Saturday, June 11, 2011


As mentioned before, a major issue for radio resource sharing is the interference. Greater efficiencies in spectrum use can be achieved by coupling channel-quality information in the resource-allocation process. Ray-tracing is an effective tool for network setup, evaluation, and optimization. To demonstrate the potential of the proposed resource sharing framework, a site specific ray-tracing propagation model is used in this section to provide realistic environment specific propagation data for the BS–MS, BS–RS, and RS–MS links.
The ray-tracing tool used in this work was verified with measurement data, and has been used in many previous WLAN and WiMAX system evaluations. The ray-tracing model takes individual buildings, trees, and terrain contours into account and determines specific multipaths based on scattering and diffraction. Mobile transitions from LoS to NLoS are naturally handled by the algorithms. In this study, we can easily utilize the ray tracer to establish the typical multihop relay application scenarios, such as coverage hole and cell edge. Also, it can directly demonstrate interference paths and strengths which is more suitable for studies presented in previous sections.


Because mobile terminals are allowed to move freely at street level in the MWA scenario, local surrounding large and small obstructions and terrain contours result in path loss, shadowing, and multipath fading. Interferences from other signals also distort the transmitted signal in an unpredictable and time-varying fashion.
A realistic MWA scenario is now analyzed based on a region of Central Bristol, United Kingdom (Figure 10.16a). A single BS location was chosen on the roof top of a tall central building (30 m above ground level). Eighty-five MS units were distributed over this geographic area at street level with heights of 1.5 m. The BS with 3-sectors was assigned an EIRP of 57.3 dBm (based on a 15 dBi 1200 sector BS antenna). First, the raw multipath components (MPCs) are created using the above ray-tracing tool. Based on isotropic ray-traced channel data, the ETSI specific antenna beam patterns are then incorporated via spatial convolution. Figure 1 presents the distribution of received power from the BS to the surrounding region at street level. A severe coverage hole is clearly visible, and this is mainly caused by variations in the terrain height. Two methods could be used to achieve acceptable WiMAX coverage in this macrocell. First, a second BS could be deployed in the coverage hole. However, this would add to the infrastructure costs and hence it may be more effective to deploy an RS node.

Figure 1: Simulated macrocell in Bristol, United Kingdom.
Figure 2 shows the locations of MSs and their affiliations to either the BS or RS. The locations of BS, RS and MSs are overlaid on a terrain map of Bristol city-centre. For each MS a line is drawn to indicate whether communication occurs via the BS or RS. The choice of connection type depends on the received SINR. At a given location, signal power is obtained by sum of all received MPCs, which are transmitted by an assigned BS sector. Interferences are caused by MPCs which comes from either RS (for BS access) or BS (for RS access) if interfering users occupy same frequency resource as the detect user. It should be noted that in NLoS conditions the MS may connect to an adjacent sector since this is determined by the direction of the strongest path.

Figure 2: BS and RS radio resource location.
Figure 2 also demonstrates subchannel allocation for a 3-users OFDMA operating with a 5 MHz channel bandwidth, based on 512-FFT DL PUSC (Partial Usage of Sub-Channels) OFDMA TDD profile as specified in 802.16e. For the 512-FFT DL PUSC OFDMA, a total of 15 subchannels are mapped (after renumbering and permuting) in one OFDMA symbol. Each sector has access to one-third of the total number of subcarriers. There are 3 groups (one per sector), with 5 subchannels in each group. Each subchannel comprises 2 clusters (14 physical subcarriers in each cluster). This results in a total of 420 subcarriers in each OFDMA symbol (360 data bearing carriers and 60 pilot carriers). When the RS is used to connect to an MS, a certain number of timeslots (or alternatively subchannels) must be assigned in the covering sector to support the BS–RS link. Here we assume that BS–RS link and BS–MS link (taken from the sector covering the RS) are in group #3. When radio resource sharing is applied, a number of RS–MS and BS–MS links are supported simultaneously using the same resources. For all the RS–MS links, and also the BS–MS links where the antenna beam is steered away from the RS, it is possible to share group #1. Group #2 is used for those MSs that connect to one of the BS sectors (i.e., but not the sector coving the RS); these can be located near to the RS if required.
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