MULTIHOP STUDY BASED ON RAY-TRACING

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.

SIMULATION SCENARIO

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.

CALCULATION OF MEAN PATH LOSS AND SHADOWING

With one measurement approximately every 2 cm at a frequency of 3.5 GHz, the fast fading is captured. To extract the slow fading for calculation of mean path loss and shadowing, the fast fading was averaged through a sliding window of 40 wavelengths with successive windows overlapping by 30 wavelengths. An example of this process is shown in Figure 1 for the BS–MS link of route 6.

 

Figure 1: Example of BS–MS fast- and slow-fading along route 6, with RS at 5 m.

The shadowing on a wireless link is the signal fluctuations around the local mean level. For the purposes of this part, the local mean level will be obtained by fitting a least-squares regression curve to the data at each location and taking this as the mean path loss around which the shadowing fluctuations occur. For the BS–MS link, the data comprising all of the runs together will be treated as a single variable, but for the RS–MS link, the data collected at each different RS height will be treated separately. An example of the resulting shadowing distribution, on normal probability axes, so that a normal distribution with the same mean and variance as the actual data would be exactly a straight line. This is evidently a normally distributed variable, and calculation shows that it has zero mean and standard deviation of 3.5 dB. Similar distributions were found for the other routes and for the RS–MS link.

HIGHLY EFFICIENT MULTIHOP RELAY TOPOLOGIES

The big challenge for broadband wireless system design comes up with the right balance between capacity and coverage that offers good quality and reliability at a reasonable cost. It is important to look at system spectral efficiency more broadly to include the notion of coverage area. Results presented in previous sections have demonstrated the high potential benefits for relay deployment with radio resource sharing, in terms of interference, MIMO combination, and multiuser transmission. To implement the radio resource reuse and achieve highly efficient relay deployments, appropriate frequency reuse and multiuser access strategies are required. Relay systems must be based on a topology that fully exploits effective resource assignment based on the spatial separation of nodes. In this section, we propose directional distributed relay for highly efficient multiuser transmission with reduced demands on radio resource.

Figure 1 depicts the directional distributed relaying architecture. This is based on a paired radio resource transmission scheme, and it is possible to achieve one radio resource to one user (or one group of users) in average, even with multihop relay. The radio resource can be defined as either frequency (e.g., subcarriers in an OFDMA symbol) or time (e.g., OFDMA time slots). Transmissions in the BS coverage are the same as the IEEE 802.16e standard. For relay links, paired transmissions are applied, where the BS forms two directional beams, or uses two sector antennas to communicate with RS1 and RS2 simultaneously. A paired radio resources are required: f₁ and f₂. The first radio resource (f₁) is applied to the RS–BS1 link and also to the RS2–MS links (in the RS2 coverage); while the second resource (f₂) is applied to the BS–RS2 link and also to the RS1–MS links (in the RS1 coverage). Radio resources are shared between the RSs and MSs. Each end-user employs a single pair of radio resources, on average.

 

Figure 1: Directional distributed relaying with paired radio resource.

Using the sharing scheme outlined above the interference can be controlled at the BS and RS nodes. In this relay configuration there are only two sets of interference, as also illustrated in Figure 1. The interference between the BS and MS groups (I₁ and I₂) can be detected and controlled by the BS. First, the BS could employ an adaptive array to exploit the spatial separation of the groups. Second, since the received power by each MS in each MS group is known to the BS, the BS can apply interference avoidance  between the two groups based on measured signal to interference plus noise ratio (SINR) and power control, where the transmit power of the two RSs are controlled for balancing the SINR according to the service requirement. Furthermore, in this scenario the expected level of interference is small because the BS connects to the MSs through a relay, which means the relay SNR-gain will be much higher than the SNR_access level. Interference between RSs (I₃ and I₄) can be reduced by array processing (including the use of sector antennas) at the RSs. Interference measurement for the efficient resource assignment can be achieved during the neighborhood discovery procedure. To achieve high levels of SINR (e.g., 10–25 dB), array processing, including the use of sector antennas at the RS, is desirable.

This proposed topology is fully compatible with the existing 802.16e standard and no modifications are required at MSs. Alternative deployments topologies are also possible based on the same concept, such as a single RS to cover a coverage hole. In such cases, the radio resource sharing is performed between the RS and its BS. It could be complicated for statistical studies as the performance is fully dependent on the deployment scenario. However, it is much more feasible in a realistic application environment by employing real channel measurements and ray tracers.