Transmit Diversity v. Spatial Multiplexing in Modern MIMO Systems

arXiv:0811.3887v2 [cs.IT] 5 Mar 2009 Transmit Diversity v. Spatial Multiplexing in Modern MIMO Systems Angel Lozano∗, and Nihar Jindal† May 29, 2018 Abstract A contemporary perspective on the tradeoff between transmit antenna diversity and spatial multiplexing is provided. It is argued that, in the context of most modern wire- less systems and for the operating points of interest, transmission techniques that utilize all available spatial degrees of freedom for multiplexing outperform techniques that explic- itly sacrifice spatial multiplexing for diversity. In the context of such systems, therefore, there essentially is no decision to be made between transmit antenna diversity and spatial multiplexing in MIMO communication. Reaching this conclusion, however, requires that the channel and some key system features be adequately modeled and that suitable perfor- mance metrics be adopted; failure to do so may bring about starkly different conclusions. As a specific example, this contrast is illustrated using the 3GPP Long-Term Evolution system design. I Introduction Multipath fading is one of the most fundamental features of wireless channels. Because multiple received replicas of the transmitted signal sometimes combine destructively, there is a significant probability of severe fades. Without any means of mitigating such fading, ensuring reasonable reliability requires hefty power margins. ∗Angel Lozano is with Universitat Pompeu Fabra, Barcelona 08005, Spain. His work is partially sup- ported by the project CONSOLIDER-INGENIO 2010 CSD2008-00010 ”COMONSENS”. †Nihar Jindal is with the University of Minnesota, Minneapolis, MN55455, USA. His work was partially conducted during a visit to UPF under the sponsorship of Project TEC2006-01428. 1 Fortunately, fades, or nulls, are very localized in space and frequency: a change in the transmitter or receiver location (on the order of a carrier wavelength) or in the frequency (on the order of the inverse of the propagation delay spread) leads to a roughly indepen- dent realization of the fading process. Motivated by this selectivity, the concept of diversity is borne: rather than making the success of a transmission entirely dependent on a single fading realization, hedge the transmission’s success across multiple realizations in order to decrease the probability of failure. Hedging or diversifying are almost universal ac- tions in the presence of uncertainty, instrumental not only in communications but also in other fields as disparate as economics or biology. In communications specifically, the term ’diversity’ has, over time, acquired different meanings, to the point of becoming overloaded. It is used to signify: • Variations of the underlying channel in time, frequency, space, etc. • Performance metrics related to the error probability. Adding nuance to the term, more than one such metric can be defined (cf. Section IV). • Transmission and/or reception techniques designed to improve the above metrics. In this paper, we carefully discriminate these meanings. We use ’selectivity’ to refer to channel features, which are determined by the environment (e.g., propagation and user mobility) and by basic system parameters (e.g., bandwidth and antenna spacing). In turn, the term ’diversity’ is reserved for performance metrics and for specific transmit/receive techniques, both of which have to do with the signal. Note that channel selectivity is a necessary condition for diversity strategies to yield an improvement in some diversity metric. A Diversity over Time Archaic electrical communication systems from a century ago already featured primitive forms of diversity, where operators manually selected the receiver with the best quality. Automatic selection of the strongest among various receivers was discussed as early as 1930 [1]. This naturally led to the suggestion of receive antenna combining, initially for microwave links [2]–[5]. MRC, by far the most ubiquitous combining scheme, was first proposed in 1954 [6]. In addition to receive antenna combining, other approaches such as the aforementioned one of repeating the signal on two or more frequency channels were also considered for microwave links [7]. (Systems were still analog and thus coding and interleaving was not an option.) Given the cost of spectrum, though, approaches that consume additional bandwidth were naturally unattractive and thus the use of antennas 2 quickly emerged as the preferred diversity approach. Recognizing this point, receive antenna combining was debated extensively in the 1950’s [8]–[11] and has since been almost universally adopted for use at base station sites. The industry, however, remained largely ambivalent about multiple antennas at mobile devices. Although featured in early AMPS trials in the 1970’s, and despite repeated favorable studies (e.g., [12]), until recently its adoption had been resisted.1 Multiple base station antennas immediately allow for uplink receive diversity. It is less clea

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