SINR order statistics in OFDMA systems

Abstract

In orthogonal frequency division multiple access downlink, the signal-to-interference-noise ratio (SINR) of every subband at each user is estimated with the aid of pilots and the channel quality (some function of SINR's) is fed back to the base station (BS). This information is utilized by the BS for downlink scheduling. However, the SINR of the bands at each user are correlated due to the common interfering BSs. In this paper, we obtain the order statistics of band SINR's by considering the the co-channel interference and taking the correlations of interference across the bands into consideration. The order statistics are then used to analyse a best-m band feedback scheme, in which every users reports the index of their best-m bands to the BS. The BS utilizes this information and does greedy allocation of the bands to the users. We observe that utilizing the partial ordering of SINR's for channel allocation provides substantial gains over random allocation of bands to the users even in the presence of interference. I. INTRODUCTIONOrthogonal frequency division multiple access (OFDMA) is the ubiquitous downlink access technique in the next generation cellular standards. In OFDMA, the users utilize the pilots to estimate the channel quality of a physical resource block which we term as "band" for convenience. The users then report the channel quality (or some functions of them, for e.g., exponential effective SNR mapping (EESM)) of the best m-bands to the BS, which uses this information for scheduling the users. Hence the order statistics of the SINR's of different bands are important in the design and the analysis of the downlink scheduler.Interference from the other BSs critically influence the joint statistics of SINRs of the bands. At each user, while the small scale fading from the interfering BSs is independent across the bands, all the bands encounter the same path loss from the interfering BSs. Hence the interference is correlated across the bands.In the earlier works, only the signal-to-noise ratio is considered and the interference from other BSs is generally neglected in the analysis of order statistics. The main contribution of the paper is obtaining the order statistics of SINR's of the bands by explicitly taking into account the interference and its correlations across the bands. We use a spatial Poisson point process [1], [2] for the BS locations. This spatial model was introduced in [3] to model the interference in a cellular network. From the order statistics, we observe that in wideband systems with high frequency selectivity (and hence a large number of bands) there is a substantial gain using the best band scheduling even with interference. The obtained order statistics can potentially be used to analyse various schedulers. As an application, we

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