Motorola i860 manual Spectrum of i DEN Quad 16QAM

OVERVIEW: iDEN Digital Modulation Technology

RL 0dBm 10 dB/

-10

-20

-30

Power(dB) -40

-50

-60

-70

-20 -10 0 10 20

Frequency from Desired Channel Center

(kHz)

Figure 1-1. Spectrum of i DEN Quad 16QAM

Quadrature Phase Shift Keying (QPSK) is one of the most common modulation techniques for satellite communications. In QPSK, a digital data stream is taken two bits at a time to generate four possible phase states of the transmitted carrier. A characteristic of this technique is its resistance to noise.

Time Division Multiple Access (TDMA) is used to allocate portions of the RF signal by dividing time into 6 slots, one for each unit. Time allocation enables each unit to transmit its voice information without interference from another unit’s transmission.

Transmission from a unit or base station is accommodated in time-slot lengths of 15 milliseconds and frame lengths of 90 milliseconds (see Figure 1-2).

Note that Rx (outbound) indicates base-to subscriber transmissions; Tx (inbound) indicates subscriber-to-base transmissions. The slots are paired and have a fixed offset of 19 milliseconds; their timings are synchronized by the iDEN system. The TDMA technique requires sophisticated

1-2 68P80400P73-A