Specifications

Interface Protocols
Operating Characteristics
August 08, 2000 68P81095E55-E 3-7
Digital Voice Processing
The iDEN system converts analog voice into digital data for transmission across the
radio link of the network. The iDEN system a standard to convert and compress the
analog voice into digital data that can be applied to the carrier using M16-QAM.
Voice sounds are converted to digital data by sampling the analog waveform and
reducing it to a set of numbers. The analog voice is interpreted by an algorithm that
measures and predicts the waveform and converts it to a digital data stream. The 8-
bit voice codec (vocoder) used is Vector Sum Excited Linear Predicting (VSELP).
By sampling the voice in small timeslices (milliseconds), summing the vectors
(changes in amplitude slope), and predicting changes (gross sum of changes), the
voice can be converted to digital with a response time to accurately reproduce
speech. The data stream is then compressed before it is applied to the 4 carrier
sidebands. This converts and compresses 90 ms of speech into 15 ms of digital data.
The resultant data packets are applied to the radio link (Figure 3-4).
Figure 3-4 iDEN VSELP Voice Compression to 6: 1the Radio Link
Radio Carrier Access Method
To support and facilitate the use of resources by more than one conversation, the
iDEN system divides the radio link data stream by time.
Time Division
Multiple Access
Time Division Multiple Access (TDMA) allows more than one user or device to
multiplex on (share) a given carrier. In the iDEN system the radio carrier’s digital
data stream is divided by time (Figure 3-5). Since the data stream runs much faster
than is required during communication, other data or conversations can be placed on
a single radio carrier (may be interleaved) without degrading or interfering with each
other. This increases the possible conversations per radio from one (analog cellular)
to:
• 3 per radio (Interconnect)
• 4 per radio (2-Dispatch and 2-Interconnect)
• 6 per radio (Dispatch only)
This provides several benefits:
• Reduced base station costs as compared to analog transmissions
• Full-duplex support — allowing the MS to switch between transmit and
receive.
• No incremental hardware to support dispatch, interconnect, and messages.
VSELP
15ms
90ms
Spoken Word
Analog to Digital Conversion
Data
Compression