Fundamental Limits to Mobile Data Access

14.2.1 Capacity and Bandwidth Efficiency

Ever since the dawn of the Information Age, capacity has been the principal metric used to assess the value of a communication system. ^[2], ^[3] Irrespective of whether it is applied to an individual radio link, to a cell, or even to an entire system, the capacity signifies the largest volume of data throughput that can be communicated with arbitrary reliability.

Because capacity grows linearly with the amount of spectrum utilized, the most immediate way in which capacity can be enlarged is by allocating additional bandwidth. However, radio spectrum is a scarce and very expensive resource at the frequencies of interest, where propagation conditions are favorable. ^[4] Hence, it is imperative that the available bandwidth is utilized as efficiently as possible. Consequently, bandwidth efficiency — defined as the capacity per unit bandwidth — has become a key figure of merit.

Besides bandwidth, the capacity also is a function of the received signal power or, more specifically, of the signal-to-interference-and-noise ratio (SINR) at the receiver.

However, unlike with bandwidth, the capacity only scales logarithmically with the SINR and thus trying to enhance the capacity by simply transmitting more power is extremely costly. Furthermore, it is futile in the context of a dense interference-limited cellular system, wherein an increase in everybody's transmit power scales up both the desired signals as well as their mutual interference, yielding no net benefit. Therefore, power increases are useless once a system has become limited in essence by its own interference. Furthermore, because mature systems designed for high capacity tend to be interference-limited, ^[5] it is power itself — in the form of interference — that ultimately limits their performance.

In order to improve bandwidth efficiency, multiple access methods — originally rather conservative in their design — have evolved toward much more sophisticated schemes. In the context of frequency division multiple access (FDMA) and time division multiple access (TDMA), this evolutionary path has led to advanced forms of dynamic channel assignment, as well as the incorporation of frequency hopping. In the context of code division multiple access (CDMA), it has led to a variety of multiuser detection and interference cancellation techniques. ^[6], ^[7] In all cases, the objective is to attain the highest possible degree of bandwidth utilization through aggressive frequency reuse.

14.2.2 Space: The Final Frontier

As a key ingredient in the design of more spectrally efficient systems, space has become, in recent years, the last frontier. Nonetheless, the use of the spatial dimension in wireless is hardly new. In fact, one could argue that the entire concept of frequency reuse on which cellular systems are based constitutes a simple way of exploiting the spatial dimension. Cell sectorization, a widespread procedure that reduces interference, can be regarded also as a form of spatial processing. These basic concepts can be taken to the limit, and the area capacity can be increased almost indefinitely, ^[8] by shrinking the cells and deploying additional base stations. ^[9] However, the cost and difficulty of deploying the vast infrastructure required to provide ubiquitous coverage using only microcells has proved prohibitive in the past; it remains to be seen whether that will change with the advent of wireless data. In light of these developments, the use of the spatial dimension is now geared mostly toward maximizing the system capacity on a per-base-station basis. ^[10] Here, base station antenna arrays are the enabling tool for a wide range of spatial processing techniques. ^[11] Because capacity grows roughly linearly with the number of sectors per cell, the most immediate use for such arrays is an increase in the number of sectors. This idea can be refined by making such sectors adaptive using beam-steering and beam-forming techniques devised to enhance desired signals and mitigate interference. All such schemes, however, are fundamentally limited by the multipath nature of the radio channel: sectors and beams are only effective as long as they are sufficiently broad with respect to the angular dispersion or spread introduced by the channel.

Any attempts to create excessively narrow sectors or beams will result in distorted patterns and unforeseen interference. This fundamental barrier, however, can be overcome by incorporating a second antenna array at the terminal.

14.2.3 Pushing the Limits with Multiantenna Technology

Until recently, the deployment of antenna arrays in mobile systems had been contemplated exclusively at base station sites because of size and cost considerations. One of the principal role of those arrays was to provide spatial diversity against signal fading. ^[12], ^[13] Such fading, arising from multipath propagation caused by scattering, had always been regarded as an impairment that had to be mitigated. However, recent advances in information theory have shown that, with the simultaneous use of antenna arrays at both base station and terminal, multipath interference cannot only be mitigated, but actually exploited to establish multiple parallel channels that operate simultaneously and in the same frequency band. ^[14], ^[15], ^[16] Based on this fundamental idea, an entire new class of multiple-transmit multiple-receive (MTMR) communications architectures has emerged. ^[17] A critical feature of these MTMR architectures is that the total radiated power is held constant irrespective of the number of transmit antennas. Extraordinary levels of bandwidth efficiency can thus be achieved without any increase in the amount of interference caused to other users.

Imagine a number of single-antenna user terminals collocated into an MTMR terminal that handled their multiple signals simultaneously. Intuitively, this would require the base station to be able to resolve the individual antennas within the terminal array, which in turn would require synthesizing an impossibly narrow beam. The novelty in MTMR communication, however, is that the scattering environment around the terminal is used as an aperture through which those antennas become effectively resolvable.

Notice that, by reusing the same frequency band at each antenna, very large increases in throughput are achieved without increasing the user bandwidth. Hence, in many respects, these MTMR schemes can be regarded as the ultimate step in the quest for ever-tighter levels of frequency reuse, for here every individual user is reusing its bandwidth multiple times.