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Monday, January 17, 2011

Cognitive Radio: The rationale of Cooperative Spectrum Sensing

With the rapid growth of wireless applications and services in the recent decade, spectrum resources are facing huge demands. The radio spectrum is a limited resource and is regulated by government agencies such as the Federal Communications Commission (FCC) in the United States. Within the current spectrum regulatory framework, all of the frequency bands are exclusively allocated to specific services and no violation from unlicensed users is allowed. The spectrum scarcity problem is getting worse due to the emergence of new wireless services.

Cognitive radio is a novel technology which improves the spectrum utilization by allowing secondary networks (users) to borrow unused radio spectrum from primary licensed networks (users) or to share the spectrum with the primary networks (users).

As an intelligent wireless communication system, cognitive radio is aware of the radio frequency environment, selects the communication parameters (such as carrier frequency, bandwidth and transmission power) to optimize the spectrum usage and adapts its transmission and reception accordingly. One of most critical components of cognitive radio technology is spectrum sensing. By sensing and adapting to the environment, a cognitive radio is able to fill in spectrum holes and serve its users without causing harmful interference to the licensed user. To do so, the cognitive radio must continuously sense the spectrum it is using in order to detect the re-appearance of the primary user. Once the primary user is detected, the cognitive radio should withdraw from the spectrum instantly so as to minimize the interference it may possibly incur.

This is a very difficult task as the various primary users will be employing different modulation schemes, data rates and transmission powers in the presence of variable propagation environments and interference generated by other secondary users. Another great challenge of implementing spectrum sensing is the hidden terminal problem ( illustrated in the figure below), which occurs when the cognitive radio is shadowed, in severe multi-path fading or inside buildings with high penetration loss while a primary user is operating in the vicinity.

Due to the hidden terminal problem, a cognitive radio fails to see the presence of the primary user and then will access the licensed channel and cause interference to the licensed users. In order to deal with the hidden terminal problem in cognitive radio networks, multiple cognitive users can cooperate to conduct spectrum sensing.

Cooperative communications has been recently recognized as a powerful solution that can overcome the limitation of wireless systems. The basic idea behind cooperative transmission rests on the observation that in a wireless environment, the signal transmitted or broadcast by a source to a destination node, each employing a single antenna, is also received by other terminals, which are often referred to as relays or partners. The relays process and retransmit the signals they receive. The destination node then combines the signals coming from the source and the partners, thereby creating spatial diversity and taking advantage of the multiple receptions of the same data at the various terminals and transmission paths. In addition, the interference among terminals can be dramatically suppressed by distributed spatial processing technology. By allowing multiple cognitive radios to cooperate in spectrum sensing, the hidden terminal problem can be addressed.

Cooperative spectrum sensing in cognitive radio networks has an analogy to a distributed decision in wireless sensor networks, where each sensor makes a local decision and those decision results are reported to a fusion center to give a final decision according to some fusion rule. The main difference between these two applications lies in the wireless environment. Compared to wireless sensor networks, cognitive radios and the fusion center (or common receiver) are distributed over a larger geographic area. This difference brings out a much more challenging problem to cooperative spectrum sensing because sensing channels (from the primary user to cognitive radios) and reporting channels (from cognitive radios to the fusion center or common receiver) are normally subject to fading or heavy shadowing.

In general, cooperative spectrum sensing is performed as follows:

Step 1: Every cognitive radio performs local spectrum measurements independently and then makes a binary decision.
Step 2: All the cognitive radios forward their binary decisions to a common receiver which is an access point (AP) in a wireless LAN or a base station (BS) in a cellular network.
Step 3: The common receiver combines those binary decisions and makes a final decision to infer the absence or presence of the primary user in the observed band.

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