Exploiting Multiuser Diversity in a Packet Data Multiantenna Downlink
Wireless communications are currently facing a tremendous boost on the advent of the worldwide introduction of third generation cellular and next-generation wireless LAN standards. With both the amount of internet traffic and cellular wireless communications exploding in the past few years, a huge market is being created for high speed wireless access. However, it is already clear today that the promise of significantly higher datarates for almost everyone and, as a consequence, new wireless services and applications, can only be kept if the available frequency resources are used in a much more efficient manner than they are today. With service providers having paid close to 50 billion Euros for the 3rd generation cellular licenses in Germany, a significant return on investment will only be possible by increasing the spectral efficiency, measured in datarate (bits/sec) per Hertz of available bandwidth.
This can only be achieved by systematically utilizing all available forms of diversity. It has been known for quite some time that in order to communicate efficiently over the wireless channel, one should exploit multipath diversity by benefiting from independently fading propagation paths. Furthermore, recent results indicate that if one uses multiple antennas at the transmitter and receiver, the spectral efficiency per link increases linearly with the number of antennas in a rich scattering scenario by means of spatial multiplexing. Alternatively, the use of space-time codes and multiple antennas exploits spatial diversity. These forms of diversity either increase the quality of a single communication link, or increase the number of terminals communicating with, say, a basestation in a cellular system, by decreasing the power requirement for each terminal.
In an environment where one transmitter communicates with many mobile receivers, another form of diversity can be exploited in order to improve spectral efficiency: multiuser diversity.
The system at hand is depicted in Fig. 1. A basestation or access point with multiple antennas serves several mobile users with a packet-based transmission protocol. If the transmitter has knowledge about each receiver's channel quality (indicated by the feedback link), a "smart" scheduling algorithm transmits data packets only to users with "good" channel conditions. As the number of users in the system increases, the probability that at least one user has good channel conditions increases and as such the overall throughput can be significantly improved compared to a fixed resource allocation scheme such as TDMA, FDMA or CDMA. The gain obtained by scheduling users in such a way is large when fading for all users is fast and has a high dynamic range. In picocells with typically slow fading or in macrocells with little scattering around the transmitter, it can be shown that employing multiple antennas along with intelligent signal processing at the transmitter increases multiuser diversity by "randomizing" the channels. This is seen in Fig. 2
Challenges for System Design
The markers in the figure indicate a certain SNR level the user's channel has to attain in order to satisfy a certain QoS requirement. It is seen that without any transmit processing (red curve), the fading level is always below the threshold. With perfect (coherent) channel knowledge, coherent beamforming would significantly increase this user's SNR (green curve) and the required threshold would be met. With ''random'' beamforming, the user's SNR is above the threshold every once in a while and the scheduler can make use of that information and schedule the user when its channel is ''good'', without the need for coherent channel knowledge. This is a completely new design paradigm, which is in contrast to conventional approaches where one attempts to exploit diversity in order to make a single link more reliable. Multiuser diversity is thus a promising way to improve the area or multiuser spectral efficiency, measured in datarate per bandwidth per area or user.
Several challenging problems arise if one is interested in exploiting multiuser diversity in a real system. One key assumption is always that the transmitter has knowledge about the channel conditions of each user, according to which the scheduler decides which user to schedule. This requires either channel estimates from a continuous uplink transmission in reversible channels, which are available only in TDD systems, or tight feedback from all users to the transmitter. Another point to keep in mind is that high rate wireless data will be transmitted with packet-based transmission protocols with quality of service (QoS) constraints, yielding additional scheduling constraints.
Our aim is thus to study the feasibility of exploiting multiuser diversity in a realistic multiantenna downlink scenario. We are interested in increasing the area spectral efficiency, taking into consideration all the challenges imposed by real system design such as channel estimation, feedback design, smart scheduling with QoS constraints and intelligent antenna processing at the transmitter. We will analyze single-cell and multicell scenarios, taking into account frequency selective fading for each user, different fading dynamics and various degrees of scattering around the transmitter.