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Downlink RSSI Design Goal
5.1 Downlink RSSI Design Goal
Wireless service providers typically provide a minimum downlink signal level and an
associated confidence factor when specifying coverage requirements. These two fig-
ures of merit are a function of wireless handset sensitivity and margins for fading and
body loss. Wireless handset sensitivity is the weakest signal that the handset can pro-
cess reliably and is a combination of the thermal noise in the channel, noise figure of
the handset receiver front end and minimum required SNR. Fade margins for multi-
path fading (fast or small-scale) and log-normal shadow fading (slow or large-scale)
are determined by the desired confidence factor, and other factors. Downlink RSSI
design goal calculations for the TDMA protocol are shown below for a 95% area cov-
erage confidence factor.
Downlink design goals on the order of –85 dBm are typical for protocols, such as
GSM and iDEN. Wireless service providers may choose a higher level to ensure that
in-building signal dominates any macro signal that may be leaking into the building.
Noise Power
10 Log (KT)+10 Log (30 KHz); K=1.38X10
–23
, T=300 degrees Kelvin
–129 dBm
Wireless Handset Noise Figure 7 dB
Required SNR 17 dB
Multipath Fade Margin
95% Reliability for Rician K=7 dB
6 dB
Log-normal Fade Margin
95% Area/88% Edge Reliability for 35 dB PLS and 9.5 dB Sigma
11 dB
Body Attenuation + 3 dB
Downlink RSSI Design Goal (P
DesignGoal
)
Signal level received by wireless handset at edge of coverage area
–85 dBm