0.16 Appendix h  (Page 16/4)

In [link] , the Basic Black Box Impairment Generator of [link] was described as a routine that transforms a M atlab script specifying the operation of the transmitter into the (received) signal thatappears at the input of the receiver. This Appendix opens up the black box, shining light on theinternal operation of the $B^3$ $I$ $G$ .

The $B^3$ $I$ $G$ is implemented in M atlab as the routine BigTransmitter.m , and it allows straightforward modeling of any (or all) of the possible impairments discussed throughout Software Receiver Design , including carrier frequency offset, baud-timing offsets, frequency selective and time-varying channels, as well as channel noise. Since many of the impairments and nonidealities that arise in acommunication system occur in the channel and RF frontend, $B^3$ $I$ $G$ is more than a transmitter: it includes the communication channel and receiver RF frontend as well. An overview of $B^3$ $I$ $G$ is shown in [link] .

The $B^3$ $I$ $G$ architecture expands on the simplified communication system of Chapter [link] and has more options than the ${\mathcal{M}}^6$ transmitter of Chapter [link] . Some of the new features are:

• Support for multiple users. The transmitter generates a signal which may contain information intended for more than one receiver.
• Assumption that the receiver employs a sampled-IF architecture. [link] presented several choices for performing downsampling in a receiver. In modern communication systems, where carrier frequencies can be on the order of several gigahertz, it is rarely feasible to build a receiver front-end that samples at the Nyquist rate. The $B^3$ $I$ $G$ Transmitter generates a received signal that has been brought down to a lower, IF frequency and then sampled at some specified rate assumed to be lower than the Nyquist rate of the IF frequency.
• Insertion of periodic preamble data. In practice, transmission of some known preamble data is required so that the receiver can learn how to combat the various impairments and non-idealities. In addition, the preamble permits the receiver to identify the start of a frame or transmission.

The $B^3$ $I$ $G$ is intended to mimic the base station in a cellular system in which the geographic coverage area is split into hexagonally shapedcells as in [link] . Thus there are many users in the same geographic region all wishingto transmit and receive simultaneously. How is a given receiver able to separate its message from thecomposite received signal? The key is to separate the transmission in either time or in frequency(or using a mixture of time and frequency called the code space). These separation techniques are called Time Division Multiple Access (TDMA),Frequency Division Multiple Access (FDMA), and Code-Division Multiple Access (CDMA).