Received Signal Strength

The received signal strength for both mobile handsets and fixed base stations in a cellular
network is widely varying and unpredictable, because of the variable nature of
the propagation path. Since the handset and Node B receiver front end must be
extremely linear to prevent intermodulation occurring, the variation of signal strength
persists through the RF stages (filter, LNA, mixer) and into the IF section.
A customary approach to IF and baseband processing in the superhet receiver is to
sample and digitize the modulated IF. The IF + modulation must be sampled at a rate
and a quality that maintains the integrity of the highest-frequency component—that is,
IF + (modulation bandwidth/2). The digitization may be performed at a similar rate
(oversampling) or at a lesser rate calculated from the modulation bandwidth (bandwidth
or undersampling). Either digitization method must fulfill the Nyquist criteria
based on the chosen process (oversampling or bandwidth sampling).
From an analysis of the sampling method, modulation bandwidth, required CNR,
and analog-to-digital converter linearity, the necessary number of ADC bits (resolution)
can be calculated. The number of bits and linearity of the ADC will give the spurious
free dynamic range (SFDR) of the conversion process.
For TDMA handset requirements, the resolution will be in the order of 8 to 10 bits. For
IMT2000DS/MC, 4 to 6 bits may be acceptable. Typical received signal strength variation
can be in excess of 100 dB. This equates to a digital dynamic range of 16 or 18 bits. The implementation option is therefore to use a 16 to 18 bit ADC, or to reduce the signal
strength variation presented to the ADC to fit within the chosen number of ADC bits.
Atypical approach to signal dynamic range reduction is to alter the RF or IF gain, or
both, inversely to the signal strength prior to analog-to-digital conversion. The process
of gain control is referred to as AGC (automatic gain control) and uses variable-gain
amplifiers controlled by the RSSI.
RSSI response time must be fast enough to track the rate of change of mean signal
strength—to prevent momentary overload of subsequent circuits—but not so fast that
it tracks the modulation envelope variation, removing or reducing modulation depth.
The RSSI function may be performed by a detector working directly on the IF or by
baseband processing that can average or integrate the signal over a period of time.
Simple diode detectors have been previously used to measure received signal
strength. They suffer from limited dynamic range (20/25 dB), poor temperature stability,
and inaccuracy. The preferred method is to use a multistage, wide-range logarithmic
amplifier. The frequency response can be hundreds of MHz with a dynamic range
typically of 80/90 dB.
The variable amplifiers require adequate frequency response, sufficient dynamic
range control (typically 60 dB+), and low distortion. Additionally, the speed of
response must track rate of mean signal level change. It is a bonus if they can directly
drive the ADC input, that is, with a minimum of external buffering.
The concept of dynamic range control has a limited application in base station
receivers, as weak and strong signals may be required simultaneously. If the gain was
reduced by a strong signal, a weak signal may be depressed below the detection
threshold. A wider dynamic range ADC must be used. 88