Design of a High Data Rate Audio Band OFDM Modem

Navalekar, Abhijit C

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Design of a High Data Rate Audio Band OFDM Modem

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Land mobile radio technology (LMR) has existed since the early 1920's. The most visible manifestation of this technology is the handheld VHF/UHF radios also referred to as 'walkie-talkie'. These handheld devices are one of the most ubiquitous forms of radio communication systems. Most of them are designed for transmitting analog voice signals. Due to an increase in the amount of digitized analog signals over the past few years complemented by a need for transmitting pure digital data, there has been a desire to transmit digital data. There are methods which allow the analog radios to transmit digital data without any modifications; however the data rate achievable using these methods is very low. In contrast, the digital variants of these hand-held radios are capable of transmitting digital data at comparatively higher data rates. However they are expensive and require major infrastructure overhauls. In this thesis, a prototype modem was developed which interfaces with an analog radio without any modifications to the radio. Furthermore, the data rates achievable are comparable with those achieved using digital radios. The modem uses Orthogonal Frequency Division Multiplexing (OFDM) technique to generate an audio band signal which is fed to the radio. The OFDM technique used to generate the audio band signal from data bits ensures maximum bandwidth efficiency. The developed modem is capable of communicating over Ethernet connection. It uses a RJ 45 interface to connect to a data source.

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