One result of the dimensioning of the access network is the required capacity by the backhaul network. A typical backhaul network consists of point-to-point or point-to-multipoint radio links or even fiber rings in recent networks. In most cases the information rate that is originated to and from the backhaul network should be calculated to evaluate an existing infrastructure or consider the cost to deploy a backhaul network along with WiMAX.
The total committed information rate of a WiMAX PoP is linear to the number of sectors and average sector throughput, which mainly depends on channel bandwidth and deployment scenario. Considering trisector cells, and average throughput of 9 Mbps (5 MHz channel bandwidth), the total TDD committed rate is 27 Mbps. To estimate the FDD equivalent, which is directed to and from the backhaul network, the total rate is multiplied by the DL/(DL + UL) ratio. Hence the traffic that should be reserved in the backhaul network for a PoP with 2/3 DL WiMAX traffic is 18 Mbps. For a dual layer cell or 10 MHz channel bandwidth the backhaul rate per PoP would be doubled, around 36 Mbps. It is acceptable to include a 5 percent margin as in practice it is common to observe small deviations in sector throughput. During the deployment and the operation the exact backhaul traffic can be monitored through the management system and design adjustments can be made accordingly.
In certain occasions the network designers are requested to propose a backhaul system along with the WiMAX network. Although this is not in the scope of this chapter, however there are some interesting observations that can be highlighted. Clearly the multipoint systems are more cost effective than point-to-point radio systems when the WiMAX PoP number is high, the PoP are quite close to each other and when their backhaul rate is such that more than 3–4 WiMAX PoP per multipoint sector can be served (around 10–12 per site). This indicates that the WiMAX site backhaul traffic should be from 15 to 25 Mbps, which mainly refers to trisector cells with 5 MHz channel. For higher bandwidth systems, such as dual layer cells or with 10 MHz bandwidth per site the point-to-point solution may offer the higher required capacity. Again in a dense urban environment where sites are usually deployed with separation of 0.3–0.7 km the use of fiber (if available) could be a better solution. Concluding, the best backhaul system strategy can be determined by evaluating the number of WiMAX PoP, their backhaul rate, and finally the PoP positioning. Note that although the WiMAX network dimensioning is done in such way to optimize the access network, this does not suggest that the backhaul network is also optimized and a separate study may be necessary.
