Networks and Mobile Systems Research Group MIT Laboratory for Computer Science nms.lcs.mit.edu RadioActive Networks: Robust Wireless Communications John Ankcorn Stephen Garland John Guttag Matt Welborn (XtremeSpectrum)

December 7, 2000DARPA Active Networks Demo MeetingSlide 1 The wireless network challenge Things change, even in wired networks Applications, people, the world,... Active networks address the challenge of change Change is faster, less predictable in wireless networks People move Condition of communications channel changes RadioActive networks address the larger challenge

December 7, 2000DARPA Active Networks Demo MeetingSlide 2 Digital wireless communication Source Modulator Sink Demodulator Channel Channel conditions change Physical causes Fading Noise Interference Other channels Multipath

December 7, 2000DARPA Active Networks Demo MeetingSlide 3 Problems with conventional networks They treat most application as having similar needs Bandwidth, latency, error rate Security, error recovery They target the “worst” (or “average”) case But there are many different ways to be bad Which rarely all happen at once Consequently, conventional static designs... Waste resources almost all the time Perform unnecessarily poorly much of the time

December 7, 2000DARPA Active Networks Demo MeetingSlide 4 Adapt dynamically to the existing case Current channel conditions Current application Some kinds of adaptation occur today Dynamic channel allocation Adaptive power control (for transmission) Meeting the wireless challenge RadioActive networks promise much more

December 7, 2000DARPA Active Networks Demo MeetingSlide 5 RadioActive networks Virtual or software radios Provide adaptive “physical” layer Two examples using SpectrumWare Dynamically changing what is transmitted Dynamically changing behavior of receiver Active networks Provide control channel John Guttag talked about this before

December 7, 2000DARPA Active Networks Demo MeetingSlide 6 An adaptive wireless network interface Data link Physical A/D conversion Freq. conversion Modulation Multiple access Line coding Link framing Channel coding Bytes Bits Symbols Discrete signal Continuous signal OSI network layersVirtual radio layers

December 7, 2000DARPA Active Networks Demo MeetingSlide 7 Ideal software radio Not exactly what we have, but close enough for today Processor and Memory D/A Amplifiers Wideband antenna Wideband analog/digital converter A/D

December 7, 2000DARPA Active Networks Demo MeetingSlide 8 An admission No live demo today We have done table-top demos at GLOMO meetings They involve considerable hardware, preparation Not appropriate for this format Will instead show videos, screen shots

December 7, 2000DARPA Active Networks Demo MeetingSlide 9 SpectrumWare radio overview (video)

December 7, 2000DARPA Active Networks Demo MeetingSlide 10 Use for healthcare Connect ambulance/hospital Universal instruments Interchangeable probes Ultrasound, EKG, EEG,… A major activity now at MIT Other software applications Connect computers Universal wireless network interface Connect people Versatile patch panel Like the good old days

December 7, 2000DARPA Active Networks Demo MeetingSlide 11 Adaptive transmission and reception Transmission Change what is transmitted Location and width of channel Coding of symbols Reception Change algorithm for processing received signal E.g., to reduce power consumption Out-of-band or in-band signaling to coordinate change Via ActiveNet technology

December 7, 2000DARPA Active Networks Demo MeetingSlide 12 Transmission Modulation represents information on physical channel Variation in amplitude and/or frequency/phase Analog information: AM, FM Digital information (symbols): PAM, QAM,... Over a clear channel Symbols can be “close together” Allows sending more data in same width band Over a noisy channel Symbols can be spread apart Reduces bit error rate

December 7, 2000DARPA Active Networks Demo MeetingSlide 13 Modulation: 8-PSK with high SNR

December 7, 2000DARPA Active Networks Demo MeetingSlide 14 Modulation: 8-PSK with low SNR

December 7, 2000DARPA Active Networks Demo MeetingSlide 15 Modulation: QPSK with low SNR

December 7, 2000DARPA Active Networks Demo MeetingSlide 16 Reception Two basic issues Channel separation (eliminate interfering signals) Demodulation (symbol detection) Receiver Shared channel Data Bits Demodulation Channel separation Data Bits Modulation

December 7, 2000DARPA Active Networks Demo MeetingSlide 17 Reception: channel separation Desirable kinds of adaptation To interference (FDMA) and noise To application requirements for output SNR Challenge Make computation depend on output sample rate Not on the higher input sample rate Example: Wideband cellular phone receiver (AMPS) Band widthSample rate AMPS band 10MHz25M/sec Single channel 30kHz60K/sec

December 7, 2000DARPA Active Networks Demo MeetingSlide 18 Channel separation: overview Frequency Frequency/N Sample rate reduction Reduce later computation Bandwidth reduction Attenuate interfering signals Frequency translation Shift signal to filter passband

December 7, 2000DARPA Active Networks Demo MeetingSlide 19 Wideband filter Sampling rate ~ input channel width Bandwidth reduction Time duration of input ~ (output channel width) -1 Filter Input Output Random subsample used to reduce computation

December 7, 2000DARPA Active Networks Demo MeetingSlide 20 To decide whether transmitted bit was “1” or “0” 1) Compute filter output Reception: symbol detection N = 2) Apply “threshold test” Sum >   decide “1” Sum < -   decide “0”

December 7, 2000DARPA Active Networks Demo MeetingSlide 21 Adaptive symbol detection Terms in sum N = Receiver uses Quality of channel Desired error rate To control Accuracy Power consumption Bit rate (with protocol) 22 Threshold N = Software optimization Sort samples! 22

December 7, 2000DARPA Active Networks Demo MeetingSlide 22 How many terms to compute? Calibrate For n = 1..N Plot bit-error rate vs.  To achieve a given BER Pick n,   n = 3 n = 4 n = 10 BER Test after n steps If |sum| < , test again after N steps n Av Average number of steps Lowest after 6th step

December 7, 2000DARPA Active Networks Demo MeetingSlide 23 Wrapping up Virtual radios allow Faster innovation Easier adaptation Graceful degradation Cross layer optimization Research areas for RadioActive networks New signal processing algorithms Network protocols that exploit flexibility End-to-end analysis of soft physical layers MIT students studying radioActive network