Opportunistic communication: A system view

The wireless medium is often called a fading channel: the pejorative adjective suggests that the intrinsic temporal and frequency variations are an impediment to reliable communication. While not untrue, the channel fluctuations are turned from foe to friend in some scenarios. A concrete situation is when the time scale of communication is much larger than that of the channel fluctuations: the so-called ergodic fading channel. In a pointto- point ergodic fading channel, the transmitter can make good use of the channel state information (CSI): by devoting more power to when the channel is good and less (or even none) when the channel is bad, the rate of reliable communication is improved. The improvement is significant when the operating signal-to-noise ratio (SNR) is small; this is a simple instance of opportunistic communication. Another important instance is in cellular communication: by scheduling user transmissions when their channel conditions are good, the system throughput is improved. This effect is called multiuser diversity and its role in cellular communication is the focus of this chapter. Multiuser diversity gain Consider the single antenna downlink flat fading channel with K users: where hk [m] m is the channel fading process of user k. There is an average power constraint of P on the transmit signal and wk [m] ∼ CN (0, N0) are i.i.d. in time m (for each user k = l, …, K).