6.1 Introduction

Cognitive radio is an emerging wireless communications paradigm in which either the network or the wireless node itself intelligently adapts particular transmission or reception parameters by sensing the environment. The goals of adaptation include maximizing spectral efficiency, minimizing interference to other cognitive radios, coexistence of licensed and unlicensed band communications, battery energy efficiency, etc. The environmental parameters that are continually sensed for adaptation include occupied radio frequency bands, user traffic, network state, etc. One promising technology that enables the implementation of a cognitive radio network is software-defined radio. The underlying theoretical principles for cognition are broadly based on signal-processing and machine-learning. The cognitive cycle [1] consists of three major components: (a) sensing of Radio Frequency (RF), (b) cognition/management, and (c) control action. In particular,

1. RF sensing:

   • estimation of the total interference in the radio environment;

   • detection of spectrum holes (or unused bands);

   • estimation of channel state information (e.g., Signal to Noise Ratio (SNR));

   • prediction of channel capacity for use by the transmitter.

1. Cognition/management:

   • spectrum management which controls the opportunistic spectrum access;

   • traffic shaping;

   • routing;

   • Quality of Service (QoS) provisioning.

1. Control actions:

   • transmit-power control;

   • adaptive modulation and coding;

   • transmission rate control.

Task (1) deals with spectrum sensing and prediction, hence, places emphasis on physical layer issues. In this chapter we will focus on...