The Direct Conversion Receiver (DCR)

We have demonstrated that the DCR is a suitable receiver configuration for singlechannel
operation in the handset. It is similarly suitable for single-channel operation in
the Node B.
How will it perform in the multichannel environment?

Consider a wideband approach to receive four simultaneous channels. The receiver
front end would still have a bandwidth of 60 MHz—to be able to operate across all 12
W-CDMA channels. The tuning front end would require a local oscillator (LO) having
three discrete frequencies in order to downconvert the band in three blocks of four
channels each. In the multicarrier receiver, the LO would be placed in the center of the
four channels to be downconverted (received), as shown in Figure 11.7.
The output of the I and Q mixers would be the four channel blocks centered around
0 Hz each time the LO was stepped, as shown in Figure 11.8. Atypical IC mixer having
a Gilbert cell configuration can only achieve at best an IQ balance of some 25 to 30 dB.
This means that if the channel 2 to channel 3 amplitude difference is greater than 30 dB,
signal energy from channel 3 will transfer into, and hence corrupt, channel 2. Similarly,
there will be an interaction between channels 1 and 4.
If we consider the problem of IQ imbalance, we find there are several causes. Typical
causes are those of IC manufacturing and process tolerance, variation with supply
voltage to the mixers, temperature variation, and other similar effects. This group of
causes are predictable, constant effects that can be characterized at the production test
stage and compensating factors inserted into the receive processing software.
Compensation can be affected in the digital processing stages by a process of vector
rotation, feeding some Q signal into I, or I into Q as required to balance the system. The
greater problem is that of IQ imbalance due to dynamic signal variation effects. These
are unbalancing effects that cause the operating point of the mixers to shift with signal
strength and radiated signal reflection and reentry effects. These effects have been
described in Chapter 2 in our discussion of DC offset problems.

Although the correction by vector rotation is a relatively simple digital process, the
difficulty lies in estimating the instantaneous degree of compensation required given
all the variables causing signal amplitude variation. For this reason, the DCR (and
near-zero IF) is not chosen for Node B designs. However, the designer should always
review the current capability of this technology at the start of any new design, as
research is certainly being undertaken to increase the application of the DCR. 249