RF Link Budget Gains & Losses
The system designer will need to determine the specific RF link budget parameters to be used when designing the system. The following figure shows the typical gains and losses that are encountered in the RF path.
An RF link budget must be determined for each sector of each site. The RF link budget for each sector must incorporate all required parameters, (such as building penetrations, antenna heights, antenna gains, cable losses, coverage criteria, coverage reliability, etc.), some of which may have different values between sites or sectors.
It is common that all sectors of a given site have the same link budget or even that several sites have the same link budget due to common installation practices being followed. If this is the case, then the same link budget can be used for all of the similarly configured sectors. However, if the parameters change from sector to sector and site to site, then separate link budgets will need to be calculated for each unique sector.
LTE RF link budgets may make simplifying assumptions regarding the values for various parameters. For instance, the Rx Sensitivity may be considered a constant. In actuality, Rx Sensitivity is not a constant value but varies with respect to MCS level, fast fade margin and diversity gain. Some of the simplifying assumptions of the link budget are addressed in the second and third level of the design.
Other items that may be simplified via the link budget but addressed in more detail in later phases of the design are:
• Base Station and User Equipment types have different link budget or RF characteristics – Transmit powers, receive sensitivities, antenna configurations
• Indoor versus Outdoor Coverage – In-building penetration margin will impact maximum allowed pathloss
• Coverage Reliability – Higher reliability requires a greater margin which results in a smaller site coverage area
• Other environment variables affecting coverage/range – Clutter, body loss, fade margins, weather, antenna heights, TDD configuration, path loss model, etc.