The Forward CDMA Channel, transmitted over the cell's carrier frequency, is separated from other cells by a specific offset of the short PN codes. A BS may also transmit more than one Forward CDMA Channel by means of frequency-division multiplexing (different RF carriers). A Forward CDMA Channel is subdivided into code channels, separated by Walsh codes. There are four types of code channels; the first three are common channels that serve all the MSs in the cell, while the fourth is dedicated to individual MSs:
1. Pilot Channel
2. Synch Channel
3. Paging Channel
Walsh code orthogonality can be exploited in the Forward CDMA Channel because MSs are phase-synchronized with the BS. Synchronization is acquired and maintained with the help of the Pilot and Sync Channels. There are 64 possible code channels in the Forward CDMA Channel, based on the number of available Walsh codes. A conceptual view of the bit manipulation process that generates the physical Forward CDMA Channel is shown in Fig. 13.2-1. Each code channel, after being spread with a Walsh code, is spread in quadrature with two short PN codes, before modulating the carrier for transmission. Quadrature spreading means that the bit stream is split into two branches, each spread separately with one of two PN sequences, and then modulated by two carriers with the same frequency but 90 degrees out of phase. The two branches, called I (in-phase) and Q (quadrature), are added together before transmission, resulting in quadrature phase-shift keying (QPSK) modulation.
Pilot Channel. This is a common channel used by the mobile station to measure the strength of the signal transmitted by a BS, to select the strongest signal, and to acquire initial synchronization with a BS via short PN code phase-offset detection. There is always one and only one Pilot Channel per Forward CDMA Channel. It carries an unmodulated signal of all zeros and is spread by Walsh code W0. This channel is sent with higher power than the other channels, so it has a high signal-to-noise ratio (SNR), making it easy to acquire.
Sync Channel. This is a common channel used by the BS to transmit (periodically) the Sync Channel message, which contains system information used by mobile stations to complete the synchronization process with the BS. There is a maximum of one Sync Channel per Forward CDMA Channel, spread by Walsh code W32.
Paging Channel. This is a common channel used by the BS to contact individual MSs (paging) and to send system information, such as traffic channel assignment. Up to seven code channels can be assigned as paging channels, which can be configured to operate at 4800 or 9600 bps. They are spread by Walsh codes W1 through W7. Channel 1 (primary paging channel) is the default channel.
Paging Channel transmission is structured as a sequence of 20-ms frames, grouped into a cycle of 80-ms slots. The MS can operate in nonslotted as well as slotted mode; that is, the MS can listen for pages in all slots or only in a predetermined set of slots, going idle for the rest of the cycle to save power.
Forward Traffic Channel. This is a group (one or more) of code channels from the BS to an individual MS. It carries primary traffic (typically voice) and signaling traffic and may also carry secondary traffic (additional user data streams). It consist of one forward fundamental channel and up to seven optional forward supplemental code channels, each structured as a sequence of 20-ms frames.
Multiple types of traffic can be multiplexed in the same code channel using blank and burst or dim and burst techniques. The initial bits of each frame are used to define the allocation of the remaining bits to primary, secondary, and signaling traffic (see example of Fig. 13.2-3). The multiplexing of traffic types in a traffic channel and their bit rates are configurable according to sixteen multiplex options. Supplemental channel assignment and multiplex options are negotiated between BS and MS during connection setup, by an exchange of messages (service negotiation messages in Tables 13.2-2 and 13.2-4).
The bit stream in the fundamental channel is punctured with a power control bit every 1.25 ms, or 800 times per second, to form a power control subchannel. A bit value of 1 causes the MS to decrease power by 1 dB; a value of 0 causes it to increases power by 1 dB.
Walsh codes W1 to W63 can be assigned to fundamental and supplemental traffic channels, except for W1-W7 and W32 when used for Paging Channels and the Sync Channel, respectively.
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