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Friday, March 11, 2011

OFDM – Why it’s a good fit for cognitive radio (Continuation – Part II)


I have earlier discussed this topic in this ARTICLE . I only made reference to spectrum sensing and awareness as well as spectrum shaping cognitive radio requirements. In this article I continue the discussion by discussing how OFDM meets other cognitive radio requirements henceforth justifying its fit highlighted below;

Advanced Antenna Techniques
Advanced antenna techniques are not necessarily required for cognitive radios. However, they are desirable as they will provide better spectral efficiency which is the primary motivation for cognitive radio. Smart antennas and MIMO systems can be used to exploit the spatial dimension of spectrum space (e.g. through beam forming) to improve the efficiency. In essence multi-antenna systems can help to find spectral opportunities in the spatial domain and can help to exploit these opportunities in full. The use of MIMO techniques offers several important advantages including spatial degree of freedom, increased spectral efficiency and diversity. These advantages can be used to increase the spectrum utilization of the overall system. Furthermore, beamforming, diversity combining, and space-time equalization can also be applied to cognitive OFDM systems. Another application of adaptive antenna techniques is the reduction of the interference in OFDM systems. MIMO systems commonly employ OFDM as their transmission technique because of the simple diversity combination and equalization, particularly at high data rates. In MIMO–OFDM, the channel response becomes a matrix. Since each tone can be equalized independently, the complexity of space–time equalizers is avoided and signals can be processed using relatively straightforward matrix algebra. Moreover, the advantages of OFDM in multipath are preserved in MIMO–OFDM system as frequency selectivity caused by multipath increases the capacity.

Multiple Accessing and Spectral Allocation
The resources available to a cognitive system often have to be shared among users. Several techniques can be used to achieve such tasks. OFDM supports the well-known multiple accessing techniques such as TDMA, FDMA, and CSMA. Moreover, CDMA can also be used together with OFDM for multiplexing different users, in which case the transmission is known as Multicarrier Code Division Multiple Access (MC–CDMA) or Multicarrier Direct Spread Code Division Multiple Access (DS–CDMA) OFDMA, a special case of FDMA, has gained tremendous attention recently with its usage in fixed and mobile Worldwide Interoperability for Microwave Access (WiMAX). In OFDMA, subcarriers are grouped into sets each of which is assigned to a different user. Interleaved, randomized, or clustered assignment schemes can be used for this purpose. Hence, it offers very flexible multiple accessing and spectral allocation capability for cognitive radios without any extra complexity or hardware. The allocation of subcarriers can be tailored according to the spectrum availability. The flexibility and support of OFDM systems for various multiple accessing enables the interoperability and increases the adoption of cognitive radio as well.


Interoperability

Another desirable feature of cognitive radio is interoperability. Interoperability can be defined as the ability of two or more systems or components to exchange information and to use the information that has been exchanged. Since cognitive radio systems have to deal with licensed users as well as other cognitive users, the ability to detect and encode existing users’ signals can expedite the adoption and improve the performance of cognitive radio systems. Furthermore, some recent unfortunate events manifested the importance of interoperability in terms of wireless communications for the first responders.

For interoperability problems, cognitive radio can improve the disaster relief operations by developing the coordination between first responders. For such tasks, OFDM is one of the best candidates as OFDM signaling has been successfully used in various technologies. Systems based on OFDM include 802.11a and 802.11g Wireless LAN standards, Digital Audio Broadcasting (DAB), DVB, and WiMAX.






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