OFDM Archives | Wireless Pi

Data symbols riding on the subcarriers experience a rotation due to residual carrier frequency offset and sampling frequency offset

August 29, 2022 Wireless/SDR

Effect of a Sampling Clock Offset on an OFDM Waveform

In an earlier article on the impact of a sampling clock offset on a single-carrier waveform, we explained the nature of a Sampling Clock Offset (SCO), i.e., a difference in sampling clock frequency between the Tx and the Rx. This is also known as a symbol timing frequency offset. The meaning of a sampling clock offset for a slow Rx clock that skips some samples within an interval is visually demonstrated in the figure below. In the context of OFDM systems, a previous article describes how the normalized Carrier Frequency Offset (CFO) and the normalized Symbol Timing Offset (STO) affect the FFT input at the Rx. Now assuming a successful timing synchronization, we study the impact of the residual normalized

Computation of the metric involves a correlation sum at a time difference of half symbol duration

June 13, 2022 Wireless/SDR

Timing Synchronization in OFDM Systems

Orthogonal Frequency Division Multiplexing (OFDM) has been the vehicle driving most high rate wireless communication systems in the world today. Some of the notable examples are our WiFi, 4G and 5G technologies. See the interesting LoRa PHY for modulation techniques based on frequency shift – chirp spread spectrum that utilize many of the concepts from OFDM for algorithm design. As a background, we have also discussed before the impact of a timing error on an OFDM signal. It was observed that an integer timing offset does have affect the performance as long as it within certain boundaries. A fractional timing offset introduces a phase rotation that becomes part of the complex channel gain. Therefore, the purpose of a timing recovery

The effect of symbol timing offset on an OFDM symbol

March 28, 2022 Wireless/SDR

Effect of Timing Mismatch in OFDM Systems

Timing synchronization is one of the most fascinating topics in the field of digital communications. The impact of symbol timing offset has been discussed in the context of single-carrier systems before. The intuition behind how an OFDM system works is also presented in a previous article. However, the problem of timing synchronization is quite different in OFDM systems as compared to single-carrier systems due to the nature of the waveform. Let us explore how a timing error impacts the demodulated waveform in such a scenario. To avoid using many indices, we skip the OFDM symbol index $m$ in the following expressions to focus on each OFDM symbol. Moreover, upconversion and downconversion through a carrier at frequency $F_C$ is also bypassed

December 29, 2020 Miscellaneous / Wireless/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 the industry, after getting my PhD. In my very first year of work at Qualcomm, I noticed how even when

February 14, 2022 Wireless/SDR

Advantages and Disadvantages of OFDM – A Summary

Orthogonal Frequency Division Multiplexing (OFDM) is a technique of choice for many high rate wireless communication systems. An overview of OFDM for a DSP/wireless beginner was given in this article where visualizations of how OFDM slices the spectrum into multiple subcarriers for one user was provided in detail. Orthogonal Frequency Division Multiple Access (OFDMA) is an extension of OFDM for multiple users, i.e., it is a multiple access technology (like TDMA and CDMA from 2G and 3G cellular systems, respectively) in which the available spectrum is divided into multiple subcarriers that are shared among multiple users. This was the choice of multiple access technology in 4G LTE systems. After the adoption of OFDM in 5G NR, it has become even