How Long and Short Code Spreading in Reverse channel CDMA
Reverse Channel Long Code Spreading
The channelization in the reverse link must provide for unique code assignments for every operational phone.
- Long Code Spreading Provides Unique Mobile Channelization
- Mobiles are Uncorrelated but not Orthogonal with Each Other
Since the long code is 42 bits in length, this allows 2^42 (4.3 billion) unique channel assignments. Thus the long code imprinted with the user’s unique mask is used to provide the channelization in the reverse link.
This allows all mobiles in even very large systems to have unique channel assignments. Of course, since the long codes are simply uncorrelated and not orthogonal to each other, the recovery and demodulation process is more difficult for CDMA base stations. The high speed searcher circuits in the base station allow it to quickly search over the wide range of long codes to lock on a particular user’s signal.
These more expensive modules represent a good design trade-off, since it is possible to design in more expense and complex hardware/software into a base station than into a mobile phone.
Reverse Channel Short Sequence Spreading
CDMA mobiles use the same PN sequences as the base for final short sequence scrambling. An extra one half period clock delay in the mobile’s Q channel produces Offset QPSK modulation rather than straight QPSK modulation.
This is done so that mobiles can use a simpler and more efficient power amplifier design. OQPSK modulation prevents the signal from going to zero magnitude and greatly reduces the dynamic range of the modulated signal. Less costly amplifiers can be used on CDMA mobiles because of the reduced linear dynamic range obtained with OQPSK modulation.
- Same PN Short Codes Are Used by Mobiles
- Short Sequence spreading Aids Base Station Signal Acquisition
- Extra 1/2 Chip Delay is Inserted into Q Path to Produce OQPSK Modulation to Simplify Power Amplifier Design