The Digitally Sampled IF Superhet

In analyzing the handset receiver/transmitter options, we recognized that the prime
constraint on any decision was that of battery power requirement. To provide for the
handset to access any of the 5 MHz channels in the 60 MHz spectrum allocation, a
receiver front end tuning with a 12-step synthesizer is necessary to downconvert the
selected channel to be passed through a 5 MHz bandwidth IF centered filter to the sampling
ADC. The digitized single 5 MHz channel is then processed digitally to retrieve
the source-coded baseband signal. This single-channel approach is adopted in the
handset in order to comply with the low-power criteria.
If this single-channel approach were adopted in the Node B, where multiple RF
channels may simultaneously be required, the requisite number of receivers would
have to be installed. As the restriction of Node B power consumption is not as severe,
an alternative approach can be considered.
The ideal approach is to implement a wideband front end, to downconvert the 12 5
MHz-wide channels, to pass a number (or all) of the channels through a wideband IF
filter, and to sample and digitize this wider bandwidth of channels. The digitized channels
would then be passed to a powerful digital processing capability that could simultaneously
extract the downconverted baseband signals. The number of channels to be
simultaneously processed would again be dependent on the power available both in
implementing an RF front end of sufficient dynamic range and an ADC/DSP combination
of sufficient processing capability.
Additionally, a greater dynamic range is required by the ADC and DSP, since in the
multichannel environment, the channels may be at substantially different signal
strengths and so dynamic range control cannot be used. If the IF gain were to be
reduced by a strong signal channel, a weak signal channel would disappear into the
noise floor.