As we discussed in Chapter 3, dynamic spectrum sharing (DSS) was perhaps the key application that drove the early interest in cognitive radios (CR). Dynamically sharing the spectrum has been around since the early days of wireless telecommunications. Indeed, any time multiple users share spectrum through some form of dynamic channel access, it does amount to DSS. For example, a communications device may use a random access protocol such as carrier sense multiple access (CSMA) to observe (or sense) whether there is an active carrier signal in a channel that it intends to transmit on (see Figure 12.1(a)). In a simpler random access protocol such as ALOHA, a radio will simply transmit without first verifying whether there is an active user in the channel or not, but it will still sense the channel to determine whether the transmission was successful or not. If it is not, it may follow up with specific protocol actions that make this also dynamically sharing the spectrum with other users. In centralized networks, for example, cellular networks, the radios will be assigned channels on the need-only basis in real time by a central controller facilitating dynamic sharing of the channels among radios.

DSS by a CR, however, differs from these usual spectrum sharing approaches in an important way as is shown in Figure 12.1(c). As we discussed in Chapter 3, CRs dynamically share the spectrum with users from other systems. ...

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