ECE4253 Digital Communications
	Department of Electrical and Computer Engineering - University of New Brunswick, Fredericton, NB, Canada

16-QAM Decoding Tool

In 16-QAM, information in conveyed through phase and amplitude variations. In each time period, the phase and/or amplitude can change once. Since there are sixteen possible phase and amplitude states, there are 4 bits of information conveyed within each time slot. The rate of change (baud) in this signal determines the signal bandwidth, but the throughput or bit rate for 16-QAM is four times the baud rate. A V.22bis modem operates at 600 baud to give 2400 bits per second throughput

The constellation shown below illustrates the available phase and amplitude states of the modulated signal. In each time interval, two bits are derived from phase change (change in quadrant) while the other two bits are found from the absolute position within a quadrant.

Phase changes (in this case, phase changes between quadrants) are mapped to dibits (DIBIT 1) identically to a V.22 modem (QPSK) which operates at 600 baud to give 1200 bits per second throughput. Two more bits (DIBIT 2) are encoded by the position of the phase state within a quadrant (00,01,10,11) as shown on the constellation. In this way, four bits are conveyed at every time slot.

For convenience, the states are labelled A..P as shown on the constellation below.

Constellation

STATES: H-C-E-J-O-P-K-D- STATE CHANGE QUADRANT CHANGE DIBIT 1 NEW STATE DIBIT 2 H to C 0 deg 01 C 10 C to E 90 deg 00 E 10 E to J 90 deg 00 J 00 J to O 90 deg 00 O 01 O to P 0 deg 01 P 11 P to K 0 deg 01 K 00 K to D 90 deg 00 D 11

Summary