Reconfigurable Multirate Systems in Cognitive Radios

One perspective in communication systems is to increase the spectrum utilization using cognitive radios. A cognitive radio is a network of intelligent co-existing radios which senses the environment to find the available frequency slots, white spaces, or the spectrum holes as noted in Akyildiz et al. (2008); Haykin (2005). Then, it modifies its transmission characteristics to use that particular frequency slot. Figure 1 illustrates the overlay spectrum sharing outlined in Cabric et al. (2006), or the opportunistic spectrum access discussed in Zhao & Sadler (2007); Zhao & Swami (2007), or the dynamic spectrum access which is considered in Sherman et al. (2008). Here, secondary users occupy the frequency slots which are not used1 by the primary users. One of the main tasks in a cognitive radio is thus the spectrum mobility as in Akyildiz et al. (2006; 2008), or the dynamic frequency allocation as in Haykin (2005), or the dynamic spectrum allocation as in Leaves et al. (2004); Zhao & Swami (2007). This chapter uses the term dynamic frequency-band allocation (DFBA). Being dynamic means that the transmission parameters, e.g., bandwidth, center frequency, transmission power, and communication standard etc., may vary with time according to Akyildiz et al. (2006). One should at least be able to change the center frequency and bandwidth although other parameters may also change. This is also referred to as the reconfigurability according to Akyildiz et al. (2008); Haykin (2005); Jondral (2005); Leaves et al. (2004); Ramacher (2007); Sherman et al. (2008).