CDG : Technology : Welcome to the World of CDMA

Forward CDMA Channel

The FORWARD CDMA CHANNEL is the cell-to-mobile direction of communication. It carries traffic, a pilot signal, and overhead information. The pilot is a spread, but otherwise unmodulated DSSS signal. The pilot and overhead channels establish the system timing and station identity. The pilot channel also is used in the mobile-assisted handoff (MAHO) process as a signal strength reference.

Frequency Plan

The base station transmit frequency is 45 MHz above the mobile station transmit frequency in the cellular service (IS-95A), and 80 MHz above 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 Parameters

The IS-95A forward link currently supports a 9600 bps rate family in the three data-bearing channel types, as shown in the table. In all cases the FEC code rate is 1/2 and the PN rate is 1.2288 MHz. Note that 1.2288 MHz = 128*9600 bps.

**Forward Link Channel Parameters, Rate Set 1**
Channel	Sync	Paging		Traffic
Data rate	1200	4800	9600	1200	2400	4800	9600	bps
Code repetition	2	2	1	8	4	2	1
Modulation symbol rate	4800	19,200	19,200	19,200	19,200	19,200	19,200	sps
PN chips/modulation symbol	256	64	64	64	64	64	64
PN chips/bit	1024	256	128	1024	512	256	128

J-STD-008 supports, in addition to the above rates, a second traffic channel rate family with a maximum rate of 14,400 bps. This is termed Rate Set 2, the original 9600 bps family being Rate Set 1. Rate Set 2 uses an FEC code rate of 3/4, created by puncturing the code used in Rate Set 1.

**Forward Link Channel Parameters, Rate Set 2**
Channel	Traffic
Data rate	1800	3600	7200	14400	bps
Code repetition	8	4	2	1
Modulation symbol rate	19,200	19,200	19,200	19,200	sps
PN chips/modulation symbol	64	64	64	64
PN chips/bit	682.67	341.33	170.67	85.33

Signal Structure

Channelization

The forward link consists of up to 64 logical channels (code channels). The channels are independent in that they carry different data streams, possibly at different rates, and are independently adjustable in amplitude (Cf. Reverse Channelization).

Coding and Interleaving

The figure shows the core processing that generates one forward code channel, rate set 1. Rate set 2 is identical except the coding rate is 3/4 rather than 1/2, yielding the same code symbol rate with 3/2 times the data rate.

Walsh Codes

The code channels, as transmitted, are mathematically orthogonal. The orthogonality is established by covering the FEC code symbols with one of a set of 64 so-called Walsh functions. "Mutually orthogonal" means that their cross correlations are small (ideally zero). Because only whole periods of the Walsh functions occur in each code symbol, the effect of the Walsh cover is to make the channels completely separable in the receiver, at least in the absence of multipath. The orthogonality not only means that there is no co-mingling of channels, it means there is no interference between users in the same cell, again in the absence of multipath. This has a substantial beneficial effect on the forward link capacity.

Multipath delay spread that exceeds a chip does introduce mutual interference between users in one cell. In any particular Rake finger the uncorrelated channels contribute an effective interference level. This level varies from zero, when there is only one multipath component, up to (N-1)/N of the total signal power if there are N discrete, equal-amplitude multipath components.

Note that one of the Walsh functions is always a constant, code number zero, by the numbering convention. This channel is always reserved to serve as the pilot.

Spreading

Each forward code channel is spread by the Short Code, which has I- and Q-components. The spreading is thus quadrature. That is, from a single binary-valued, covered, symbol stream, two binary sequences are generated by mod 2 addition of the short code PN sequences, as shown in the figure.

Modulation

The two coded, covered, and spread streams are vector-modulated on the RF carrier. The spreading modulation is thus QPSK, superimposed on a BPSK code symbol stream.

The spectrum shaping of the forward link is carefully prescribed in the IS-95A air interface and the IS-97 performance specification. The latter is in terms of the so-called Rho meter, a measurement of the correlation between the actual transmitter output with the ideal transmitter output. The air interface also specifies a slightly nonlinear phase characteristic the purpose of which is partial pre-equalization of the mobile receiver.

In-band ripple is specified as less than ±1.5 dB. Stopband rejection is 40 dB beginning 740 kHz from band center. An equi-ripple, 48 tap FIR baseband filter is suggested, although not required.

Overhead channels

There are three types of overhead channel in the forward link: pilot, sync, and paging. The pilot is required in every station.

Pilot Channel

The pilot channel is always code channel zero. It is both a demodulation reference for the mobile receivers, and for handoff level measurements, and thus must be present in every station. It carries no information. It is pure short code, with no additional cover or information content.

The amplitude of the pilot and its spatial distribution must be carefully controlled because their relative amplitudes control handoff boundaries between stations. The PN_I and PN_Q modified linear feedback shift register sequences that comprise the short code have period 2¹⁵ chips, which is 80/3 = 26.667 ms at the 1.2288 MHz chip rate.

All stations use the same short code, and thus have the same pilot waveform. They are distinguished from one another only by the phase of the pilot. The short period of the short code, 2¹⁵, facilitates rapid pilot searches by the mobiles.

The air interfaces stipulate that pilot phases always be assigned to stations in multiples of 64 chips, giving a total of 2^15-6 = 512 possible assignments. The 9-bit number that identifies the pilot phase assignment is called the Pilot Offset.

Sync Channel

The sync channel carries a repeating message that identifies the station, and the absolute phase of the pilot sequence. The data rate is always 1200 bps. The interleaver period is 80/3 = 26.667 ms, equal to the period of the short code. This simplifies finding frame boundaries, once the mobile has located the pilot.

The Sync Channel carries a single, repeating message that conveys the timing and system configuration information to the mobile station. The mobile station can derive accurate system time, to within a fraction of a spreading chip (833 ns) by synchronizing to the short code. The short code synchronization and the pilot offset, which is part of the sync message, fix system time modulo 26.667 ms. The code period ambiguity is then resolved by the long code state and system time fields that are also part of the sync message.

Paging Channel

The paging channel is the vehicle for communicating with mobile stations when they are not assigned to a traffic channel. As the name implies, its primary purpose is to convey pages, that is, notifications of incoming calls, to the mobile stations. It carries the responses to mobile station accesses, both page responses and unsolicited originations. Successful accesses are normally followed by an assignment to a dedicated traffic channel. Once on a traffic channel, signaling traffic between base and mobile can continued interspersed with the user traffic.

The paging channel may run at either 4800 or 9600 bps.

Each base station must have at least one paging channel per sector, on at least one of the frequencies in use. All paging can be done on one frequency, or it can be distributed over multiple frequencies.

Traffic Channel

Traffic channels are assigned dynamically, in response to mobile station accesses, to specific mobile stations. The mobile station is informed, via a paging channel message, which code channel it is to receive (it is tempting, but inappropriate to use the word "tune"!).

The traffic channel always carries data in 20 ms frames. Frames at the higher rates of Rate Set 1, and in all frames of Rate Set 2, include CRC codes to help assess the frame quality in the receiver.

Soft Handoff

During soft handoffs each base station participating in the handoff transmits the same traffic over its assigned code channel. The code channel assignments are independent, and in general will be different in each cell. Whatever code channels are not in use for overhead channels are available, up to either a total of 64 or the available equipment limit, whichever is smaller.

Rate

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.

The rate variation is accomplished by 1, 2, 4, or 8-way repetition of code symbols. Transmission is continuous, with the amplitude reduced at the lower rates so as to keep the energy per bit approximately constant, regardless of rate. The rate is independently variable in each 20 ms frame.

Power Control Subchannel

The 800 bps reverse link power control subchannel is carried on the traffic channel by puncturing 2 from every 24 symbols transmitted. The punctured symbols both carry the same power control bit, so they can be coherently combined by the receiver. Each base station participating in a soft handoff makes its own power control decision, independent of the others, unless they are different sectors of the same cell, in which case they all transmit a common decision. This special circumstance is made known to the mobile when the handoff is set up.

Timing

All base stations must be synchronized within a few microseconds for the station identification mechanisms to work reliably and without ambiguity. Any convenient mechanism can be used for this purpose, but the system was designed under the assumption that the Global Positioning System (GPS) would be used. This is a family of low-earth-orbit satellites that broadcast a spread-spectrum signal and ephemeris information from which a sophisticated Kalman filter algorithm in a receiver can derive both a very accurate position and a very accurate time.