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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 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.
RF Transmission Bursts
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 (GPS)
system. Each burst is numbered; the number is referred to as the slot number. All bursts occurring
at a given time carry the same slot number.
Inbound transmission bursts (sent from the unit) are offset 19 milliseconds from the outbound
burst; the inbound burst begins 4 milliseconds after the end of the outbound burst. See Figure 1.
This offset in time allows the unit to appear capable of transmitting and receiving at the same time
(it actually is switching rapidly back and forth between receiving and transmitting).
In i425 iDEN receive frequency range is 851-870 MHz and the transmit frequency range is 45
MHz below the receive band at 806 to 825 MHz.
An iDEN channel is created by grouping bursts, so their slot numbers differ by the repetition rate.
For dispatch calls the i425/i290 uses a single frequency that can handle six calls using a 6:1
repetition rate with the 4.2 Kbps coder or a 3:1 repetition rate with the 8.0 kbps. The audio
quality with the 3:1 repetition rate is superior to the 6:1 rate.
The dual band unit will power up in the 800 MHz band and will use control channels, Primary
Control Channel (PCCH) and Broadcast Control Channel (BCCH). The iDEN system can operate
three voice channels simultaneously in dispatch mode on a single RF carrier at 25 kHz bandwidth.
6880401P26-O
OVERVIEW: iDEN Digital Modulation Technology
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