1.2: Radio Architecture

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A radio device is comprised of reasonably well-defined units, see Figure 1.1.7. The analog baseband signal can have frequency components that range from DC to many megahertz.

Up until the mid-1970s most wireless communications were based on centralized high-power transmitters and reception was expected until the signal level fell below a noise threshold. These systems were particularly sensitive to interference, therefore systems transmitting at the same frequency were geographically separated so that signals fell below the background noise threshold before there was a chance of interfering with a neighboring system operating at the same frequency. This situation is illustrated in Figure \(\PageIndex{1}\).

Cellular communications is based on the concept of cells in which a terminal unit communicates with a basestation at the center of a cell. For communication in closely spaced cells to work, interference from other radios must be managed. This is facilitated using the ability to recover from errors available with error correction schemes.

In 1981 the U.S. FCC defined cellular radio as “a high capacity land mobile system in which assigned spectrum is divided into discrete channels which are assigned in groups to geographic cells covering a cellular geographic area. The discrete channels are capable of being reused within the service area.” The key attributes here are (a) the concept of cells arranged in clusters and the total number of channels available is divided among the cells in

Figure \(\PageIndex{1}\): Interference in a conventional radio system. The two transmitters, \(1\), are at the centers of the coverage circles defined by the background noise threshold.

Figure \(\PageIndex{2}\): Interference in a cellular radio system.

a cluster and the full set is repeated in each cluster, e.g. in Figure \(\PageIndex{2}\) a 7- cell cluster is shown; and (b) frequency reuse. with frequencies used in one cell are reused in the corresponding cell in another cluster. As the cells are relatively close, it is important to dynamically control the power radiated by each radio, as radios in one cell will produce interference in other clusters.

Achieving maximum frequency reuse is essential. In a cellular system, there is a radical departure in concept from this. Consider the interference in a cellular system as shown in Figure \(\PageIndex{2}\). The signals in corresponding cells in different clusters interfere with each other and the interference is much larger than that of the background noise. Error correction coding to correct errors resulting from interference.