User Manual
Airbridge cBTS3612 CDMA Base Station
System Description
4 System Function
4-12
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Long code scrambling
In the forward channel, long code scrambling is used to scramble the user data to
provide encryption function.
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Walsh code spreading
In the forward channel, Walsh code is used to identify each user.
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QPSK modulation
Quadrature Phase Shift Keying (QPSK) modulation is used in the forward channel.
PN short code is used in the modulation for scrambling and providing cell ID.
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Baseband filtering
This process implements pulse shaping without inter-code interference and
suppression of out-band signal.
II. Reverse channel baseband processing
Baseband processing in the reverse channel includes multi-path signal
demodulation, signal de-interleaveing, channel decoding, extraction of frame
information data, as shown in Figure 4-9.
Multi-path
signal
demodulation
De-interleave
Channel
decoding
Extract
information
bit
Figure 4-9
Reverse channel processing flow
1) Multi-path demodulation
With Rake technology, BTS can demodulate the radio multi-path signals and
effectively combine multi-path energy.
2) De-interleaving
Signals received from an MS have been interleaved, so de-interleaving must be
performed by BTS to recover the signals.
3) Channel decoding
MS uses convolutional code or Turbo code for channel encoding before
transmission, while BTS decodes with Viterbi decoder or Turbo decoder at the
receiving end based on the channel code type of the MS.
4) Extraction of frame information data
When transmission, MS adds Cyclic Redundancy Check (CRC) bits and a number
of all-zero tail bits at the end of information bits to compose a transmitting frame. On
receiving the frame, BTS performs CRC check and removes the non-information bit
