In WCDMA capacity and coverage in the downlink are highly interdependent, because multiple users share the same power resource in the downlink. Uplink power limits coverage, while too little downlink power may curb capacity.
As a result, the power level required from the RAN varies with the type of services required by the users, the number of simultaneous users, as well as their exact location inside the cells. High-bit-rate services require greater power levels from the radio base station.
In WCDMA, the total cell capacity is affected by the decline in signal strength over distance to each user. When there is significant decline, compensation is needed from a comparably larger proportion of the downlink resource. This leaves less of the downlink resource for other users.
Moreover, indoor users are, by definition, always close to the cell border, even if a building happens to be close to a base station. A high proportion of the high-bit-rate data users are probably located inside buildings, argues Ekstrand. "In an ideal, theoretical setting, power doesn't matter but in the real world, it does," he says. "In a typical network, between 80 and 100 percent are indoors, at least in cities."
With the imminent introduction of the next phase of WCDMA, High Speed Downlink Packet Access (HSDPA), the capacity gains from using high-power base stations will be even more significant to a successful implementation. Ericsson is currently the only vendor offering 30W base stations in addition to the standard option of 20W.
Besides higher output power, Ericsson's RAN solutions include unique software features that ensure as many users get access to as much bandwidth as possible at the same time, in order to preclude the proportion of dropped calls or interrupted services. One of these is Channel Rate Switching, a software feature that automatically lets the network continuously increase or decrease the bit rate, depending on radio requirements from the service demanded as well as on the number of simultaneous users.
In addition, the trade-off between coverage and capacity in WCDMA systems requires an admission control functionality to avoid system overload and to provide the planned coverage. Ekstrand explains how Ericsson's features help the operator maximize network usage: "When a new subscriber seeks access to the network, admission control estimates the network load and based on the new, expected load, the subscriber is either admitted or blocked out."
But as users move from one area to another, admission control must be complemented by congestion control, which may entail a handover rejection if there is insufficient power in the adjacent cell. Channel-rate switching will enable a smoother and more effective handover by adjusting the bit rate to the actual power resources instead of maintaining the same rate, which may lead to an overall lower capacity in the network.
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