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The sync channel is used to provide time and frame synchronization to the receiver. Cellular CDMA networks typically use GPS as the reference timing source, and the BTS sends the system time to the receiver over the sync channel [49]. Other information such as the pilot PN offset and the long code state are also provided on the sync channel [47]. The long code is a PN sequence and is used to spread the reverse link signal (i.e. UE to BTS) and the paging channel message. The long code has a chip rate of 1.2288 Mcps and is generated using 42 LFSRs. The outputs of the registers are masked and modulo‐two added together to form the long code. The latter has a period of more than 41 days; hence, the states of the 42 LFSRs and the mask are transmitted to the receiver so that it can readily achieve long code synchronization. The sync message encoding before transmission is shown in Figure 38.5.

The initial message, which is at 1.2 Ksps, is convolutionally encoded at a rate r = (1/2) with generator functions g₀ = 753 (octal) and g₁ = 561 (octal) [48]. The state of the encoder is not reset during the transmission of a message capsule. The resulting symbols are repeated twice, and the resulting frames, which are 128 symbols long, are block‐interleaved using the bit reversal method [47]. The modulated symbols, which have a rate of 4.8 Ksps, are spread with Walsh code 32. The sync message is divided into 80 ms superframes, and each superframe is divided into three frames. The first bit of each frame is called the start of message (SOM). The beginning of the sync message is set to be on the first frame of each superframe, and the SOM of this frame is set to one. The BTS sets the other SOMs to zero. The sync channel message capsule is composed of the message length, the message body, cyclic redundancy check (CRC), and zero padding. The length of the zero padding is such that the message capsule extends up to the start of the next superframe. A 30‐bit CRC is computed for each sync channel message with the generator polynomial

The SOM bits are dropped by the receiver, and the frames bodies are combined to form a sync channel capsule. The sync message structure is summarized in Figure 38.6.