Praise for Fundamentals of WiMAX

12.1 Wireless Channel Modeling 403or 50 percent cell-edge outage probability, it can be shown that coverage probability over the entirecell is given by1 1Outage cellarea = -- 1 + exp ----2 ⎝⎛ ⎠⎞ 1 Q --⎝ ⎛ b⎠⎞ , where bb 2=10αloge-------------------- . (12.2)σ S 2The relationship is even more complex if S is not equal to 0dB and the cell-edge outage isless than 50 percent. For example, for α = 4 and σ S = 8dB, a 25 percent cell-edge outage probabilitywill translate to a 6 percentage area outage, or 94 percentage area coverage. Similarly ifα = 2 and σ S = 8dB, a 25 percentage cell-edge outage probability will translate to a 9 percentagearea outage, or 91 percentage area coverage. Typical cellular designs aim for a 90 percent to 99percent coverage probability, which often requires a shadow margin of 6dB–12dB. Determiningthe median pathloss and the shadow fading using these models is critical for network design andplanning, as it often directly dictates the BS density required to provide reliable signal quality tothe desired area of coverage.The third level of spatial scale at which a radio channel can be characterized is the variationin signal strength observed over a small scale. As discussed in Section 3.4, the phenomenon ofmultipath propagation means that the amplitude of the received radio signal can vary significantly(several tens of dBs) over very small distances on the order of wavelengths or inches. Agood understanding of multipath fading and its impact on system performance is required todesign a wireless network.As explained in Section 3.2, multipath channels are often modeled using tap-delay lineswith noninfinitesimal amplitude response over a span of ν taps:h( t,τ ) = h 0 ()δτ t ( – τ 0 ) + h 1 ()δτ t ( – τ 1 ) + … + h ν – 1 ()δτ t ( – τ ν – 1 ). (12.3)Here, t indicates the time variable and captures the time variability of the impulse response ofeach multipath component modeled typically as Rayleigh or Rician fading, and τ indicates thedelay associated with each multipath. Empirical multipath channels are often specified using thenumber of taps ν and the relative average power and delay associated with each tap. For purposesof modeling in a simulation environment, the most frequently used power-delay profiles are thosespecified by ITU. ITU has specified two multipath profiles, A and B, for vehicular, pedestrian,and indoor channels. Channel B has a much longer delay spread than channel A and is generallyaccepted as a good representative of urban macro-cellular environment. Channel A, on the otherhand, is accepted as a good representative of rural macrocellular environment. Channel A is alsorecommended for microcellular scenarios in which the cell radius is less than 500m. The specifiedvalues of delay and the relative power associated with each of these profiles are listed inTable 12.1. Most simulation results presented in this chapter are based on the pedestrian channelB (referred to as Ped B) model, since it is commonly accepted as representative of an environmentsuitable for broadband wireless communications. Some results, however, have been provided forother multipath channels such as pedestrian channel A (referred to as Ped A) to illustrate theimpact of multipath propagation on the system-level behavior of a WIMAX network.