0.16 Appendix h  (Page 2/4)

Within a single cell, the transmitter implements multiple users that send messages by sequencing them in time.In TDMA, where the signal intended for each user is transmitted during a designated time slot, all users employ the same band of frequencies.The frame structure for the $B^3$ $I$ $G$ TDMA is shown in [link] . This may be contrasted with the FDMA situation where users each employtheir own frequency range, as was discussed in [link] .

Typically, adjacent cells transmit on different frequencies, though frequencies may be reused in cells that are sufficiently far away.Adjacent cells may use frequencies that are close to each other – and may even overlap slightly due to relaxed spectral masks.Consequently, it is not unreasonable to expect some interference from adjacent cells. $B^3$ $I$ $G$ makes it easy to study the effects of this kind of interference.

In Code Division Multiple Access (CDMA), each user is assigned a code which is mutually orthogonal to all of the other users. All users transmit at the same time and in the same frequency range, and the decoding relies on the orthogonality of the codes. $B^3$ $I$ $G$ does not currently implement a CDMA protocol.

Constructing the received signal

The $B^3$ $I$ $G$ Transmitter (i.e., the base station) simulates $K$ simultaneous users using TDMA. Each receiver (i.e., each mobile device in [link] ) retains and decodes only its intended portion of the signal. For single-user operation, $K=1$ .

Each of the three blocks in [link] is itself built from several components that are familiar from previous chapters. The system that generatesthe analog received signal (i.e. the actual transmitter and channel) is shown in block diagram form in [link] . This is, of course, the magical ${\mathcal{M}}^6$ Transmitter from [link] . When the signal arrives at the front end of the receiver,it is transformed into a sampled IF signal as in [link] .

The original message in [link] is a character string of English text. Each character is mapped into asequence of 4-PAM symbols. The mappings may be made using the M atlab function letters2pam.m , which converts each letter to an eight-bit binary string, and hence tofour 4-PAM symbols as in [link] . Alternatively, the mappings may be made with the more terse text2bin.m , which converts each letter to a 7-bit binary string, thus assigning seven 4-PAM symbols to every pair of letters.This mapping is familiar from [link] .

In order to decode the message at the receiver, the recovered symbols must be properly grouped, thestart of each group must be located, and the desired user data must be extracted. To aid this TDMA frame synchronization, apreamble is inserted in the symbol stream at the start of every frame. The preamble is a short text message that starts each frameand it is assumed to be known at the receiver. Like the message, it is encoded in 4-PAM symbols.The unknown messages for each user are placed in sequence after the preamble, as in [link] .