Multi-hop Cellular OFDMA Systems
4.3 Performances Improvement by Multi-hop Relay
4.3.2 Simulation Results A. Simulation Environments
A downlink two-hop cellular network is simulated in Manhattan-like environment, where 14 RSs are deployed within the coverage of an MR-BS. Total of 19 cells with three sectors per cell are considered. The RS’s locations are as depicted in Figure 15, which follows the deployment method proposed in previous section. The MR-BS and RSs are deployed above rooftop, so that the relay links have line-of-sight (LOS) condition. The access links between MR-BS and MS are assumed to be non line-of-sight (NLOS). For the access links between RS and MS, LOS is assumed if they locate on the same street and the distance is less than 150m. The path-loss and shadow fading models are referenced from the multi-hop relay system evaluation
methodology in [41]. The total transmit power of the MR-BS is 45 Watts with 1 km cell coverage, and the RS/MS maximum transmit power are 5 Watts and 0.5 Watts respectively.
Two frequency reuse methods for the MR network are simulated to investigate the potential benefit of the proposed RS deployment method. The first method does not reuse the frequencies within the same MR-cell, where the orthogonal frequency channels are allocated to each relay link/access link in relay zone/access zone respectively. In this case, there is no intra-cell interference. The second method is to reuse the frequency channels within the same MR-cell, the channels for relay links are reused in each sector and the ones for access links are reused by every RSs and MR-BSs.
At the beginning of simulation, MSs are generated by Poisson process and randomly located on the street. During the simulation, the MS moves along the streets and communicates with an RS or the MR-BS based on the received signal quality. The modulation and coding scheme is adjusted on a frame-by-frame basis according to signal quality. For each frame, the number of bits successfully received in access links is recorded, and at the end of simulation, the cell throughput is calculated by dividing the number of successfully received bits in access links by the overall simulation time.
The more detailed simulation parameters are given in [42].
B. Simulation Results
In Figure 17(a), the CDF (Cumulated Distributed Function) of downlink received signal quality is presented for different relay methods and different frequency reuse factors. The simulation results show that the received signal quality can be
a MS has 23%, 81% and 74% probability to have a received Eb/(I0+N0) higher than 20 dB in the cases of no relay, conventional relay, and the proposed relay methods, respectively. It means that the received signal quality is improved by deploying RSs, which is mostly achieved by the lower propagation loss from serving station to MS.
Therefore, per user throughput can be significantly increased by using higher order MCS in response to such a high SINR. On the other hand, if we draw a horizontal line at CDF=10% and compare the curves with K =1. It shows that a mobile station has 10% probability to have signal quality lower than 4dB, 15dB and 11dB for the case with no relay, conventional resource allocation method and proposed resource allocation method, respectively. If the coverage planning criterion is to ensure mobile station has 90% probability to have signal quality better than 4dB, the conventional method and proposed method can lead to additional 11dB and 7 dB margin in link budget for coverage extension.
Figure 17. (a) CDF of downlink received signal quality and (b) the downlink cell capacity
The downlink cell capacity is presented in Figure 17 (b), which shows that the proposed resource allocation method and RS deployment method can lead to substantial capacity improvement than conventional method and the case with no relay. For two-hop case, a data bit is transmitted from BS to RS in 1st hop and RS to
nd
the results show that the conventional method has smaller cell capacity because all traffic in relay links shall be treated as overhead, and the improvement by higher transmission rate cannot break even with the lost on radio resources for relay links.
The conventional method has -4.81%, -27.63% and -33.92 cell capacity with respect to the case with no relay for reuse factors K =1,3 and 7, respectively. On the other hand, the proposed resource reuse method along with RS deployment method can achieve 116.4%, 68.33% and 56.03% capacity gain for reuse factors K =1,3 and 7, respectively. The cell capacity is improved because radio resources are reused in different relay links and access links within each cell and the proposed RS deployment method also utilize the severe shadow fading effect to isolate the interfering signals.
In Figure 18(a), the CDF of uplink transmission power for MS is presented.
Because the power control is considered in uplink transmission, therefore, the reduction on propagation loss by RS deployment will lead to lower transmit power.
For K = , MSs have the probabilities 36.43%, 74.23% and 63.10% to consume the 1 transmission power lower than 10dBm for the case of no relay, conventional method, and the proposed method, respectively. According to the better propagation condition and lower propagation loss, the uplink transmission power can be substantially reduced by RS deployment.
The uplink capacity is presented in Figure 18(b), which is defined as the effectively received bits at MSs within each cell in uplink access zone. Compared with the conventional method, it shows that 2.54%, 35.84% and 44.11% capacity gain can be achieved by the proposed resource reuse method for the cases of K =1, 3 and 7, respectively. On the other hand, degradation on system capacity is observed in the MR network due to additional resource used for multi-hop transmission for the case
Figure 18. (a) The CDF of uplink MS transmit power and (b) uplink cell capacity