain of each
logical channel, including pilot, sync, paging, and all traffic channels, is first
adjusted by the gain control function. The gain of each channel dictates how
much power is to be transmitted for that channel. The gains for the individual
traffic channels are dynamically changing (i.e., they are controlled by the forward
Power-control process). After the channel gains are adjusted, the signals are
coherently added together to form the composite spread-spectrum signal. After
the summation, both the I and the Q paths are up-converted by their respective
carriers. The up-converted signals then are added together to form the final pass
band QPSK signal.

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Reverse CDMA Channel

The reverse link supports two types of logical channels: access channels and
traffic channels. Because of the noncoherent nature of the reverse link, Walsh
functions are not used for channelization. Instead, long PN sequences are used
to distinguish the users from one another.

1. Access Channel

The access channel is used by the mobile to communicate with the base station
when the mobile doesn’t have a traffic channel assigned. The mobile uses this
channel to make call originations and respond to pages and orders. The
baseband data rate of the access channel is fixed at 4.8 Kbps. The baseband
information is first error protected by an R = 1/3 convolutional encoder. The lower
encoding rate makes error protection more robust on the reverse link, which is
often the weaker of the two links. The symbol repetition function repeats the
symbol once, yielding a code symbol rate of 28.8 Ksps. The data is then

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interleaved to combat fading. Following interleaving, the data is coded by a 64-
ary orthogonal modulator. The set of 64 Walsh functions is used, but here the
Walsh functions are used to modulate, or represent, groups of six symbols. The
reason for orthogonal modulation of the symbols is again due to the noncoherent
nature of reverse link. When a user’s transmission is not coherent, the receiver
(at the base station) still has to detect each symbol correctly. Making a decision
of whether or not a symbol is +1 or -1 may be difficult during one symbol period.
However, if a group of six symbols is represented by a unique Walsh function,
then the base station can easily detect six symbols at a time by deciding which
Walsh function is sent during that period. The receiver can easily decide which
Walsh function is sent by correlating the received sequence with the set of 64
known Walsh functions. Note that on the forward link, Walsh functions are used
to distinguish among the different channels. On the reverse link, Walsh functions
are used to distinguish among the different symbols (or among groups of six
symbols).

2. Traffic Channel

The reverse traffic channel is used to transmit user data and voice; signaling messages are also
sent over the traffic channel. The structure of the reverse traffic channel is similar to that of the
access channel. The major difference is that the reverse traffic channel contains a data burst
randomizer. The orthogonally modulated data is fed into the data burst randomizer. The function
of the data burst randomizer is to take advantage of the voice activity factor on the reverse link.
Forward link uses a different scheme to take advantage of the voice activity factor—when the
vocoder is operating at a lower rate, the forward link transmits the repeated symbols at a reduced
energy per symbol and thereby reduces the forward-link power during any given period.

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