Link budget design in wireless communication
Answers
. Requirements of wireless networks
i. A wireless network requires planning for its setup, functioning and operation.
ii. The wireless network planning process can be divided into three phases: the initial planning (dimensioning) phase, the detailed radio network planning, and the operation and optimization phase.
iii. The dimensioning phase is used to estimate the approximate number of cell sites required, base station configurations, and number of network elements in order to determine the projected costs and associated investments.
iv. In the dimensioning phase, concentration is to provide near ubiquitous coverage. When the coverage demands are met, there is focus on traffic demands.
v. Network plans change throughout the life of a wireless network. The initial focus is on coverage and expansion. For coverage, it is essential to know the link budget for both the uplink and the downlink.
vi. Link budget calculates all the power gains and power losses on both the links. This calculation helps to estimate the link capacity and hence the coverage.
vii. The detailed planning phase takes into account the real site locations, real propagation conditions, and real user distributions based on the operator’s traffic forecasts.
viii. The network operation and optimization phase deals with individual cell and system testing.
ix. Each phase requires additional support functions such as propagation measurements, key performance indicator (KPI) definitions, and so on.
x. The planning involves study of the following:
Network topology: base-station transceiver-base station controller-mobile switching center (BTS-BSC-MSC)
Link capacity: BTS-BSC, BSC-MSC
BSC sizing
MSC sizing
The number of users
Users’ behavior, i.e., heavy or light users
Busy-hour traffic as a percentage of the total traffic
Users’ distribution over the service area
Division of the service area into zones of different traffic density
Increase in number of users
xi. Thus, setting up a wireless network requires a study of the extent of radio coverage, the quality of service for different environments, efficient use of spectrum, and the evolution of the network.
2. Link budget analysis
i. For a cellular network to provide good quality of service (QoS) in indoor and outdoor environments, flexibility and resilience should be incorporated into the design of radio link.
ii. The link budget analysis should be such that there is little restriction on making or receiving calls within a building or travelling in a vehicle using handsets.
iii. In normal system designs, it is assumed that the gain of a mobile antenna is 0 dBi. In reality, mobile antenna gain could be as low as -6 to -8 dBi when in building or in a moving vehicle.
iv. In the radio coverage planning process, the selection of the propagation model is one of the most important considerations. The accuracy of the prediction by a particular model depends on its ability to account for the details of terrain, vegetation, and buildings.
v. Another critical factor that affects radio coverage is penetration loss for both buildings and vehicles. For buildings, penetration loss of 10 to 12 dB can be assumed. However, if calls are originated as well as received in the inner core of the building, this loss can be as high as 16 to 20 dB. For vehicles like cars, penetration loss of 3 to 6 dB can be assumed. However, in vans and buses, it can be as high as 10 to 12 dB.
vi. To ensure that the signal level in 90% of the cell area is equal to or above the specified threshold, a shadow margin, which is dependent on the standard deviation of the signal level, must be included in the link budget calculations.
vii. Bit Error Rate (BER) should be calculated based on fading. Fade margin is determined from the combined statistics of both large-scale and small-scale signal variations based on the lognormal and Rayleigh distribution, respectively.