Welcome to the World of CDMA
Reverse CDMA Channel
The REVERSE CDMA CHANNEL is the mobile-to-cell direction of communication. It carries traffic and signaling. Any particular reverse channel is active only during calls to the associated mobile station, or when access channel signaling is taking place to the associated base station.
The mobile station transmit frequency is 45 MHz below the base station
transmit frequency in the cellular service (IS-95A), and 80
MHz below in the PCS service (ANSI J-STD-008). Permissible
frequency assignments are on 30 kHz increments in cellular
and 50 kHz in PCS. See Frequency Plans
for further details.
Transmission ParametersThe IS-95A Reverse CDMA Channel currently supports a 9,600 bps rate family in the Access Channel and Traffic Channels, as shown in the table. The transmission duty cycle varies with data rate. In all cases the FEC code rate is 1/3, the code symbol rate is always 28,800 symbols per second after there are 6 code symbols per modulation symbol, and the PN rate is 1.2288 MHz. The modulation is 64-ary orthogonal, using the same Walsh functions that are used in the forward link for channelization. Each period of the Walsh function is repeated for four chips of the PN code. The Walsh symbol rate is thus 1.2288 MHz/(4 chips per Walsh chip)/(64 Walsh chips per Walsh symbol) = 4,800 modulation symbols per second. Note that 1.2288 MHz = 128*9,600 bps.
The Reverse CDMA Channel consists of 242-1 logical channels. One of these logical channel is permanently and uniquely associated with each mobile station. That logical channel is used by the mobile whenever it passes traffic. The channel does not change upon handoff (Cf. Forward Channelization). Other logical channels are associated with base stations for system access. This reverse link addressing is accomplished through manipulation of period 242-1 Long Code, which is part of the spreading process.
Coding and InterleavingFigure 1 shows the core processing that generates one Reverse CDMA Channel.
Figure 1. Reverse CDMA Channel signal generation.
Separation of Users
The reverse CDMA Channel, in contrast to the Forward CDMA Channel, does not use strict orthogonality in any sense to separate logical channels. Rather, it uses a very long period spreading code, in distinct phases. The correlations between stations are not zero, but they are acceptably small.
Reverse link data modulation is 64-ary orthogonal, and is applied prior to the spreading. Groups of six code symbols select one of 64 orthogonal sequences. The 64-ary orthogonal sequences are the same Walsh functions that are used in the Forward CDMA Channel, here are used for a totally different purpose. Each period of the Walsh sequence (a Walsh Chip) is four PN chips in duration. The modulation symbol rate is thus always 4,800 sps.
Each Reverse CDMA Channel is spread by both the channel-unique
Long Code and the Short Code, which
has I- and Q-components. The spreading is thus quadrature.
That is, from a single binary-valued symbol stream, two binary
sequences are generated by mod 2 addition of the short code
PN sequences (Figures 1 and 2). The effect of adding long
and short codes is to produce a supersequence that has an
extraordinarily long period, about 257, or 3700
years at the 1.2288 MHz spreading rate (see Reverse
The two coded, covered, and spread streams are vector-modulated
on the RF carrier. The Q-axis modulation is delayed by 1/2 chip. The
spreading modulation is thus offset QPSK. Offset modulation was chosen
in an effort to reduce the envelop modulation of the RF signal and reducing
performance requirements on the power amplifiers in the subscriber station.
There is only one type of overhead channel in the
Reverse CDMA Channel: the Access Channel.
Traffic channels are in the Reverse CDMA Channel are mobile-unique.
That is, each station has a unique Long
Code Mask, based on its electronic serial number. Whenever
the mobile is assigned to traffic, it uses its specific long
During soft handoffs a mobile station transmits the same information that it would in the absence of the handoff, that is, there is no change in the content of the mobile transmissions. There is a change in the way that reverse link power control is applied to the reverse link. Power control "down" commands from all participants in the handoff are logically "or'ed" together - if any of the handoff participant base stations says "down," then the mobile station is required to reduce its reverse link power.
Traffic channels carry variable rate traffic frames,
either 1, 1/2, 1/4, or 1/8 of the maximum rate. In IS-95A only a 9600
bps rate family is currently available in the standard. In J-STD-008
a second rate set, based on a maximum rate of 14,400 bps is available.
The Rate Set 2 will be added in a future revision of IS-95.
TimingMobile stations are required to adjust their transmission time according to the timing that they are able to derive from the pilot and sync channels, and adjusted for the know base station pilot offset. That is, the pilot offset index read from the sync channel must be used to correct transmitter timing so that the Reverse CDMA Channel signal arrives at the base station aligned in time with system time, to within a round-trip of the air interface propagation delay.
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