Paper Summary on: P. V. Viswanath, D. Tse and R. Laroia, Opportunistic Beamforming Using Dumb Antennas

Multiuser diversity is a form of diversity inherent in a wireless network, provided by independent timevarying channels across the di erent users. The diversity bene t is exploited by tracking the channel uctuations of the users and scheduling transmissions to users when their instantaneous channel quality is near the peak. The diversity gain increases with the dynamic range of the uctuations and is thus limited in environments with little scattering and/or slow fading. In such environments, the use of multiple transmit antennas is proposed to induce large and fast channel uctuations so that multiuser diversity can still be exploited. The scheme can be interpreted as opportunistic beamforming and true beamforming gains can be achieved when there are su cient users, even though very limited channel feedback is needed. I Information Theoretical Results on Multiuser Diversity The random, time-varying channel characteristics between a transmitter and a receiver pose many challenges to reliable wireless communications. A key challenge lies in variable fading of the received signal power which can lead to variable performance over the communication link. Classical diversitymethods attempt to counter this disparity; in particular, they focus on reducing the uctuations of received signal power by combining multiple signal copies experiencing independent fading. Another form of diversity, multiuser diversity also arises from the existence of independently faded signals, although the signals are coming from di erent users in the network. Unlike the classical diversity techniques aiming to improve the reliability of point-to-point communication, the main objective of the multiuser diversity is to increase the total system throughput; while classical diversity diversity techniques are designed to overcome the adverse e ect of fading, multiuser diversity exploits channel fading. The multiuser diversity is motivated by recent information theoretical results for Gaussian multiple access channel (MAC) and broadcast channel (BC): For both uplink [2] and downlink [3] of a wireless cellular system, the achievable sum rate is maximized when the transmission takes place only between the base station and the user with the strongest channel at each channel state, given each user is equipped with single antenna. To extract the multiuser diversity bene ts in cellular system, there are two fundamental system requirement:  The base station has access to channel quality measurements;  The base station is capable of scheduling transmissions among the users as well as adapting the data rate as a function of the instantaneous channel quality. II Proportional Fair Scheduling The strategy of multiuser diversity is only concerned with maximizing long-term average system throughput; in practice, there are latency requirements, in which case the average throughput over the delay time-scale is the performance metric of interest. Moreover, since in reality the users' fading statistics are not symmetric, the strategy of communicating with the user with the best channel favors the users who are closer to the