Channel Objects

In many wireless environments, the propagation environment or channel ultimately determines the operational characteristics and performance of the communications system. The wireless channel is usually complex due to time-varying multipath and fading. These considerations lead to a variety of questions. For example:

• How well does a given SDR design perform in a given environment?

• How does the performance of the radio change as the environment in which the radio is being deployed changes?

• How robust is the radio to environmental changes?

• For what choices of modulation/waveforms is robustness enhanced?

Including a channel object within a simulation testbed that realizes the channel model for a variety of specific environments provides a methodology for addressing these important concerns.

The inclusion of a channel object in an SDR testbed is illustrated in Figure 4. In Figure 4 we see a testbed containing two radios with one operating as a transmitter and the other operating as a receiver. Both the baseband (BB) and the radio frequency (RF) portions are illustrated. The channel object is realized as a lowpass equivalent of the channel. The parameters defining the channel object are determined by measurement (e.g., a power-delay profile) or are defined through the use of standard statistical models (e.g., Rayleigh, Ricean, Rummler, etc.).

Figure 4 suggests a simulation environment in which the baseband portions of actual radios are used for the transmitter and receiver. The signal processing algorithms used in the transmitter and in the receiver can be varied in response to changing channel model parameters. Since, in a simulation environment, the actual transmitted signal is available, it is possible to measure the link quality of service (QoS). The QoS could take many forms. Examples are the bit or symbol error rate, the frame error rate, signal-to-noise ratio at the receiver input, average or maximum fade duration, or outage probability. As suggested by Figure 4, the algorithms used in the transmitter and receiver are adjusted in response to the QoS measure in an attempt to maximize the QoS as the channel characteristics change. These algorithms determine the standard within which the radio is operating and also allows for modifications within a standard, such as filter bandwidths, data rates, coding algorithms, etc. These types of studies provide a path for evolving standards.

The concepts discussed here are especially attractive for the design and performance evaluation of software radios.

Figure 4– SDR testbed with channel object.