Link budget planning is part of the network planning process, which helps to dimension the required coverage, capacity and quality of service requirement in the network. UMTS WCDMA macro cell coverage is uplink limited, because mobiles power level is limited to (voice terminal 125mW). Downlink direction limits the available capacity of the cell, as BTS transmission power (typically 20-40W) has to be divided to all users. In a network environment both coverage and capacity are interlinked by interference. So by improving one side of the equation would decrease the other side. System is loosely balanced by design. The object of the link budget design is to calculate maximum cell size under given criteria:
Type of service (data type and speed)
Type of environment (terrain, building penetration)
Behavior and type of mobile (speed, max power level)
System configuration (BTS antennas, BTS power, cable losses, handover gain)
Required coverage probability
Financial and economical factors (use of more expensive and better quality equipment or not the cheapest installation method)
and to match all of those to the required system coverage, capacity and quality needs with each area and service.
In an urban area, capacity will be the limiting factor, so inner city cells will be dimensioned by required Erlangs/km² for voice and data. Even using 25dB as inbuilding penetration loss into the building core area, link budget would typically allow about 300m cell range, which is a way too much for a capacity purposes. In a rural area uplink power budget will determine the maximum cell range, when typically cells are less congested. A typical cell range in rural areas will be several kilometers depending on a terrain.
Below is an example of how WCDMA voice call link budget can be done. Some of the values can be debated, including the propagation model, but it gives an idea of the calculation methods.
发表于 2005-4-16 22:27:00
