A protocol suite for cognitive radios in dynamic spectrum access networks

Abstract

In this chapter, a protocol suite is presented for initiating and controlling transmissions among cognitive radios in dynamic spectrum access networks. A framework is provided for the selection of the initial modulation to be used in a session after a frequency band has been selected. During the first few packet transmissions in a new session, a power-adjustment protocol compensates for uncertainties in the interference and propagation characteristics in the designated frequency band. Throughout the session, an adaptive transmission protocol compensates for variations in the communications environment. Because increases in transmitter power can disrupt other sessions that are underway in the network, adaptation of modulation and coding is the preferred mechanism for responding to increased interference or propagation loss. For a wide range of modulation techniques and channel models, performance results for our protocols are compared with performance results for ideal protocols that are furnished perfect channel-state information. A suite of protocols is required for cognitive packet radios that wish to initiate and maintain reliable communications in a wireless ad hoc dynamic spectrum access network. The session initiation process begins when one wireless communications device, referred to as the source, wishes to set up a session to send a sequence of packets to another wireless communications device, the destination. For the packet sizes that we consider, a session for the transfer of a 1MB file requires the delivery of approximately 2000-8000 packets, depending on the rate of the error-control code. Other wireless communications devices that are within range of the source are referred to as unintended receivers. These devices may receive interference from the source's transmissions if they are operating in the same frequency band. In this chapter, it is assumed that the source and destination are within range of each other; relaying of packets is not required. The wireless communications devices employ half-duplex packet transmission, so simultaneous two-way communication is not possible. The only feedback information is provided in acknowledgment packets, so the goal of the adaptive transmission protocol is to respond to such channel 140 M. B. Pursley, T. C. Royster variations as those due to changes in shadow loss or propagation distance between the source and destination. Once an available frequency band is identified for the session, a modulationselection protocol must choose a modulation technique according to the capabilities of the radios, the established etiquette for transmission in the network, and the quality-of-service (QoS) priorities for the session. Because of the uncertainties in the propagation characteristics in the frequency band that is selected for the session, a power-adjustment protocol must adjust the transmitter power during the first few packets. In this chapter, a packet is an information packet unless specified otherwise (e.g., an acknowledgment packet). The adjusted power level must be high enough to provide dependable delivery of packets to the destination but not so high that the transmissions waste energy and cause unnecessary interference to other radios in the network. As the session continues, the communications environment may change, so an adaptive transmission protocol must modify the modulation and coding as needed to maintain reliable communications without increasing interference to other radios. At the end of the session, the adaptive transmission protocol can supply information to the modulation-selection and power-adjustment protocols if there is to be another session in the same frequency band involving the same source and destination. The radios in the network are cognitive radios (e.g., [1] or [2]) that extract information about their environments. The information they extract permits other radios to adapt their transmission parameters to changes in propagation loss or interference. For our protocols, the cognitive radios are not required to employ channel estimation techniques or even make measurements of received power. Instead, the destination obtains simple statistics from its demodulator and decoder, chooses the modulation format and code for the next packet, and communicates these choices to the source in the acknowledgment packet. We avoid the use of complex channel measurements so that the protocols can be implemented with modest complexity and employed in half-duplex packet radios using current and near-term future technology. We provide a framework for modulation selection that accounts for the three primary spectrum etiquette parameters (time, bandwidth, and power) for each of the modulation formats. We define and evaluate a power-adjustment protocol that converges within the first few packets of a session, and we describe a family of adaptive transmission protocols and provide performance evaluations for static channels with unknown parameters and dynamic channels with time-varying parameters. Our protocols are applicable to any modulation format. For illustrative purposes, we give performance results for a wide range of modulation formats, including quadratureamplitude modulation (QAM), phase-shift keying (PSK), binary orthogonal modulation, nonbinary biorthogonal modulation, and complementary code keying (CCK). Each of the modulation techniques can be employed with direct-sequence (DS) or frequency-hop (FH) spread spectrum. Because of the nature of dynamic spectrum access networks, we do not consider adaptive transmission protocols that increase transmitter power as the primary response to deteriorating channel conditions. The increased interference that results from ramping up the transmitter power degrades frequency reuse in the network and it may disrupt ongoing sessions and prevent the initiation of new ones. The preferred alternative is to use adaptive coding and modulation to respond to an increase in propagation loss or interference. The desire to minimize interference to other radios and the need for energy conservation, particularly for hand-held mobile communications devices, lead us to design protocols that increase the transmitter power only as a last resort. © 2007 Springer Science+Business Media, LLC.