QPSK Module 5 3 5 Signal Space Example

QPSK Module 5

3. 5 Signal Space Example: QPSK What is QPSK ? Ø In quadric phase-shift keying (QPSK), four possible symbols are transmitted: where E is the transmitted symbol energy, T is the symbol duration and for some fixed integer Nc (no. of cycles for each symbol). Ø Choose a Gray-encoded set of dibits such that

3. 5 Signal Space Example: QPSK Signal-Space Characterisation for QPSK § § Note how even and odd numbered bits affect si 1 and si 2 respectively.

3. 5 Signal Space Example: QPSK Signal-Space Diagram for QPSK System § Four message points and associated vectors defined by

3. 5 Signal Space Example: QPSK Example for Sequence 01101000 Input binary sequence Odd-numbered sequence Polarity of coefficient si 1 Even-numbered sequence Polarity of coefficient si 2 0 1 Dibit 01 0 - 1 1 + 0 Dibit 10 0 0 - 0 Dibit 00 0 - § § Note how even-numbered bits are associated with , while odd-numbered bits are associated with.

3. 5 Signal Space Example: QPSK Generation of QPSK Signals Binary data sequen ce Polar nonreturn -tozero level encoder + Demultiplexe r + QPSK signal s(t) t § §

3. 5 Signal Space Example: QPSK Generation of QPSK Signals Ø Consider the first dibit of s(t) {i. e. ‘ 01’} corresponding to Ø No w

3. 5 Signal Space Example: QPSK Coherent Detection of QPSK Signals Threshold = 0 x 1 Decisio x Receiv ed signal In-phase Channel x t 011 0 Estimate of transmitted binary sequence Multiplex er Quadrature Channel (No channel distortion) § § n device Decisio x 2 n device Threshold = 0 100 0 011010 00

3. 5 Signal Space Example: QPSK Coherent Detection of QPSK Signals Ø Consider the first dibit of s(t) {i. e. ‘ 01’} corresponding to symbol Ø 0 (why? ) Ø Similarly Ø Verify in QPSK signal-space diagram.