Numerical results are performed to evaluate the throughput performance of the con-ventional direct transmission and the proposed FCC and BCC protocols. The network scenario for performance evaluation and comparison is described as follows. Similar to Fig. 2.1, a single destination is assumed to locate at the center of the considered network, which confines a circular region with radius equal to 50 meters. The source nodes, which denotes the users, are randomly located within the area between 30 and 50 meters from the destination. It is assumed that all source nodes adopt the pro-posed protocols in order to observe the effectiveness of the propro-posed protocols when source nodes are far away from the destination. Based on the observation from [32], the suitable decision for relay deployment is to place the relays around the intermedi-ate location between the users and destination in order to appropriintermedi-ately enhance the network throughput. Therefore, stationary relays are uniformly distributed around the circle which is 20 meters from the destination. Various numbers of sources and relays will be considered under different simulation cases. It is also noted that the passloss exponent is set to be 4 in the following simulations. The other parameters adopted in the simulations is selected the same as in Table I.
30 32 34 36 38 40 2
2.5 3 3.5 4 4.5
σDB (dB)
Throughput (Mbps)
Number of Relays = 4 Number of Relays = 6 Number of Relays = 8
Figure 5.1: Throughput performance versus the average SNR value of boundary node σDB using FCC protocol (number of sources = 30).
Figs. 5.1 and 5.2 illustrate the throughput performance for the proposed FCC and BCC protocols respectively under different SNR values σDB. The parameter σDB is defined as the average received SNR between the destination and a source located at the boundary of destination’s transmission range. Noted that the average received SNRs of other links can also be computed according to the distances of the links compared to that located at the boundary with σDB. The total number of relays are selected as 4, 6, and 8 in both cases; while that for the sources is chosen as 30. It is intuitive to observe in both figures that the throughput performance is increased with both schemes as the value of σDB is augmented. However, in the proposed FCC protocol, the increased number of relays will degrade the throughput performance as shown in Fig. 5.1. The main reason is due to the requirement to transmit additional rRTS frames by adopting the FCC method as the number of relays is increased. The throughput performance will consequently be degraded since excessive overhead are introduced by the elongated CSI-acquiring period. On the other hand, the throughput performance is enhanced as the number of relays is increased by exploiting the proposed BCC protocol. The
30 32 34 36 38 40 2.5
3 3.5 4 4.5 5
σDB (dB)
Throughput (Mbps)
Number of Relays = 4 Number of Relays = 6 Number of Relays = 8
Figure 5.2: Throughput performance versus the average SNR value of boundary node σDB using BCC protocol (number of sources = 30).
reason is that the additional relays can provide data forwarding services for more users with the pre-specified fixed CSI-acquiring period. Even though only partial channel information are available by adopting the relay contention process within the BCC scheme, the resulting throughput performance can still be improved with augmented number of relays. Furthermore, as shown in Fig. 5.2, the throughput enhancement due to the increased number of relays becomes insignificant as σDB is augmented, i.e. all three lines converge as σDB is around 40 dB. This is attributed to the situation with sufficiently good channel quality (i.e. with larger σDB values) where direct transmission will mostly be activated by the destination. As a result, the number of relays will result in less impact on the throughput performance.
Figs. 5.3 and 5.4 are illustrated to compare the throughput performance of the pro-posed protocols with various number of relays. The total number of sources is selected as 20, 30, and 40 for both cases. It is noted that the SNR value σDB is chosen as 30 dB for observing the effectiveness of proposed schemes under poor channel quality. It can be discovered that the throughput performance can be enhanced as the number of
1 2 3 4 5 6 7 8 9 10 2
2.1 2.2 2.3 2.4 2.5 2.6 2.7
Relay Number
Throughput (Mbps) 20 nodes
30 nodes 40 nodes
Figure 5.3: Throughput performance versus the number of relays using FCC protocol with different number of sources (σDB = 30 dB).
relays is smaller than 4 in both proposed protocols. However, as the number of relays is larger than 4, the throughput obtained from the FCC scheme decreases as the relay numbers are augmented. The BCC protocol, on the other hand, can still result in enhanced throughput performance as the number of relays is increased (in Fig. 5.4).
Similar to the reasons as mentioned in the previous paragraph, the FCC protocol in-troduces more overhead by sending excessive rRTS frames as the number of relays is increasing. A harmful effect will occur when the control overheads can not be compen-sated by the enhancement of throughput resulted from the cooperative communication.
In contrary, the BCC protocol will still be beneficial from the additional relays due to the limited CSI-acquiring period. Furthermore, similar trend can be obtained in both figures as the number of users is increased. The network throughput will be enhanced with the increasing of sources, however, the amount of improvement becomes smaller as the number of sources continues to grow.
Figs. 5.5 and 5.6 show the throughput improvement of the proposed protocols compared to the direct transmission under different number of relays and number of
1 2 3 4 5 6 7 8 9 10
Figure 5.4: Throughput performance versus the number of relays using BCC protocol with different number of sources (σDB = 30 dB).
users, respectively. the SNR value σDB is also chosen as 30 dB in both figures. The total number of users is set to be 30 in Fig. 5.5; while the number of relays is selected as 5 in Fig. 5.6. As shown in Fig. 5.5, it can be seen that FCC method can provide better performance than BCC method as the number of relays is equal to 2. The reason is that the FCC protocol collects full of channel information and makes the best decision on the selection of relays. However, the BCC scheme choose the feasible relay only based on the channel quality of the source-relay links, which may not result in the best relay considering both source-relay and relay-destination channels in the cooperative communication. Nevertheless, when the number of relays increases, the performance from FCC protocol degrades due to the excessive overhead caused by transmitting the channel information via the rRTS frames. The BCC protocol can outperform the FCC scheme in throughput performance owing to its efficient design of the relay contention process. Furthermore, both of the proposed protocols can still provide better performance than that from the direct transmission in spite of the number of relays. It is noticed that this figure can be utilized as a reference plot to determine
1 2 3 4 5 6 7 8 9 10
Figure 5.5: Throughput comparison versus the number of relays with the direct trans-mission and proposed protocols (σDB = 30 dB).
the number of relays required to be deployed in order to obtain the required throughput performance. The impact of the number of users on the system throughput performance is illustrated in Fig. 5.6. It is intuitive that the total network throughput will be increased as the augmentation of the users. However, owing to potential frame collision, the resulting throughput performance may reach its saturation point or even decrease as the number of users is increased. Nevertheless, the proposed BCC scheme can provide much better throughput performance compared to the FCC protocol and the conventional direct communication.
Fig. 5.7 shows the average number of retransmissions by adopting the direct trans-mission and the proposed protocols under different channel conditions. It is noted that the number of relays are selected as 5 in both FCC and BCC protocols. It can be discov-ered that both proposed protocols can effectively reduce the number of retransmissions especially under the poor channel condition, e.g. around 1.3 less retransmissions under σDB = 30 dB. Both FCC and BCC algorithms result in similar number of retransmis-sions under different SNR values. On the other hand, although the acquisition of the
10 15 20 25 30 35 40 1
1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8
Number of Users
Throughput (Mbps)
Direct Transmission FCC
BCC
Figure 5.6: Throughput comparison versus the number of users with the direct trans-mission and proposed protocols (σDB = 30 dB).
full channel information can be beneficial to the number of retransmissions, the pro-longed transmission time resulting from the FCC protocol will degrade the throughput performance as shown in Fig. 5.8. This figure illustrates the dependency of throughput performance with the conventional direct transmission and the proposed protocols un-der different channel conditions. Both the FCC and BCC protocols can provide better performance than the direct transmission as the direct link is not good enough to trans-mit data successfully. It is also noted that the BCC protocol can further improve the throughput performance compared to the FCC protocol due to its limited transmission time for relay contention. As the channel condition improves, the transmission overhead introduced by the proposed protocols will result in decreased throughput performance.
Nevertheless, the proposed cooperative MAC protocols will still be advantageous espe-cially under the environments with comparably poor channel conditions.
28 30 32 34 36 38 40
Figure 5.7: The average number of retransmissions versus the average SNR value of boundary node σDB via the direct transmission and proposed protocols.
28 30 32 34 36 38 40
Figure 5.8: Throughput performance versus the average SNR value of boundary node σDB via the direct transmission and proposed protocols.
Chapter 6 Conclusion
This thesis presents the performance analysis and protocol designs of the cooperative communication from the medium access control (MAC) perspectives. An analytical model which consists of both the conventional direct communication and the cooper-ative mechanism is proposed to evaluate the suitability for adopting the coopercooper-ative scheme. In order to enhance the network throughput, it is suggested in this thesis that not only the cooperative diversity (with the resulting frame error rate) but also the transmission delay should be considered in the design of cooperative communications.
Moreover, the full-CSI based cooperative (FCC) MAC protocol and bitwise competition based cooperative (BCC) MAC protocol are proposed to adaptively choose appropriate relay for packet forwarding against the variation of channel conditions. Although FCC protocol can acquire full channel information, the overhead introduced by the exchange of control frames can result in degraded throughput performance. On the other hand, the BCC protocol adopts the relay contention process to limit the period for acquiring channel information, which effectively reduces the communication overhead. Simula-tion results show that the proposed BCC protocol can effectively provide enhanced throughput performance compared to the direct communication.
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