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.
RF Link Budget Gains and Losses
When you plan an LTE network, you always calculate the RF link budget to understand how much power gets from the transmitter (eNodeB) to the receiver (UE). I’ll show you how to think about it simply: you start with the transmit power, then subtract all the losses, and add any gains, to see if the signal can reach the receiver with enough strength.
Gains: These are helpful boosts that push your signal further. For example, if you’re using a high-gain antenna at the eNodeB, that focuses energy and adds gain. Sometimes you also get gain from diversity techniques or tower height.
Losses: This is where your signal gets weaker. The main one is path loss – the farther the UE is, the more the signal spreads out and loses strength. Then you’ve got building penetration loss, especially if your user is indoors. You also lose some power in cables and connectors, and even atmospheric conditions can affect it.
Why it matters: You calculate the link budget to make sure that the received signal is strong enough for the UE to decode. If it’s too weak, you’ll get poor performance or no connection. If you know the losses, you can adjust things like antenna power or placement to improve coverage.
So in short, I think of link budget as a simple power balance — transmit power minus all the losses plus all the gains. If what’s left meets the receiver sensitivity, the link works. If not, we fix it by changing power, antennas, or placement.