Linearity and Modulation Quality

If higher levels of modulation are to be supported together with multiple codes per
user on the uplink, then significant attention must be paid to PA linearity, both in the
handset and the base station, if clipping is to be avoided. As mentioned, each coded
channel represents a phase argument. The phase argument can be compromised by a
loss of code-to-code orthogonality or AM to PM effects introduced by a PA in compression.
It is important also to keep the code power well contained.
On the downlink, the BTS specification specifies that 91.2 percent of the correlated
pilot power should be contained in the total transmission power. This means that 8.8
percent of the power produced potentially causes interference to other Walsh channels
and embarrassment to the handset receive process.
As with IMT2000DS, modulation quality equates directly to capacity and coverage
capability. Modulation quality in CDMA2000 is measured using RHO (equivalent to
EVM). Causes of poor RHO could include RF PA compression, amplitude and phase
errors in the IQ modulators, carrier feed-through, and spurious Tx signals. 104