Software Defined Wireless Communication Beyond the RF Spectrum

New England Workshop for SDR May 11, 2012

Michael Rahaim, PhD Candidate

Consider This… How have we been affected by the recent strides in wireless

communication and WLANs?

Email Web Browsing News Feeds Internet Radio Video Calling Gaming Video Streaming HD Video

⁞ Applications Link Capacity Devices

Motivation: The Wireless Traffic Jam Cisco’s Visual Network Index (VNI) is an initiative to track and

forecast the impact of visual networking applications.

Annual global IP traffic is expected to reach the zetabyte threshold.

User demand is increasing faster than gains in spectral efficiency.

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These changes in how we use wireless systems will require drastic changes in how

we think of wireless communication!

Alexander Graham Bells Photophone (1880)

Visible Light Communication (VLC) Intensity Modulation / Direct Detection (IM/DD)

Dual Purpose System Fully functional lighting system Wireless data communication

Benefits Dual use Secure connections Unregulated spectrum Spatial Reuse High bandwidth density

Software Defined VLC There is a large body of research in VLC over the past few years.

Objective: Replace RF antennas with an optical front end

A software defined system offers: Fast implementation of signal processing techniques Parallel development of signal processing and front end hardware

Radio

VLC

Hardware - Transmitter We use the LFTX daughter card in order

to account for low bandwidth of the LED.

Output signal is biased to operate in the linear region of the LED driver.

First revision utilized a single LED for characterization.

Second revision utilizes an LED array in order to increase output and improve communication distance.

Single LED Driver

Array LED Driver

PC USRP (LFTX) Bias T LED

Driver

Transmitter Flow Chart

Transmitter Characteristics LED’s typically observe voltage-to-current and current-to-flux

conversions as shown below.

Source: Philips Lumileds – Luxeon Rebel

Transmitter Characteristics Combined, this presents a nonlinear conversion as seen below.

The ideal optical output is linearly related to drive signal.

The graph below shows the illuminance of two LED’s with similar drive circuits as drive voltage ranges from 0.5VDC to 5VDC

Transmitter Characteristics For a given DC offset (VDC) and analog signal range (Rangesig),

each LED can produce a specific optical output range (RangeLED).

Ideally, this range is nearly linear as seen for the two LED’s below.

Thorlabs PDA36A Transimpedance Aplifier

Optical Lenses

Hardware - Receiver Photodiode Receiver – off the shelf component from Thorlabs

Optical Lensing – Concentrate signal power on the photodiode

We use the LFRX to demodulate signals with carrier frequency in the range 5-10 MHz

Recent receiver work by a group at Cooper Union Dr. Sam Keene Kevin Tien Calvin Ball

PC USRP (LFRX)

Optical Receiver

Optical Lensing

Receiver Flow Chart

Software Original implementation used GNURadio to test modulation,

filtering, data rate, etc.

Video Streaming demonstration offers proof of concept for VLC Gstreamer is a library of media-handling components We use it to set the video resolution and playback a video stream to a buffer

which is read by GNU Radio.

Webcam GStreamer GNU Radio USRP / Front End Tx

Transmitter Flow Chart

Mplayer GNU Radio USRP / Front End Tx

Receiver Flow Chart

Software – GNU Radio

Transmitter Flow Chart

Receiver Flow Chart

System The modified front end along with

GNU Radio offers a fully functional VLC link capable of 4Mb/s data rates.

T1

T2

T3

R3

R1 / R2

Transmitter (T)

1. USRPTX 2. Bias T 3. LED Driver

Receiver (R)

1. Optics 2. Photodiode 3. USRPRX

Conclusions SDR systems provide an excellent opportunity for research

and analysis of communication systems.

Similar concepts can be applied beyond the RF spectrum.

A fully functional system provides the means to educate others and raise interest in new topics of research.

Such a system has also provided the basis for future efforts relating to high data rate VLC implementations

Questions

Pros and Cons of VLC VLC benefits include massive open spectrum and spatial reuse –

leading to increased Bandwidth Density (Mb/s/m2).

VLC is often criticized for susceptibility to blocking, intrusive uplink, and marketability as a competitor to WiFi.

20 Mb/s

20 Mb/s 20 Mb/s 20 Mb/s

Visible Light Communication 60Mb/s Aggregate Bandwidth

RF Wireless 20Mb/s Aggregate Bandwidth

Cisco VNI Forecast – Trends By 2015, traffic from wireless devices is expected to exceed

traffic from wired devices.

Most of these devices are used for fixed position wireless.

Cisco VNI Forecast – Trends Internet video is expected to account for a majority of IP

traffic in the upcoming years.

Video streaming is an ideal use for asymmetric channels.

Proposed Hybrid System Consider a room enabled with broadcast VLC hotspots and a

single WiFi AP.

Wired Interconnect

VLC “hotspots”

Network

Router and WiFi Access Point