Sensing Options

With numerous broadband radio design challenges and the need for fast and accurate sensing, the CR community began to think “outside the box.” The brute force approach and linear scaling of the technology fails to deliver the required performance for a sensing receiver. The CR community pushed towards system and architectural improvements instead of circuit design incremental improvements.

3.2.1 Spectrum Etiquette and “no talk” Zones

The simplest form of cognitive radio comes in the form of spectrum etiquette where the CR uses a “listen before talk” strategy. This strategy is commonly used in application such as spectrum leasing. The concept of the spectrum etiquette is not new. In 2007, the FCC has proposed etiquette for the ISM band [76]. The NPRM seeks to establish spectrum etiquette for unlicensed transmitters that operate under Sections 15.247 and 15.249 of the rules in the unlicensed bands at 915MHz, 2.4GHz, and 5.8GHz. Therefore, the NPRM had a direct effect on popular standards such as IEEE 802.11, 802.15.4, 802.16, and 802.18. However, [77] showed that spectrum etiquette improves spectrum network performance at the expense of an increased overhead required for information exchange.

3.2.2 Cooperative Spectrum Sensing

Cooperative spectrum sensing is a solution that relies on the variability of signals strength in various locations within a CR network [69]. The concept assumes a large number of users and sensing information is exchanged between neighbors. Cooperative spectrum sensing is a solution that has the potential to accelerate sensing while reducing the workload on the individual sensing.

The solution assumes the presence of access points [71], a gateway and nodes denoted as end users [70]. This model employs an overlay of several techniques such MIMO, cognitive radio, mesh network, and cross layer communication to improve performance. An implementation is

Cognitive Radio

Access Point Cognitive Radio

Access Point

Figure 3-10 Cooperative Sensing in Cognitive Radio

The main concern of the cooperative sensing is about increasing the probability of detection of a PU while collaborating with other PU. The results from [70] are shown in Figure 3-11. The simulation assumes BPSK modulation operating at 2.4 GHz.

where PFA is the probability of false alarm and NCS is a non cooperative network. The improvement of probability detection is shown in Figure 3-11. However, there are some key questions about application and realization of the cooperative sensing:

1. How can cognitive radios cooperate? 2. How much can be gained from cooperation 3. What is the overhead associated with cooperation? 4. How can cooperative sensing improve the sensing time?

These questions above remain unanswered. Although improving detection probability is important, detection time and accuracy remain a challenge. Also, the addition of access points adds to the complexity, deployment, maintenance, and cost of the CR solution.

3.2.3 Centralized CR Network and Spectrum Leasing

The FCC published a Notice of Proposed Rulemaking (NPRM) [75] in which the FCC presented four scenarios on effective spectrum lease to cognitive radios:

1. The most common understanding of cognitive radios operation is the use of white spaces. This common understanding of CR use assumes does not rely on authorization from the PU as long as the interference is held in check. This is called the Commons Model where the PU is oblivious to the presence of secondary users in the spectrum.

2. The PU is aware of the presence of secondary users in the band and chooses to lease its spectrum under three different models. This is called the Property-rights or Spectrum leasing Model.

a. A frequency band is licensed to a single user. The user may use CR technology to increase spectrum efficiency. Unlicensed CRs are not authorized to use the channel.

time and use spectrum only if an agreement is reached between devices and/or their owners.

c. The cognitive radio technologies can facilitate automated frequency coordination among co-primary spectrum users. This scenario can be extended to spectrum sensing cooperation as well.

In a centralized CR network, a spectrum coordinator assigns short term spectrum leases to end users [73,74]. Unlike cellular spectrum licenses, where licenses are multi-year in large contiguous spectrum chunks, the CR spectrum leases are short term leases to cooperating secondary users. The spectrum bandwidth and center frequency vary from lease to lease. The broker must deal with two conflicting objectives: 1) maximizing spectrum use, and 2) minimizing interference. Under this model, the PU can dictate the model parameters. In this model, the PU sets a censored or a “no talk” zone such that the CR is not allowed to transmit [72].

Similar to other sensing solutions, this technique improves sensing but does not address the delay or overhead associated with cooperation. Worse yet, in the case of the spectrum leasing model, the primary user solely makes the decision if and when the CR is allowed to operate. For example, assume that the PU is a cellular network in peak time. Chances are the PU denies service to SUs during the peak utilization time. This delay renders the CR highly ineffective.

3.2.4 Control Channel

Under this model, the CR may predetermine an agreed upon channel to facilitate collaboration and cooperation. The control channel can either be implemented as a dedicated frequency channel or as an underlay UWB channel [71]. Wideband RF frontend tuners/filters can be shared between the UWB control channel and normal cognitive radio reception/transmission. Control channels facilitate communication and control among users and help avoid interference or clashes.

The control channel again does not address the sensing time. This option has many drawbacks including having a master control decide when a CR is allowed to initiate transmission. This solution does not address sensing time nor does it solve the real time operation of a CR.

3.3 Summary

From RF impairments to complex spectrum power measurement techniques, the challenges to perform wideband sensing are many. In order for the CR to become a practical technology, it must overcome the sensing challenges. The CR must perform the measurements swiftly, make a decision, and tune to available bands over wide frequency bands or return to a previous channel. Several research topics in a distributed or centralized network have yielded improvements in important parameters such as detection probability, low SNR detection, false alarm minimization, collision avoidance, and scheduling. However, none of these techniques have focused on minimizing the sensing time. As a matter of fact, several of the ideas described earlier have actually increased the sensing time by adding overhead to the network.

In the next section, we show RF impairment effects highly reduced with the addition of a Dedicated Sensing Receiver (DSR). The DSR’s main function is power detection and spectrum estimation. It possesses the ability to detect the presence of other users in the band. It does not however need to perform demodulation of the signal. In order to meet the timing restrictions, novel techniques must be integrated into the sensing receiver. The addition of the DSR provides additional benefits to the overall radio. For example, the presence of two independent receive chains in the system provides channel diversity, as well as the ability to share the work load between the two receivers.

With the stated challenges and delay restrictions, the architecture of the DSR and a sensing algorithm is presented in the next chapter. The performance improvements are also made evident by the results.