Pranavi Boyalakuntla Principles of Wireless Spring 2021

I. Introduction

One of the downsides to a single carrier communication system is that the symbol period must be high in order to avoid inter-symbol interference (ISI). When the symbol period is higher, the data rate of the system is lower.

One way of increasing data rate is using a multiple carrier communication system. Using more than one carrier allows for information to be transmitted at multiple frequency intervals at the same time. In order to ensure there is no intercarrier interference (ICI), guard bands, or unused bandwidth between channels, are used. Note that guard bands are not unique to multicarrier systems. This partitioning scheme is called Frequency Division Multiplexing (FDM). Fig. 1 shows a multiple carrier communication system using OFDM.

Fig. 1: A multiple carrier communications system that utilizes guard bands.

Fig. 1: A multiple carrier communications system that utilizes guard bands.

The use of guard bands requires that extra bandwidth be available. In the case where bandwidth is more limited, orthogonal frequency division multiplexing (OFDM) can be used.

OFDM utilizes the concept of orthogonality to allow channels to overlap while minimizing ICI. When two channels are orthogonal to one another, their zero crossings align and the peak of one channel does not interfere with the peak of another channel. In Fig. 2, each channel is represented by a sinc function in the frequency domain. Each zero crossing aligns. This means that the channels are orthogonal. The peak of one channel also exists over lower values of other channels. Although these channels overlap, the ICI is minimized by maintaining orthogonality.

Fig. 2: Orthogonal channels that overlap.

Fig. 2: Orthogonal channels that overlap.

OFDM uses the available bandwidth more efficiently than FDM as there are no unused portions of the spectrum. As a result, OFDM is used in popular communication systems like WiFi and LTE.

The goal of this project is to implement an OFDM system on a Universal Serial Radio Protocol (USRP). This paper will cover the three steps that were taken to reach this goal:

  1. Simulation of a timing synchronized OFDM communication system.
  2. Simulation of an OFDM system with timing errors.
  3. Implemented OFDM system using the USRP.

II. Overview of OFDM

A. Flat-Fading Assumption

The frequency flat-fading assumption means that the channel's transfer function can be assumed to be constant over a frequency band. Assuming that all frequencies in a region fade in the same way is advantageous because all frequencies can be demodulated in the same way; it is known how to adjust each distinct region.