Inter-Symbol Interference (ISI) Archives

Eye diagram of CQPSK demodulation with a P25 C4FM receiver

June 3, 2026 Wireless and SDR

Demodulating P25 CQPSK Signals Using a C4FM Receiver

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. The main purpose of its development was to ensure seamless interoperability among federal, state and local agencies. Hence, it enables radios from different manufacturers to communicate on the same network. The standard is split into two phases: Phase 1 and Phase 2. Our focus today is on Phase 1. There are two modulation schemes defined in the standard, C4FM (Compatible 4-Level Frequency Modulation) and CQPSK (Compatible Quadrature Phase Shift Keying), both of which operate at

December 29, 2020 Miscellaneous / Wireless and SDR

CDMA or OFDM

Reading about interference cancellation techniques today, I recalled an interesting article by Sridhar Vembu titled Two Philosophies in CDMA: A Stroll Down Memory Lane. Vembu is the founder and CEO of Zoho Corporation, a venture which has turned him into a billionaire. He spent time both in academia (at Princeton) and in industry (at Qualcomm) working with the likes of Sergio Verdu in one camp and Andrew Viterbi in the other. Here are some excerpts from his article which is not available online anymore at the time of this writing. I have now worked a little over 10 years in

Constructive and destructive interference arising from the different delays of multipath

December 1, 2021 Wireless and SDR

Small-Scale Fading in a Wireless Channel

Small-scale fading is a phenomenon that arises due to the unguided nature of the wireless medium. Dramatic variations in signal amplitude occur at the Rx from constructive and destructive interference of multipath components originating from the surrounding environment that give rise to small-scale fading. This is the main challenge for designing efficient high-rate wireless communication systems which spawned an array of research activities in the past 50 years aimed to bring the wireless transmission rates closer to their wire counterparts. The technologies for 5G systems have been chosen with the benefit of experience gained from actual implementations over these years.

A V-BLAST architecture for 4 Tx antennas

December 15, 2021 Wireless and SDR

V-BLAST with Successive Interference Cancelation

In the article on Zero-Forcing detector for MIMO receivers, we have seen that the performance of linear detectors is unsatisfactory for actual implementations of conventional MIMO systems. Their main attraction comes from their low computational complexity. To strike a nice balance between performance and complexity, a neat trick is employed by the algorithm known as Successive Interference Cancelation (SIC). The concept was devised by Gerard Foschini from Bell Labs, although it was not a new idea. Successive interference cancelation was already proposed for the detection algorithms in CDMA systems. Again, the fundamental idea was borrowed from decision feedback equalization schemes

At the boundary of two OFDM symbols, pulse shaping smoothes the edges, resulting in the avoidance of spectral regrowth

February 26, 2020 Wireless and SDR

Windowing an OFDM Signal in Time Domain

Orthogonal Frequency Division Multiplexing (OFDM) has been introduced in a previous article as a technique suitable for high data-rate transmissions over a wireless channel. The two main advantages I mentioned were as follows: Simple one-tap equalization, and Ability to slice the spectrum and utilize each slice in an independent manner. Due to these advantages, it was adopted as the preferred modulation in WiFi and 4G-LTE systems. The interesting part is that while many new waveforms were proposed to replace it in 5G NR, OFDM was still the modulation of choice for both downlink and uplink directions with some minor changes.