The explosive growth of both the wireless industry and the Internet is creating a huge market opportunity for wireless data access. Limited internet access, at very low speeds, is already available as an enhancement to some existing cellular systems. However those systems were designed with purpose of providing voice services and at most short messaging, but not fast data transfer. Traditional wireless technologies are not very well suited to meet the demanding requirements of providing very high data rates with the ubiquity, mobility and portability characteristics of cellular systems. Increased use of antenna arrays appears to be the only means of enabling the type of data rates and capacities needed for wireless internet and multimedia services. While the deployment of base station arrays is becoming universal it is really the simultaneous deployment of base station and terminal arrays that can unleash unprecedented levels of performance by opening up multiple spatial signaling dimensions .Theoretically, user data rates as high as 2 Mb/sec will be supported in certain environments, although recent studies have shown that approaching those might only be feasible under extremely favorable conditions-in the vicinity of the base station and with no other users competing for band width. Some fundamental barriers related to the nature of radio channel as well as to the limited band width availability at the frequencies of interest stand in the way of high data rates and low cost associated with wide access.
FUNDAMENTAL LIMITATIONS IN WIRELESS DATA ACESS
Ever since the dawn of information age, capacity has been the principal metric used to asses the value of a communication system. Since the existing cellular system were devised almost exclusively for telephony, user data rates low .Infact the user data were reduced to the minimum level and traded for additional users. The value of a system is no longer defined only by how many users it can support, but also by its ability to provide high peak rates to individual users. Thus in the age of wireless data, user data rates surges as an important metric.
Trying to increase the data rates by simply transmitting more; Power is extremely costly. Furthermore it is futile in the contest of wherein an increase in everybodyâ„¢s transmit power scales up both the desired signals as well as their mutual interference yielding no net benefit.
Increasing signal bandwidth along with the power is a more effective way of augmenting the data rate. However radio spectrum is a scarce and very expensive resource.Moreover increasing the signal bandwidth beyond the coherent bandwidth of the wireless channel results in frequency selectively. Although well-established technique such as equalization and OFDM can address this issue, their complexity grows with the signal bandwidth. Spectral efficiency defined as the capacity per unit bandwidth has become another key metric by which wireless systems are measured. In the contest of FDMA and TDMA, the evolutionary path has led to advanced forms of dynamic channel assessment that enable adaptive and more aggressive frequency reuse.In the context of multi-user detection and interference cancellation techniques.
SPACE: THE LAST FRONTIER
As a key ingredient in the design of more spectrally efficient systems. In recent years space has become the last frontier. The entire concept of frequency reuse on which cellular systems are based constitutes a simple way to exploit the spatial dimension. Cell sectorisation, a widespread procedure that reduces interference can also be regarded as a form of spatial processing. Moreover, even though the system capacity is ultimately bounded, the area capacity on a per base station basis. Here, base station antenna array are the enabling tools for wide range of spatial processing techniques devised to enhance desired to enhance desired signals and mitigate interference. Coverage can be extended and tighter user packaging becomes possible, enabling in turn larger cell sizes and higher capacity can be extended even beyond the point at which every unit of bandwidth is effectively used in every sector through space division multiple access (SDMA), which enables the reuse of the same bandwidth by multiple users within a given sector as long as they can be spatially discriminated.
LIFTING THE LIMITS WITH TRANSMIT AND RECEIVE ARRAYS
Until recently, the deployment of antenna arrays in mobile systems was contemplated-because of size and cost considerations-exclusively at base station sites. The principle role of those arrays, long before interference suppression and other signal processing advances were conceived, was to provide spatial diversity against fading.
Download Full Report And Abstract