The challenge to broadband wireless comes from the scarcity of radio-spectrum resources. Regulatory bodies around the world have allocated only a limited amount of spectrum for commercial use. The need to accommodate an ever-increasing number of users and offering
bandwidth-rich applications using a limited spectrum challenges the system designer to continuously search for solutions that use the spectrum more efficiently. Spectral-efficiency considerations impact many aspects of broadband wireless system design.
The most fundamental tool used to achieve higher system-wide spectral efficiency is the concept of a cellular architecture, whereby instead of using a single high-powered transmitter to cover a large geographic area, several lower-power transmitters that each cover a smaller area, called a cell, are used. The cells themselves are often subdivided into a few sectors through the use of directional antennas. Typically, a small group of cells or sectors form a cluster, and the available frequency spectrum is divided among the cells or sectors in a cluster and allocated intelligently to minimize interference to one another.
The pattern of frequency allocation within a cluster is then repeated throughout the desired service area and is termed frequency reuse. For higher capacity and spectral efficiency, frequency reuse must be maximized. Increasing reuse, however, leads to a larger potential for interference. Therefore, to facilitate tighter reuse, the challenge is to design transmission and reception schemes that can operate under lower signal-to-interference-plus-noise ratio (SINR) conditions or implement effective methods to deal with interference.
One effective way to deal with interference is to use multiple-antenna processing. Beyond using the cellular architecture and maximizing frequency reuse, several other signal processing techniques can be used to maximize the spectral efficiency and hence capacity of the system. Many of these techniques exploit channel information to maximize capacity.
Let’s Check Example.
Adaptive modulation and coding: The idea is to vary the modulation and coding rate on per user and/or per packet basis based on the prevailing SINR conditions. By using the highest level modulation and coding rate that can be supported by the SINR, the user data rates—and hence capacity—can be maximized. Adaptive modulation and coding is part of the WiMAX standard.
The idea behind spatial multiplexing is that multiple independent streams can be transmitted in parallel over multiple antennas and can be separated at the receiver using multiple receive chains through appropriate signal processing. This can be done as long as the multipath channels as seen by the various antennas are sufficiently decorrelated, as would be the case in a scattering-rich environment.
Spatial multiplexing provides data rate and capacity gains proportional to the number of antennas used. This and other multi antenna techniques are covered in other part.
Efficient multi access techniques: Besides ensuring that each user uses the spectrum as efficiently as possible, effective methods must be devised to share the resources among the multiple users efficiently. This is the challenge addressed at the MAC layer of the system. Greater efficiencies in spectrum use can be achieved by coupling channel-quality information in the resource-allocation process. MAC-layer techniques are discussed in more detail in other part.
It should be emphasized that capacity and spectral efficiency cannot be divorced from the need to provide adequate coverage. If one were concerned purely with high spectral efficiency or capacity, an obvious way to achieve that would be to decrease the cell radius or to pack more base stations per unit area.
Obviously, this is an expensive way to improve capacity. Therefore, it is important to look at spectral efficiency more broadly to include the notion of coverage area. The big challenge for broadband wireless system design is to come up with the right balance between capacity and coverage that offers good quality and reliability at a reasonable cost.