Quadrature Phase Shift Keying (QPSK) Archives

March 23, 2025 DSP / Wireless and SDR

Two Birds with One Tone: I/Q Signals and Fourier Transform – Part 1

When a new member arrives at the Signal Processing Club, this is what they find at the club gate: I/Q signals. Perhaps a secret plot to keep most people out of the party? Some return from here to try another area (e.g., machine learning, which pays more and is easier to understand but less interesting than signal processing). Others persist enough to push the gate open for implementation purposes (even a little understanding is sufficient for this task) but never fully grasp the main idea. So what exactly makes this topic so mysterious? To investigate the answer, we start with

Eye diagram of CQPSK demodulation with a P25 C4FM receiver

June 3, 2026 Wireless and SDR

Demodulating P25 CQPSK Signals Using a C4FM Receiver

Usually a modulation of one species can only be demodulated with the radio signal processing chain of the same species. But P25 has a very interesting and unusual concept of demodulating a signal from one modulation species using the receiver of another species. APCO Project 25, commonly known as P25, is a suite of digital radio standards designed for public safety and first responder communications across North America which is similar to European Terrestrial Trunked Radio (TETRA). The main purpose of its development was to ensure seamless interoperability among federal, state and local agencies. Hence, it enables radios from different

Eye diagram for a 4-QAM modulated signal and a simple channel impulse response

December 30, 2016 Wireless and SDR

Impact of Multipath on the Received Signal

In this article, we describe the impact of multipath caused by the wireless channel on the signal arriving at the receiver from a constellation viewpoint. Recall that an eye diagram, a transition diagram and a scatter plot are the stethoscopes of a communication system and hence it is imperative to bring in that perspective for a Tx signal convolved with the channel impulse response. This is because a wireless channel can be seen as a Finite Impulse Response (FIR) filter with the result that the sampled Rx signal is a convolution between taps of this FIR filter and the Tx

Phase jumps at every zero crossing from modulating data onto the carrier phase for a QPSK waveform

October 7, 2016 Wireless and SDR

The Fundamental Problem of Synchronization

We have seen in the effect of phase rotation that the matched filter outputs do not map back perfectly onto the expected constellation, even in the absence of noise and no other distortion. Unless this rotation is small enough, it causes the symbol-spaced optimal samples to cross the decision boundary and fall in the wrong decision zone. And even for small rotations, relatively less amount of noise can cause decision errors in this case, i.e., noise margin is reduced. In fact, for higher-order modulation, the rotation becomes even worse because the signals are closely spaced with each other for the

A general QAM detector with respective waveforms at each block

October 7, 2016 Wireless and SDR

Quadrature Amplitude Modulation (QAM)

Quadrature Amplitude Modulation (QAM) is a spectrally efficient modulation scheme used in most of the high-speed wireless networks today. We discussed earlier that Pulse Amplitude Modulation (PAM) transmits information through amplitude scaling of the pulse $p(nT_S)$ according to the symbol value. To understand QAM, two routes need to be traversed. Route 1 We start the first route with differentiating between baseband and passband signals. A baseband signal has a spectral magnitude that is nonzero only for frequencies around origin ($F=0$) and negligible elsewhere. An example spectral plot for a PAM waveform is shown below for 500 2-PAM symbols shaped by