Cognitive Radio - Underlay, Overlay or Interweave: How the spectrum is shared?

Cognitive radio brings a concept shift as a well regarded agile technology that allows opening up the frequency bands to concurrent operating users in a non-interfering mode. Accordingly to make possible spectrum sharing without causing harmful interference to existing traffics, cognitive users should possess a minimum of information about their surrounding non cognitive users. Depending on the knowledge that is needed to coexist with the primary network, cognitive radio approaches fall into three classes: Underlay, Overlay and Interweave [1].

Underlay approach: Simultaneous cognitive and non cognitive transmissions are allowed as long as the interference level at the primary user side remains acceptable. Exceeding the predefined tolerable interference threshold may degrade dramatically the primary signal. In recent literature, many advanced signal processing techniques have proven to be very efficient for interference avoidance and mitigation, among which we find the beamforming and the spread spectrum. Beamforming consists on exploiting the superposition concept of waves to guide the signal toward a specific receiver using multiple antennas. More importantly, in a cognitive context constructive or destructive interference is provoked at the intended cognitive receiver in order to lessen the interference caused to non cognitive users while focusing the signal energy in the direction of secondary users. Using the spread spectrum technique, the secondary signal is multiplied by a spreading code to obtain a weaker signal with wider band. The resulting spread signal causes lower interference level to non cognitive users. The original secondary signal is recovered at the receiver side by simply multiplying the input signal with the same spreading code. The spread spectrum technique is also useful for alleviating the interference caused by the primary signals to the secondary ones. Another common solution could be limiting the power of the secondary signal to keep the interference level at the primary side bounded albeit restricting the secondary transmissions to short range communications.

Overlay approach: In this approach, non cognitive users share knowledge of their signal codebooks and messages with the cognitive users. Thus, the cognitive devices may enhance and assist the non cognitive transmission rather than vying for spectrum access. More precisely, cognitive devices overhear the messages sent by non cognitive sources and use these messages either to eliminate the interference generated by the primary communication at the cognitive receiver side or to improve the performance of the primary transmission through relaying the accumulated messages to the primary receiver. The latter case allows the cognitive device to transmit at the same time as the non cognitive transmitter provided that its overall transmit power is fairly covering the energy needs of its own communication as well as its relaying operation. A trade-off should be carefully designed between the interference induced on the primary signal and the improvement brought to it to achieve a stagnant SNR.

Interweave approach: This spectral coexistence approach has been proposed in the objective of enabling devices to occupy the spectrum rooms that has been left vacant by non cognitive users. The surrounding environment should be observed to be able to predict the state of each portion of the frequency spectrum, portions of spectrum that are considered as being under-utilized may be accessed by secondary users as long as the primary activity remains idle. In order to facilitate the coexistence of both primary and secondary traffics within the same network in an opportunistic transmission mode, spectrum opportunities should be actively identified and monitored. On one hand, cognitive users may conduct sensing operations permanently and reliably, different dimensions have to be explored to find the abundant spectrum gaps. Legacy sensing algorithms monitor and supervise the spectrum through three conventional dimensions: frequency, time and space domains. However, other degrees of freedom (DoF) such as the used code and the angle of arrival may be examined. On the other hand, geographic coordination through a central database to identify the vacant gaps is a good substitute for selfish spectral sensing. Combining both methods may be also envisaged.

Hybrid schemes using a combination of the aforementioned paradigms [2] have a great potential to improve the efficiency of spectrum sharing. The benefit of such schemes is that it allows secondary users to maximize their transmission rate once a spectrum opportunity is detected.

BIBLIOGRAPHY

[1] A. Goldsmith, S.A. Jafar, I. Maric, and S. Srinivasa, "Breaking spectrum gridlock with cognitive radios: An information theoretic perspective," Proc. IEEE, vol. 97, no. 5, pp. 894-914, May 2009.

[2] Z. Wu and B. Natarajan, "Interference Tolerant Agile Cognitive Radio: Maximize Channel Capacity of Cognitive Radio," Consumer Communications and Networking Conference, pp. 1027-1031, January 2007.