Before describing the details of proposed protocols in Chapter 4, preliminary analyt-ical results will be observed and validated via simulations in this section. From the throughput perspective, the feasible situations to adopt either the cooperative or the conventional direction communication will be discussed. The saturation throughput S(Rcg, Pf (dir), Pf (coop)) as defined in (3.10) can be obtained according to the average FER values computed via respective direct (i.e. from (3.3)) and cooperative links (i.e.
from (3.4)). In order to validate the analytical model, the network scenario adopted in the simulations includes 30 user nodes with a fixed relay node. Table I illustrates the relevant parameters that are utilized in the analysis and simulations. Notice that the parameters α, g, and γt in Table I can be obtained based on the least-squire fitting method from [31] while the QPSK modulation is adopted. The other parameters are acquired from the IEEE 802.11a standard.
30 32 34 36 38 40
Figure 3.1: Throughput Performance versus the channel quality of direct link σSD under various values of Rcg.
Table I : System Parameters
Approximation Parameter (α) 1.07439 Approximation Parameter (g) 0.000112484 SNR Threshold (γt) 28.0477 dB Number of Nodes (N ) 30 Minimum Window Size (W ) 32 Maximum Backoff Stage (m) 5 Maximum Retry Limit (r) 2
MAC Header 224 bits
Fig. 3.1 shows the throughput performance and validation under different values of average SNR σSDfrom the source-destination link and the ratio Rcg. It is noted that the
saturation throughput S is obtained from (3.10) under pre-defined channel conditions of the source-relay and relay-destination links, i.e. σSD = σSD = 40 dB. Within the total of 30 network nodes, the number of nodes in the cooperative group are selected as 0, 15, and 30 which result in Rcg = 0, 0.5, and 1. In other words, there are Rcg ratio of nodes in the network conducting their packet transmission based on cooperative manner. As shown in Fig. 3.1. there exists a crossing point around 36 dB of σSD that illustrates the decision point regarding the feasible situation to activate the cooperative communication. With a larger number of nodes within the cooperative group (e.g.
the curve with Rcg = 1), degraded throughput performance is observed as the average SNR of the source-destination link σSD is larger than 36 dB. In other words, direct transmission should be adopted under comparably better channel conditions between the source and destination since the exploration of cooperative communication will result in prolonged transmission time, which causes degraded effect on the throughput performance. Nevertheless, with a worse channel condition for direct link (i.e. below 36 dB in this case), the usage of cooperative communication will significantly improve the resulting throughput performance.
In addition, since coding schemes are not exploited in the derived analytical model, the average SNR σSD shown in Fig. 3.1 will be in general overestimated. In other words, the require SNR σSD for achieving the same throughput will be reduced while a specific coding strategy is adopted. Therefore, similar trend as in Fig. 3.1 can also be derived with the exploitation of a specific coding scheme. Furthermore, it can be observed from Fig. 3.1 that the results obtained from both simulations and analytical model coincide with each other under different SNR values of σSD. Noted that the slight discrepancy at higher σSD values is mainly contributed by the usage of approximated FER calculation presented in Section 3.1. Since the exponential function (as in (3.1)) results in faster decay in FER than that in realistic cases as the SNR values
0 0.5 1
Figure 3.2: Throughput performance versus different values of the ratio Rcg (left plot:
worse direct channel quality σSD = 35 dB; right plot: better direct channel quality σSD= 40 dB).
are increased, the throughput acquired from analytical model will possess slightly larger value than that from simulations under higher values of σSD as in Fig. 3.1. However, this negligible modeling difference does not deteriorate the advantage of exploiting the exponential FER approximation due to its simplicity and efficiency.
A closer examination of the dependency between the ratio Rcg and the through-put performance is provided in Fig. 3.2. It illustrates the saturation throughthrough-put achieved by the combined direct/cooperative communication system, which includes several nodes conducting direct transmission while others transmit their packets via cooperative communication. The left plot shows the case with worse direct channel quality (i.e. σSD = 35 dB); while better channel condition (i.e. σSD = 40 dB) is il-lustrated in the right plot. As shown in the left plot in Fig. 3.2, it can be observed that more nodes in the cooperative group, i.e. with larger Rcg value, will increase the throughput performance under worse direct channel conditions. Conversely, the throughput performance will be significantly degraded as the ratio Rcg is increased when the quality of source-destination channel improves as can be seen from the right
plot of Fig. 3.2. Specifically, as the channel quality of direct link is good enough, trans-missions from the source directly to the destination is considered a better choice since the decreased FER resulted from the cooperative communication may not be signifi-cant. On the other hand, the prolonged transmission time induced by the cooperative communication can cause negative effect on the throughput performance. Therefore, whether a node should join the cooperative group depends on the channel qualities of the direct and cooperative links.
It is also noted that more throughput improvement can be achieved with better source-relay link compared to the relay-destination link. As shown in the left plot in Fig. 3.2, the combination of σSR = 45 and σRD = 40 dB results in higher throughput performance comparing with the case with σSR = 40 and σRD = 45 dB. This results can be explained by the adoption of decode-and-forward scheme within cooperative communication. The source-relay link should provide good enough channel quality such that the relay can correctly decode the corresponding frame. Otherwise, full diversity gain will not be achieved with the exploration of cooperative communication. Therefore, the source-relay channel plays a more important role than the relay-destination channel for throughput enhancement, especially under poor channel quality of the direct link.
In other words, as the source is suffering from severe fading channel and noises to the destination, a better source-relay channel is considered more important compared to the relay-destination channel in order to allow the destination to acquire another copy of data frames. This results will further be explored in the design of proposed BCC MAC protocol in order to provide efficient channel acquisition process, which will be explained in Chapter 4.
Fig. 3.3 shows the occasions for the cooperative mechanism to have a better per-formance than the direct communication under different SNR values. With pre-defined average SNR values of the source-destination and the source-relay channels, the
theo-25 30 35 40 45 50 20
25 30 35 40 45
σSR (dB) Required σRD (dB)
σSD=28dB σSD=30dB σSD=32dB
Figure 3.3: Required average SNR σRD via cooperative communication for achieving the same throughput as that with direct transmission under specific σSD and σSR values.
retically required average SNR of the relay-destination channel is obtained through the cooperative communication in order to have the same throughput as that via the direct transmission, i.e. Sdir = Scoop where Sdir and Scoop are acquired from (3.17) and (3.18) respectively. For each specific σSD and σSR, each point on the curves represents the value of σRD that satisfies the following condition:
sup {σRD : Scoop(σSD, σSR, σRD) ≥ Sdir(σSD)} (3.19)
For example, as σSD = 30 dB and σSR = 40 dB, the cooperative scheme with σRD > 30 dB can outperform the conventional direct communication in network throughput. Each curve in Fig. 3.3 can also be explained as the case while Scoop = Sdir for a specific average SNR of the direct link. The region above the curve represents the situations of Scoop > Sdir. Moreover, it is especially noticed that Fig. 3.3 can be utilized as a reference plot to determine the suitability for adopting the cooperative schemes as opposed to the direct communication, which will further be explored in the design of cooperative MAC protocols in the next chapter.