Adaptive Radio Resource Allocation is essential for guaranteeing high bandwidth and power utilization as well as satisfying heterogeneous Quality-of-Service requests regarding next generation broadband multicarrier wireless access networks like LTE and Mobile WiMAX. A downlink OFDMA single-cell scenario is considered where heterogeneous Constant-Bit-Rate and Best-Effort QoS profiles coexist and the power is uniformly spread over the system bandwidth utilizing a Uniform Power Loading (UPL) scenario. We express this particular QoS provision scenario in mathematical terms, as a variation of the well-known generalized assignment problem answered in the combinatorial optimization field. Based on this concept, we propose two heuristic search algorithms for dynamically allocating subchannels to the competing QoS classes and users which are executed under polynomially-bounded cost. We also propose an Integer Linear Programming model for optimally solving and acquiring a performance upper bound for the same problem at reasonable yet high execution times. Through extensive simulation results we show that the proposed algorithms exhibit high close-to-optimal performance, thus comprising attractive candidates for implementation in modern OFDMA-based systems.
In @cite_21 the above concepts were thoroughly presented first for single- and then for multi-antenna wireless mobile systems. According to this concept each receiver monitors the experienced SNR levels, feeds them back to the BS, while the BS schedules transmissions and adapts users' bit rates depending on the particular channel quality reports. Similar arguments were raised in @cite_2 where it was demonstrated that for 2G 3G systems the cellular spectral efficiency may significantly improve, even double at certain conditions, when the BS utilizes the per-user channel rate information. In @cite_12 the ideas were extended to multi-channel OFDM wireless systems like the one we examine in this paper. The widely used term "opportunistic" bears a strong relation with our work, since we tend to allocate subchannels with high channel conditions (near to their peak) to the corresponding users (frequency-domain opportunism), while in @cite_21 a similar policy is employed in the time-domain.
@cite_21 @cite_12 @cite_2
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Multiuser diversity is a form of diversity inherent in a wireless network, provided by independent time-varying channels across the different users. The diversity benefit is exploited by tracking the channel fluctuations 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 fluctuations and is thus limited in environments with little scattering and or slow fading. In such environments, we propose the use of multiple transmit antennas to induce large and fast channel fluctuations so that multiuser diversity can still be exploited. The scheme can be interpreted as opportunistic beamforming and we show that true beamforming gains can be achieved when there are sufficient users, even though very limited channel feedback is needed. Furthermore, in a cellular system, the scheme plays an additional role of opportunistic nulling of the interference created on users of adjacent cells. We discuss the design implications of implementing. this scheme in a complete wireless system. Recently, a lot of research effort has been spent on cross-layer system design. It has been shown that cross-layer mechanisms (i.e., policies) potentially provide significant performance gains for various systems. In this article we review several aspects of cross-layer system optimization regarding wireless OFDM systems. We discuss basic optimization models and present selected heuristic approaches realizing cross-layer policies by means of dynamic resource allocation. Two specific areas are treated separately: models and dynamic approaches for single transmitter receiver pairs (i.e., a point-to-point communication scenario) as well as models and approaches for point-to-multipoint communication scenarios (e.g., the downlink of a wireless cell). This article provides basic knowledge in order to investigate future OFDM cross-layer-optimization issues Today's cellular systems are designed to achieve 90-95 percent coverage for voice users (i.e., the ratio of signal to interference plus noise must be above a design target over 90 to 95 percent of the cell area). This ensures that the desired data rate which achieves good voice quality can be provided "everywhere". As a result, SINRs that are much larger than the target are achieved over a large portion of the cellular coverage area. For a packet data service, the larger SINR can be used to provide higher data rates by reducing coding or spreading and or increasing the constellation density. It is straight-forward to see that cellular spectral efficiency (in terms of b s Hz sector) can be increased by a factor of two or more if users with better links are served at higher data rates. Procedures that exploit this are already in place for all the major cellular standards in the world. In this article, we describe data rate adaptation procedures for CDMA (IS-95), wideband CDMA (cdma2000 and UMTS WCDMA), TDMA (IS-136), and GSM (GPRS and EDGE).