Admin. OFDM, Mobile Software Development Framework Homework to be posted by Friday Start to think about project 9/7/01 Y. Richard Yang 1 Recap Inter-Symbol Interference (ISI) Handle band limit ISI Handle multipath ISI Viterbi problems: Its complexity grows exponentially with D (the number of multipaths taps relative to the symbol time) Q: how to reduce D? OFDM: Basic Idea Uses multiple carriers modulation (MCM) each carrier (called a subcarrier) uses a low symbol rate for N parallel subcarriers, the symbol time can be N times longer spread symbols across multiple subcarriers also gains frequency diversity 3 4 Benefit of Symbol Rate on ISI Multiple Carrier Modulation 1 3 4 1 3 4 1 1 5 6 1
Multiple Carrier Modulation (MCM): Problem Despite wave shaping, there can be leak from one subcarrier to another subcarrier i j Objective: Avoid subcarrier interference Interference of subcarrier i on subcarrier j i j Conventional design: guard bands to avoid interference among subcarriers Guard band wastes spectrum 7 Assume no pulse wave shaping, matched filter T sin(π f i t +φ i )sin(π f j t +φ j ) 0 T = 1 cos[π ( f i f j )t +φ i φ j ]+ cos[π ( f i + f j )t +φ i +φ j ) 0 Condition for the interference to be always 0? 8 Objective: Avoid subcarrier interference if integer number of cycles in [0, T] T cos[π ft +φ]dt = 0 0 # cycles in T is T * f => T * f = integer OFDM Key Idea: Orthogonal Subcarriers Each subcarrier frequency is chosen so that an integral number of cycles in a symbol period, i.e., subcarrier freq = k 1/T 9 They do not need to have the same phase, so long integral number of cycles in symbol time T! 10 OFDM Modulation Orthogonal Frequency Division Multiplexing OFDM allows overlapping subcarriers frequencies http://www1.linksys.com/products/images/ofdm.gif 80.11a 11 1
OFDM Implementation Take N symbols and place one symbol on each subcarrier (freq.) Freq 0 e jπ 0 f sc 0 e jπ 0 f scts e jπ 0 f sc Ts OFDM: Implementation Issue Hardware implementation can be expensive if we use one oscillator for each subcarrier Software implementation requires N multiplications per time output => N multi. per N outputs Freq N-1 e jπ () f sc 0 e jπ () f scts e jπ () f sc Ts Freq 0 e jπ 0 f sc 0 e jπ 0 f scts e jπ 0 f sc Ts Freq N-1 e jπ () f sc 0 e jπ () f scts e jπ () f sc Ts Q: complexity of the implementation strategy? 13 14 OFDM: Key Idea out k = d n e jπ (nf sc )kts Assume N outputs per symbol time T, f sc =1/T out k = d n e jπ (nf sc )kts = d n e jπ (nf sc )k N = d n e jπ Consider data as coefficients in the frequency domain, use inverse Fourier transform to generate time-domain sequence T 1 N nkf sct = d n e jπ 1 N nk OFDM Implementation: FFT channel 15 16 OFDM Implementation Parallel data streams are used as inputs to an IFFT IFFT does multiplexing and modulation in one step! Guard Interval: Removing ISI Orthogonal subcarriers remove intercarrier interference Slow symbol rate reduces inter-symbol interference, but may still have ISI 1 1 17 Basic idea of GI: skip the first part damaged signal More details: Chap. 13.1.4 Gast 18 3
OFDM Guard Interval OFDM Implementation http://proquest.safaribooksonline.com/059610053?tocview=true http://www.dsplog.com/008/0/17/cylcic-prefix-in-orthogonal-frequency-division-multiplexing/ 19 0 OFDM in 80.11a Other Multipath Techniques Subcarrier frequency spacing 31.5KHz 1/31.5KHz = 3.us 64 samples FFT 16 samples Guard Interval There are other techniques to handle multipath such as Rake Receiver See backup slides for some details http://standards.ieee.org/ getieee80/download/ 80.11a-1999.pdf 1 Summary of PHY Wireless PHY @Mbps @Mbps @3Mbps @35Mbps @1.4Gbps Transmitter: Direct Sequence Symbol Wave Scramble DQPSK Mod Spread Spectrum Shaping To RF From MAC @1.4Gbps @35Mbps @3Mbps @Mbps @Mbps Receiver: Decimation Despreading DQPSK Demod Descramble From RF To MAC (a) IEEE 80.11b Mbps @4Mbps @4Mbps @48Mbps @48Mbps @384Mbps @51Mbps @640Mbps @1.8Gbps Convolutional Symbol Wave Scramble Interleaving QAM Mod IFFT GI Addition Transmitter: encoder Shaping To RF From MAC @1.8Gbps @640Mbps @51Mbps @384Mbps Receiver: @48Mbps @4Mbps @4Mbps Demod + Viterbi Decimation Remove GI FFT Descramble Interleaving decoding From RF To MAC PHY (b) IEEE 80.11a/g 4Mbps http://setemagali.com/009/10/1/climbing-the-mountain-everyday/ 3 4 4
Big Picture Overview Applications Wireless/Mobile Application Development Framework Foundational Services: Communications, Location, Service Discovery, UI/Media, Power Management, Security 5 Mobile/Wireless software development framework for mobile wireless applications is a quite large topic We have already seen Gnuradio as an example framework We will cover more examples TinyOS, JME, Android, IOS Approach for designing/evaluating each software development framework: Focus on the key concepts introduced by each framework 6 Outline GNURadio: Design Objective A software development toolkit that provides signal processing blocks to implement software-defined radio systems. 7 8 Outline GNURadio Hardware Arch RF Frontend (Daugtherboard) Hardware Frontend ADC/DAC and Digital Frontend (USRP) Host Computer GNU Radio Software 9 http://mobiledevices.kom.aau.dk/fileadmin/mobiledevices/teaching/software_testing/gnu_radio_lecture.pdf 5
Outline Software concepts Basic Software Concepts Block Flow graph 31 Basic Software Concepts http://gnuradio.org/doc/doxygen/ classgr block.html gr_basic_block (name, in/out signature, msg queue) gr_block (Leaf block; key functions forecast/ general_work) Example: http://www.gnu.org/software/gnuradio/ doc/howto-write-a-block.html gr_hier_block (container block; key functions: connect/disconnect/lock/unlock) gr_top_block (flow graph; start/stop/wait) Software/Execution Model Software model Python Application management (e.g., GUI) Flow graph construction Non-streaming code (e.g., MAC-layer) C++ Signal processing blocks Certain routines also coded in assembly q Execution model q Python thread for each top_block Discussion: benefits/issues of the hybrid software structure? Python Application development Flow graph construction C++ Signal processing blocks Summary: GNURadio Interesting/key software design techniques you learned from GNURadio? Outline Software concepts TinyOS 35 36 6
Design Goal Hardware A free and open source component based operating system and platform targeting wireless sensor networks (WSNs) Example app Environment monitoring, e.g., measure temperature, lighting values/events periodically transmit measurements/events to a base station forward data for other nodes that are out of range of the base station http://www.tinyos.net/tinyos-1.x/doc/tutorial/ 37 1.5 x 1.5 Assembled from off-the-shelf components 4Mhz, 8bit MCU (ATMEL) 51 bytes RAM, 8KB ROM Devices serial Port temperature sensor & light sensor 900Mhz Radio (RF monolithics) 10-100 ft. range LED outputs 38 Schematic Diagram of a Mote Outline Software concepts TinyOS Software concepts 39 40 Requirements on Software Dev. Framework TinyOS: Software Concept Flexible configuration of attached devices Small foot print devices have limited memory and power resources TinyOS: Generate customized OS + application for each given scenario support one application at a time but flexible reprogramming 41 4 7
Schematic Diagram TinyOS: Software Concepts A TinyOS consists of one or more components linked together software components motivated by hardware component Each component specifies that it provides some interfaces allows other components to control it also uses some interfaces control other components 43 44 Interface Interface: Examples An interface declares a set of functions called commands that provider must implement another set of functions called events that the interface user must implement A uses interfaces I1 and I I1 I commands events commands events B provides I1 C provides I C provides I3 45 StdControl.nc interface StdControl { command result_t init(); command result_t start(); command result_t stop(); } ADC.nc interface ADC { async command result_t getdata(); } async command result_t getcontinuousdata(); event result_t dataready(uint 16_t data); Timer.nc interface Timer { command result_t start( char type, uint3_t interval); command result_t stop(); event result_t fired(); } 46 Backup Slides Rake Receiver 47 48 8
Multipath Diversity: Rake Receiver Multipath Diversity: Rake Receiver Instead of considering delay spread as an issue, use multipath signals to recover the original signal Used in IS-95 CDMA, 3G CDMA, and 80.11 Invented by Price and Green in 1958 R. Price and P. E. Green, "A communication technique for multipath channels," Proc. of the IRE, pp. 555--570, 1958 Use several "sub-receivers" each delayed slightly to tune in to the individual multipath components Each component is decoded independently, but at a later stage combined this could very well result in higher SNR in a multipath environment than in a "clean" environment LOS pulse multipath pulses 49 50 Rake Receiver Blocks Rake Receiver: Matched Filter Correlator Finger 1 Finger Finger 3 Combiner Impulse response measurement Tracks and monitors peaks with a measurement rate depending on speeds of mobile station and on propagation environment Allocate fingers: largest peaks to RAKE fingers 51 5 Rake Receiver: Combiner Comparison [PAH95] The weighting coefficients are based on the power or the SNR from each correlator output If the power or SNR is small out of a particular finger, it will be assigned a smaller weight: Zm αm = M Zi i= 1 53 MCM is OFDM 54 9