Cognitive radios hold a promise to increase spectral utilization with acceptable degradation if any, in the licensed and unlicensed bands. The license holder buys the exclusive rights to the spectral band. However, more often than not, the spectral utilization is in the low teens. Cognitive radio network provides guidance in choosing the transmission parameters such that the interference with other users is minimal. The cognitive radio uses its ability to adapt to the environment and chooses its parameters to meet its needs. For instance, assume that the CR receiver is far from the CR transmitter. The CR pair must agree on 1) power level, 2) modulation scheme, and 3) transmission schedule. As the physical distance between the transmitter and the receiver increases, the transmission link can be maintained via 1) additional transmission power, 2) increase in effective signal power due to additional gain at antenna such as directional
antenna, multiple antenna or beam steering techniques, 3) choice of robust modulation scheme better suited for low signal to noise ration (SNR) such as OFDM or BPSK, 4) use of signal processing techniques such error correction, and 5) collaboration with other CRs or a controller within the network. The receiver must also contend with classical wireless issues such as fading, interference, multipath effect, and shadowing [37].
With the central goal of increasing channel capacity, the CR network uses several techniques to increase spectral capacity [36].
2.2.2.1 Gap Filling Approach (White Space Filling)
This is the most common and most obvious approach. The CR transmission occurs during the spatial, temporal or spectral voids. This interference avoidance is predicated on the ability of the CR to accurately and quickly sense available gaps in transmission [38]. This approach was explained in the example earlier in the section on Cognitive Radio .
2.2.2.2 Simultaneous Controlled Transmission
In Figure 2-9, we demonstrated an example where PUs, SUs and CRs may operate simultaneously. In this case, the CR must adjust its transmit power level such that the PU receiver is able to operate with acceptable interference or acceptable noise temperature from other occupants [39].
2.2.2.3 Opportunistic Interference Cancellation
This approach assumes that the CR has prior knowledge of the PU link [40]. Assume that the CR is operating in a GSM band. The CR listens to the basestation command and control information and decides when and where to transmit in the band. If a CR has the ability to demodulate and decode the information contained in the TS0 slot in a GSM frame [21], it would possess the ability to predict where and when a GSM users would transmit. Hence, the CR is able to use this knowledge to avoid interference. Another example is if the PU is a CDMA user. In this case, the CR would have the ability to use one of the available orthogonal spreading codes so that the interference is minimized [40].
This approach adds additional intelligence to the cooperation in a cognitive network. Cognitive transmission assumes cooperation between the PU and the CR transmitter [41]. It assumes that the CR knows the transmission message and is able to provide orthogonal messages in essence cancelling the effect at the PU receiver. The advantage of this technique is that the CR is able to transmit at full power with real effect on the PU receiver.
2.2.2.5 Network with Beacon
This approach is an extension of the interference avoidance described earlier. The PU transmits a beacon before each transmission. The beacon signal warns cognitive users of an upcoming PU transmission [42]. This early warning minimizes the probability of interference.
2.2.2.6 Network with Primary Exclusive Regions
This approach is more suitable in a broadcast environment where one PU transmitter is communicating with multiple receivers. The network imposes exclusive regions near the PU transmitter that forbid CR transmission [43].
2.2.2.7 Single or Multi- Hop Network
This is a cooperative model where the CRs agree to communicate with a nearby receiver with self imposed transmit power. With such limit on the transmit signal, the distance between the transmitter and receiver is limited [45]. In multi-hop networks, there is a stronger cooperation among users. As an example, the network allows nearby receivers in a coalition to strip off stronger transmissions.
It is hard to rank the above network approaches. Each network has advantages and disadvantages based on different environment parameters such as the intended use, geographical location, legacy products, and regulatory restrictions. The right answer for a CR network in downtown Tokyo might be different than that of downtown Manhattan, KS. Regardless of the network type, the CR device must perform a very basic function: the ability to learn and adapt to its environment.
2.3 Cognitive Radio Architecture
One of the key elements to Cognitive Radio is its ability to learn and adapt to the environment. Non-smart radios are usually constrained by a master controller. In cellular networks, the frequencies are hardwired during the frequency planning stages and the basestations provide rights of transmissions. In BT, a device becomes a master and directs traffic to avoid interference. In theory, the CR is frequency agnostic and is free to choose the link parameters as long as the device operates below a given threshold. Based on its sensing algorithm, the CR has the flexibility to choose [46] link parameters that affect bandwidth use, spectrum efficiency, bit error rate, probability of dropped call, throughput, goodput (as opposed to throughput), power consumption, and system delay / computational complexity. For example, a CR may choose the frequency, pulse shape, symbol rate, modulation, frame length, power level, power guard bands and timing of the transmission.
The sensing function is distributed across the physical and the MAC layers. Garbic et al. [47] proposed a cross layer functionality shown in Figure 2-10.
RF Receiver: -Wideband -Agile Detection: -Energy -Timing -Verification Optimization: -Power -Modulation -Filtering -Frequency Cooperation:
Combination of sensing measurements and spectrum allocation
Sensing Cognition Adaptation
PHY La ye r MAC La ye r RF Receiver: -Wideband -Agile Detection: -Energy -Timing -Verification Optimization: -Power -Modulation -Filtering -Frequency Cooperation:
Combination of sensing measurements and spectrum allocation
Sensing Cognition Adaptation
PHY La ye r MAC La ye r