Total Throughput of Cognitive Ad Hoc Networks Overlaying

Concurrent Transmission

Figure 3.8 demonstrates the total throughput of the CR-based ad hoc link and the infrastructure-based uplink transmissions for various numbers of ad hoc users and different locations of primary users. The total throughput is normalized to the infrastructure-based uplink capacity. As shown in the figure, in the worst case at r₃ = 50 meters the total throughput with the concurrent transmission is still 145%

higher than the pure infrastructure-based uplink, and the total throughput reaches a maximum of 173% at r₃ = 10 meters.

Figure 3.9 shows the total throughput performance of the concurrent transmis-sion of infrastructure-based downlink and ad hoc link. In this case, the concurrent transmission probability is constant for various locations of primary users as shown in Fig. 3.5. Thus the throughput is mainly affected by the number of ad hoc users.

For N_CR = 50, the total throughput is 157% when 10 < r₃ < 100 meters. However, when r₃ = 50 meters, the total throughput improves from 148% to 173% as N_CR is changed from 100 to 10.

0 10 20 30 40 50 60 70 80 90 100

Reliability of Concurrent Transmission, FCT

r₃

Reliability of Concurrent Transmission, FCT

r₂

Fig. 3.7: Impacts of shadowing on the reliability of downlink F_CT^(d) (solid line) and uplink F_CT^(u)(dotted line) concurrent transmission against the locations of (a) the primary user MS₃and (b) the ad hoc user MS₂in the cases of σ_ξ= 1 and 6 dB, respectively.

0 20

40 60

80 1000

50

100 1.5

1.55 1.6 1.65 1.7 1.75 1.8

r3

# of users

Normalized Throughput

Fig. 3.8: Total throughput performance of the uplink concurrent transmission.

0 20

40 60

80 1000

50

100 1.5

1.55 1.6 1.65 1.7 1.75 1.8

r3

# of users

Normalized Throughput

Fig. 3.9: Total throughput performance of the downlink concurrent transmission.

3.6 Conclusions

In this part, we identified a critical region R_CT in which the overlaying cognitive ad hoc users and the primary user can concurrently transmit data without causing interference to each other. If the location information of other nodes is available, such a concurrent transmission region can be easily identified. There are two major advantages of identifying the concurrent transmission opportunity. First, the overall throughput of the concurrently transmitted data obtained by combining both the overlaying cognitive ad hoc networks and the legacy infrastructure-based system is much higher than that of the pure infrastructure-based system. Our numerical results show that, in the uplink case, the concurrent transmission region subject to 1 dB and 6 dB shadowing standard deviation can be up to 45% out of the entire cell area with about 90% and 60% reliability, respectively. Second, if such a concurrent transmission opportunity can be identified first, it is clear that the need of the time- and energy-consuming wide-band spectrum scanning process required by most existing cognitive radio systems can be reduced dramatically.

Chapter 4

Neighbor-aware Cognitive Spectrum Access with QoS Provisioning

In this chapter, we focus on the cognitive MAC protocol design, which is different to the conventional scheme with two objectives: the avoidance of primary user’s transmissions and short access delay. In additional to the objectives of high spectrum utilization and QoS provisioning in traditional networks, the cognitive MAC protocol for CR devices has to determine whether its transmission will interfere the primary user at the current and future time period. Moreover, the CR device also demands to access the channel only in a short period of time because primary users have the highest priority to access the channel. Due to the short available transmission time, the fairness from the aspect of access delay among users is more important than the amount of delivered bit for the cognitive MAC protocol, which is also different from the requirement in the legacy MAC design.

4.1 Motivation

According to [7, 8, 13], the main functionality of a cognitive MAC protocol, as shown in Fig. 1.1, can be summarized as follows:

• observe stage - to sense the surrounding environment and record the spectrum usage of the existing legacy systems;

• plan stage - to evaluate if a temporary ad hoc link can be established without interfering current users;

• decide stage - to determine the transmit power, frequency, the time and the duration of the frame transmission;

• act stage - to perform transmission with specified resources at the scheduled time.

To achieve the aforementioned objectives, we design an enhanced CSMA/CA MAC protocol for the spectrum access of secondary users. The CSMA/CA MAC protocol has the preliminary function of spectrum avoidance to the primary users.

To start with, we examine the CSMA/CA MAC protocol by referring the four stages of the cognition cycle in Fig. 1.1. First, from the viewpoint of the observe stage, the cognitive MAC protocol is required to record the spectrum usage of primary users and to collect the traffic characteristics, such as the delay-sensitive or non-real-time data traffic. For the CSMA/CA MAC protocol, most recent research results, instead of identifying the interference, focus on either sensing the carrier transmission in the surrounding environment or avoiding collisions [105–107]. Thus, the functions of recording the spectrum usage and traffic characteristics are not fully considered in the current CSMA/CA MAC protocols.

Second, in the plan stage of the cognition cycle, the cognitive MAC protocol shall determine whether the requested frame transmission from the secondary user will interfere the primary user’s connection. Because the cognitive MAC protocol only permits the secondary user to utilize the spectrum of the legacy system during the spare time of the primary user’s transmissions, the access delay in the cognitive MAC protocol for secondary users shall be small. The standard deviation of the access delay in a cognitive MAC should be reduced to make all the secondary users have the equal opportunities of accessing the channel. However, the fairness problem in terms of equal access delay is not emphasized in many modified CSMA/CA MAC protocols [108–111]. Furthermore, a cognitive MAC protocol shall differentiate the priority for various traffic types with QoS provisioning. Although the authors in [107, 112–115]

suggested adjusting the transmission probability with different contention window (CW) sizes and different lengths of black bursts to differentiate the traffic types, the issue of avoiding interference to the legacy system was not fully considered yet.

Third, in the decide stage, the cognitive MAC protocol schedules frame trans-missions for secondary users to satisfy the QoS requirement, especially for delay-sensitive traffic. In previous works, some researchers suggested to reserve time slots prior to delay-sensitive frame transmissions [116–121]. However, such reservation methods require a polling process or additional handshaking procedure to coordinate frame transmissions. These methods consume battery energy and waste the valu-able bandwidth in sending management frames. Thus, how to design a distributed mechanism to reserve the transmissions for high priority frames becomes an issue.

At last, in the act stage of the cognition cycle, the cognitive MAC protocol synchronizes stations and execute the transmission at the specified time. To syn-chronize the clock of each station, the methods designating a centralized controller to broadcast “beacon” signals or utilizing the global clock provided by Global Position-ing System (GPS) were suggested in [12, 120, 121]. However, both methods require additional devices.

Here, we propose such a generic cognitive MAC protocol in overlaying ad hoc networks with emphases on achieving the aforementioned objectives: high spectrum utilization, QoS satisfactory and short access delay. Specifically, in the observe stage, we propose a mechanism of establishing the neighbor list to help stations to recognize the spectrum opportunities. In the plan stage, an improved contention resolution mechanism, consisting of the gating mechanism, linear backoff algorithm and stall avoidance scheme, is suggested to enhance the performance of throughput, access delay and fairness from the aspect of short access delay for CR devices. In the decide stage, a novel invited reservation procedure is developed to ensure a secondary user with QoS provisioning. At last, in the act stage, a distributed frame synchronization mechanism is proposed to coordinate frame transmissions among secondary users without a centralized controller.