Fundamental design tradeoffs in cognitive radio systems

Abstract

Under the current system of spectrum allocation, rigid partitioning has resulted in vastly underutilized spectrum bands, even in urban locales. Cognitive radios have been proposed as a way to reuse this underutilized spectrum in an opportunistic manner. To achieve this reuse while guaranteeing non-interference with the primary user, cognitive radios must detect very weak primary signals. However, uncertainties in the noise+interference impose a limit on how low of a primary signal can be robustly detected.In this paper, we show that the presence/absence of possible interference from other opportunistic spectrum users represents a major component of the uncertainty limiting the ability of a cognitive radio network to reclaim a band for its use. Coordination among nearby cognitive radios is required to control this uncertainty. While this coordination can take a form similar to a traditional MAC protocol for data communication, its role is different in that it aims to reduce the uncertainty about interference rather than just reducing the interference itself.We show how the degree of coordination required can vary based on the coherence times and bandwidths involved, as well as the complexity of the detectors themselves. The simplest sensing strategies end up needing the most coordination, while more complex strategies involving adaptive coherent processing and interference prediction can be individually more robust and thereby reduce the need for coordination across different networks. We also show the existence of a coordination radius wall which limits secondary user densities that can be supported irrespective of coordination involved. Furthermore, local cooperation among cognitive radios for collective decision making can reduce the fading margins we need to budget for. This cooperation benefits from increased secondary user densities and hence induces a minima in the power-coordination tradeoff.

A Survey on Coexistence in Heterogeneous Wireless Networks in TV White Spaces

“…Such interference, may also affect primary signal detection as shown in [52]. The problem is worsened in areas with limited spectrum availability where many devices might choose the same channel or they will have to work on adjacent or cochannels.…”

“…To make this possible, the secondary user must have the channel gain information between its transmitter and the primary receiver [60]. Different interference measurement schemes have been proposed in [52,61].…”

Fundamental Limitations in Energy Detection for Spectrum Sensing

“…This is because secondary users generally have very limited knowledge about the whole spectrum, which may leave the spectrum sensing results far from accurate. Some existing spectrum sensing methods in the literature are by way of matched filtering, waveform-based sensing [2], cyclostationary-based sensing [3,4], eigenvalue-based method [5,6], energy detection [7][8][9][10][11][12][13], etc. Obviously, energy detection is the most popular and simple way for spectrum sensing.…”

“…This is actually not a well-posed presentation though, since there may be too many solutions or no solution sometimes to the inequality in Equation (1) with respect to λ for a given noise variance σ 2 n , signal to noise ratio (SNR), and data size M.…”

Consensus-based decentralized clustering for cooperative spectrum sensing in cognitive radio networks

“…To determine the temporal or/and spatial spectrum holes, a main challenge is reliable and efficient spectrum sensing. Compared with the single-SU spectrum sensing methods [2], cooperative spectrum sensing (CSS) among multiple SUs can be employed to tackle the problem of hidden primary receivers [3] and wireless channel fading (e.g., shadowing and multi-path fading) [4] by exploiting multi-user spatial diversity [5]. There have been recent comprehensive surveys on CSS in the literature [6,7].…”