TABLE OF CONTENTS All systems examined in the previous sections were single user systems. We consider now the optimal transmission strategies for the multiuser case. In this section, we study the multiple access and the broadcast channels. In the next section, we will consider the so called "cocktail party problem", where there are many sources that wish to talk to as many destinations and it is necessary to find out the optimal access strategy. In all cases, we assume, as in the previous sections, that the channels are known at the transmitter side.
In this section, we evaluate the maximum information rates achievable in the multiple access channel. We consider the situation where both access point and user terminals have single antenna transceivers. The extension to the multiantenna case was considered in [20]. Within the single-antenna scenario, we consider the scalar case, where each user transmits one symbol per each channel use, and the vector case, where each user adopts a block transmission strategy.
Let us start with the Gaussian multiple access channel, where each received sample Y is the superposition of M independent symbols X m, coming from as many users, plus additive white Gaussian noise (we drop the dependence on time, as we consider the stationary case):
| (6.120) |  |
where h m is the flat-fading coefficient between the mth user and the access point; V is additive Gaussian noise. Each user is...
