Modulation - QPSK

Quadrature Phase Shift Keying is effectively two independent BPSK systems (I and Q), and therefore exhibits the same performance but twice the bandwidth efficiency.Quadrature Phase Shift Keying can be filtered using raised cosine filters to achieve excellent out of band suppression.Large envelope variations occur during phase transitions, thus requiring linear amplification.

Cos Wc t

900

Odd Data

Even Data

(NRZ)

(NRZ)

QPSK

Q-Channel

I-Channel

Q

(-1,-1)

(-1,1)

I

(1,1)

(1,-1)

Wc = Carrier Frequency, I = In phase channel, Q = Quadrature channel

Multi-level (M-ary) Phase and Amplitude Modulation

Amplitude and phase shift keying can be combined to transmit several bits per symbol (in this case M=4). These modulation schemes are often refered to as linear, as they require linear amplification.16QAM has the largest distance between points, but requires very linear amplification. 16PSK has less stringent linearity requirements, but has less spacing between constellation points, and is therefore more affected by noise.M-ary schemes are more bandwidth efficient, but more susceptible to noise.

16 PSK 16 APSK16 QAM