Distributed spectrum leasing via cooperation

“Cognitive radio” networks enable the coexistence of primary (licensed) and secondary (unlicensed) terminals. Conventional frameworks, namely commons and property-rights models, while being promising in certain aspects, appear to have significant drawbacks for implementation of large-scale distributed cognitive radio networks, due to the technological and theoretical limits on the ability of secondary activity to perform effective spectrum sensing and on the stringent constraints on protocols and architectures. To address the problems highlighted above, the framework of distributed spectrum leasing via cross-layer cooperation (DiSC) has been recently proposed as a basic mechanism to guide the design of decentralized cognitive radio networks. According to this framework, each primary terminal can ”lease” a transmission opportunity to a local secondary terminal in exchange for cooperation (relaying) as long as secondary quality-of-service (QoS) requirements are satisfied. The dissertation starts by investigating the performance bounds from an information-theoretical standpoint by focusing on the scenario of a single primary user and multiple secondary users with private messages. Achievable rate regions are derived for discrete memoryless and Gaussian models by considering Decode-and-Forward (DF), with both standard and parity-forwarding techniques, and Compress-and-Forward (CF), along with superposition coding at the secondary nodes. Then a framework is proposed that extends the analysis to multiple primary users and multiple secondary users by leveraging the concept of Generalized Nash Equilibrium. Accordingly, multiple primary users, each owning its own spectral resource, compete for the cooperation of the available secondary users under a shared constraint on all spectrum leasing decisions set by the secondary QoS requirements. A general formulation of the problem is given and solutions are proposed with different signaling requirements among the primary users. The novel idea of interference forwarding as a mechanism to enable DiSC is proposed, whereby primary users lease part of their spectrum to the secondary users if the latter assist by forwarding information about the interference to enable interference mitigation at the primary receivers. Finally, an application of DiSC in multi-tier wireless networks such as femtocells overlaid by macrocells whereby the femtocell base station acts as a relay for the macrocell users is presented. The performance advantages of the proposed application are evaluated by studying the transmission reliability of macro and femto users for a quasi-static fading channel in terms of outage probability and diversity-multiplexing trade-off for uplink and, more briefly, for downlink

Gamal, “On the utility of frequency reuse in cognitive radio channels

Abstract-We consider the generalized cognitive radio channel where the secondary user is allowed to reuse the frequency during the active periods of the primary user, as long as the primary rate remains the same. In this setting, the optimal power allocation policy with a single antenna secondary transmitter (and receiver) is explored. Interestingly, we show that the offered gain resulting from the frequency reuse during the active periods of the spectrum disappears in both the low and high signal-tonoise ratio (SNR) regimes. This drawback, however, is shown to disappear with multi-antenna nodes by using simple zero-forcing strategies at both ends of the secondary channel. I. BACKGROUND In the classical cognitive radio set-up, the secondary users must first sense the wireless channel to determine the unused parts of the spectrum. Those users will then transmit their own messages during these white spaces in order to increase the overall spectral efficiency. In other words, the cognitive radios can only transmit over those particular frequency bands (or time intervals) which the licensed (primary) users are not transmitting. In contrast to the classical cognitive radio approach, recent studies have introduced cognitive channels in which the secondary user exploits the active areas in the spectrum (i.e., simultaneously transmits with the primary users) as long as certain constraints are satisfied [1]- In particular, we consider a four-terminal network, in which the primary transmitter and receiver are Node 1 and Node 3, respectively; whereas their secondary counterparts are Node 2 and Node 4. All nodes are assumed to be half-duplex and the transmitters are limited by individual long-term average power constraints. In the classical cognitive radio channel, the secondary user is only allowed to transmit during state 1, in which the primary transmitter is silent. Here, we consider the generalized cognitive radio, where we allow the secondary user to transmit also in state 2, in which the primary user is active, as long as the following coexistence constraints are satisfied [4]. • Primary link has the same structure (encoder and decoder) as in the non-cognitive network. • Primary users have the same performance (instantaneous achievable rate) as in the non-cognitive network. The fundamental difference between our work and [4] is the relaxation of the unrealistic assumption that the secondary transmitter knows a-priori the signal to be transmitted over the primary link in the generalized setting above. In fact, by explicitly accounting for the time needed to decode the primary messages, at the single-antenna secondary transmitter, it is shown in the sequel that the gain offered by frequency re-use in state 2 disappears in the high and low SNR regimes. Furthermore, by equipping the secondary transmitter and receiver with multiple antennas, we show how this problem can be overcome in the high SNR regime. II. SISO COGNITIVE CHANNEL We adopt the asymptotic assumption of M → ∞ blocks with N → ∞ channel uses per block. It is further assumed that the primary transmitter is silent, i.e., in state 1, in any particular block with probability p and the cognitive user is informed a-priori with only the states of the different blocks. Mathematically, we denote the instantaneous cognitive rate during state 1 and state 2 as R 1 (P 1 ) and R 2 (P 2 ), respectively. We also assume that the power of the secondary user linearly scales with the power of the primary user and denote P and βP as the total (long-term) average power constraints of the cognitive and primary transmitters, respectively. Thus, in this setting, the following cognitive rate is achievable if the coexistence constraints are satisfied with a choice of power allocation parameter t ∈ [0, 1]: In this section, we analyze this power allocation problem under the assumption of a single-input-single-output (SISO) cognitive link. The main hurdle now is to identify the optimal coding strategy when the secondary transmitter is re-usin