OVERVIEW: iDEN Voice Compression Technology
68P80400P73-A 1-3
algorithms and one digital-signal processor (DSP) to perform voice compression/decompression
and RF modulation/demodulation.
Figure 1-2. iDEN TDMA Format
1.2 iDEN Voice Compression Technology
Voice is converted into a digital bit stream by sampling the voice signal at a high rate and
converting the samples into numbers, which are represented by bits. A sample consists of 8 bits.
Approximately 8000 samples per second (64 Kbps) are required to maintain a reasonable quality.
Voice compression reduces the number of bits per second while maintaining the voice at an
acceptable quality level. The iDEN system uses a coding technique called Vector Sum Excited
Linear Prediction (VSELP) to compress voice to 4.2 or 8.0 Kbps. The compressed voice-data bits
modulate the RF signal. The compression rate is based on the type of call (dispatch or interconnect)
and the network configuration established by the local service provider.
Adding error-correction bits to the coded-voice bits increases the required RF-transmission bit rate
to approximately 7.4 Kbps for the 4.2 Kbps voice coder and 14.8 Kbps for the 8.0 Kbps voice coder.
1.2.1 RF Transmission Bursts
All RF transmissions within an iDEN system occur in 15-millisecond blocks called bursts. An
iDEN base station transmits bursts continuously on each RF frequency it uses. Transmission bursts
from the stations are synchronized in time by signals received from the global positioning satellite
15ms15ms 15ms15ms 15ms 15ms 15ms 15ms 15ms 15ms 15ms 15ms 15ms
15ms
90ms 90ms
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15ms
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Tx
Rx
Receiver 4ms delayed
Transmitting 6 of 6 slots continually.
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15ms
90ms 90ms
15ms15ms 15ms 15ms 15ms 15ms 15ms 15ms 15ms 15ms 15ms 15ms
15ms
90ms 90ms
Rx
Tx
Transmitter 4ms delayed
When turned on, scans for control station, then transmits one slot every six slots.
Base Station Control Channel
Portable Unit
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