A Software Defined Radio Implementation for Voice

Transmission over Wireless Ad-hoc Networks

Jason TranSURF-IT 2009 Fellow

Mentors: Dr. Homayoun Yousefi’zadehDr. Hamid Jafarkhani

Graduate Student: Ala Khalifeh

Introduction

– Most radios today can only serve as one kind of radio.• Cell phones cell phones• Walkie talkies walkie talkies• FM Transmitter FM Receiver

– Imagine a radio that can be programmed.– The Software Defined Radio (SDR)

• Example: Gobi - Qualcomm’s SDR can switch between EVDO (Verizon/Sprint) and HSDPA (AT&T)

My SURF-IT Research

• We’ve implemented a voice transmission scheme over a software defined radio platform using the Universal Software Radio Peripheral (USRP) programmed by GNU Radio.

• The goal of this project was to optimize voice transmission using voice codecs and forward error correction (FEC). These will be discussed in more detail later.

Motivation/Definitions

• Why Reprogram the Walkie Talkie?• Fundamentally we are going from

Analog -> Digital (like our new TV broadcasts).

• Three Advantages:– Efficient: less data is needed to be sent

(compression)– Reliable: digital can overcome errors/noise– Flexible: no needed extra hardware to change

signals which could save money

Analog vs. DigitalWhat’s the Difference?!

• We only care about information at certain points.

• Information is now “discrete” or “digital”, but still transmitted in analog.

Converting to Digital

(aka Sampling)

Our Software Defined Radio (SDR)

• Our SDR = GNU Radio on Linux (Software) + Laptop USRP

• GNU Radio is an open source Linux software used to program the USRP.

• The USRP consists of digital to analog converters and a radio frequency front-end that sends the data over the air.

+

Network ModesIntranet

(Infrastructure) Ad-Hoc

The Project (Streaming Voice)

• We stream by packetizing a certain amount of data.– For example6000Bytes 1500Bytes 1500 Bytesor 6KBytes 1500 Bytes 1500 Bytes

• Two usual transport protocols used for streaming are transmission control protocol (TCP) and user datagram protocol (UDP).– TCP: sends a packet and waits for acknowledgement

before sending another– UDP: continuously sends without acknowledgement

Transmission Diagram

Transmission Errors

• Two types we are concerned about:– Packet Erasures (late or lost packets)– Bit errors

• Bit Errors (flipped or unknown bits)– Packets [3 byte example(8 bits per byte)]

10101001|00010100|1010101010x010x1|x0010x00|10x010x0

Regular Packet

Packet with Errors

One Byte

Error Handling

• Voice Codec: Speex (data compression)– Uses less bandwidth and reduces packet erasures– Variable Bitrate and Echo Cancellation

• Forward Error Correction (FEC): Reed Solomon Codes– Creates redundant bits:

• Received “000” is seen as “0” “110” is seen as “1”• Received “010” is seen as “0” “011” is seen as “1”

• With these, we’re able to optimize voice streaming.

Additional and Future Work

• This project is one part of a larger project of delivering multimedia content in real-time (voice, audio, video) and non-real time (HTTP and FTP) on a Mobile Ad-hoc Network (MANET).

Applications

• Mobile ad-hoc networks (MANETs) can establish a network where there is no infrastructure– Military (Joint Tactical Radio System (JTRS) )– Emergencies– Medical

• SDRs enable“cognitive” radios– Self-adjusting radios

Thank You!Thank you SURF-IT and UROP

coordinators!

Questions?

Future Work and Applications

• Other current applications include cellular GSM base station, GPS receiver, FM transmitter/receiver, digital television decoder, and passive radar.

Metrics for Measuring Transmission Performance

• Packet Loss/Throughput • Perceptual Evaluation of Speech Quality

– Family of standards for a test methodology of evaluating speech quality.

– Compare speech before and after transmission.– Mean Opinion Score (MOS)

• Packet Delay Variation – Jitter – IP Packet Delay Variation for IP Performance Metrics

(IPPM)

Background Traffic

(200/400/600/800kbps)

Performance Analysis • We know what link speed we will choose (1Mbps)• In real life, we’ll have a lot of background traffic. • We want to measure performance of voice stream under

different network before and after using error handling schemes (encoder/FEC).

• Experiment setup: Measure voice transmission from computer 1 to computer 2.

Voice

Transmission

Computer 3Computer 2Computer 1

Throughput vs. Raw Bitrate

Packet Error Rate (PER) vs. Raw Bitrate

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Networking and the OSI Model

Linux

Reed Solomon Codes

GNU Radio

USRP