PHY LAYER SPECIFICATIONS IN THE STANDARD

Operations in high frequencies ranging between 10 and 66 GHz were initially specified in the earlier versions of the 802.16 standard for fixed access. With this specification, only line-of-sight (LOS) signal propagation, with unobstructed path from the transmitter to the receiver, is feasible. Though high-frequency operations have the advantage of less interference, however most wireless technologies prefer lower frequencies because RF signals penetrate structures much better at low frequencies, enabling non-LOS propagation techniques. In non-LOS or multipath propagation modes, the transmitted signals are scattered, reflected, and diffracted by objects in the propagation paths between the transmitter and the receiver as shown in Figure 1. Thus, the receiver receives multiple copies of the transmitted signal, each arriving with different amplitude and phase or delay. These multipath signals may combine destructively at the receiver resulting in severe signal fades. To accommodate services in non-LOS conditions in the WiMAX system, 802.16-2004 standard subsequently specifies operations at lower frequencies, between 2 and 11GHz. Single-carrier transmission, known as wirelessMAN-SC, as well as two multicarrier transmissions, wirelessMAN-OFDM (orthogonal frequency division multiplexing) and wirelessMAN-OFDMA (orthogonal frequency division multiple access) are also specified. The WiMAX system also specified a number of advanced PHY layer and antenna technologies, both fixed and adaptive, to combat the severe fading effect of the multipath propagation channel, to enhance system performance.

 
Figure 1: Non-LOS propagation and intersymbol interference (ISI).

EQUIPMENT SPECIFICATIONS

From a WiMAX access network design perspective, the most important parameters are related to the PHY and MAC characteristics of the air-interface. Typically all parameters that affect the link gain budget, such as transmit power, antenna gains, receiver sensitivities, advanced antenna systems are of great importance. Consider an example of a WiMAX access network that is intended to serve stationary indoor terminals in a typical suburban environment (i.e., SUI-C channel), hence a coverage-limited scenario. Comparing two systems with MIMO 2x and BF 8x configuration,the system range is around 2.2 and 3.7 km, respectively. This result is for the same amplifier output where the 6 dB system gain is due to the difference in antenna elements. Note that the diversity or BF gains are not considered for the range estimations since they are only applied in user traffic and not in the signaling part of the frame that usually limits the range. Applying Equation 13.5, the footprint is estimated as 12.5 and 35.5 km², respectively. Results indicate that for coverage limited scenarios the higher system gain of a BF configuration can significantly reduce the network size. On the other hand, for mobility scenarios the use of MIMO is a more suitable approach. In general WiMAX has several air-interface profiles that may be best suited according to the deployment scenario.

In addition to the link gain budget, the sector capacity is equally significant for capacity-limited scenarios. A comparison of the spectral efficiency per modulation and the required SINR thresholds could indicate that some products may operate with higher efficiency than others. Capacity is further increased by advanced antenna systems, where spatial multiplexing could even double the spectral efficiency, while BF could reduce interference levels and upgrade the PHY mode.

A list of important parameters, with impact on coverage, capacity, and QoS are shown in Figure 1. These parameters should be provided for all combinations of operating frequency band-channel bandwidth, both for the DL and UL and for different terminal profiles. A vital parameter for dimensioning is the number of subscribers that can be supported in a sector due to availability of service flows. This number depends on the number of service types per user. Additionally, a description of the capabilities and performance of the radio resource management (scheduler) would be beneficial to the designers. It is common during dimensioning to assume that the system can preserve QoS but in many occasions a safety margin (i.e., 5 percent) is required. The scheduler’s performance may downgrade close to full capacity load or for high number of subscribers per sector. Considering BE traffic, the impact is not as significant, however, this is not the case for VoIP and other delay-sensitive services. When handover is involved, it becomes more challenging to preserve QoS. It is evident from this section that WiMAX networks designers should have a thorough under-standing of technology so as to foresee potential issues during network dimensioning and hence consider appropriate margins. A good insight into WiMAX air-interface performance issues.

 

Figure 1: Important WiMAX air-interface parameters.