2 Transmission of Data Through Frequency Selective Time Varying Channels We have seen a wireless channel is characterized by time spread and frequency spread.Time SpreadFrequency Spread
3 Single Carrier Modulation in Flat Fading Channels if symbol duration >> time spread then there is almost no Inter Symbol Interference (ISI).channeltime11phase still recognizableProblem with this: Low Data Rate!!!
4 … in the Frequency Domain this corresponds to Flat FadingchannelFrequencyFrequencyFlat Freq. ResponseFrequency
5 Single Carrier Modulation in Frequency Selective Channels if symbol duration ~ time spread then there is considerable Inter Symbol Interference (ISI).channeltime??1phase not recognizable
6 One Solution: we need equalization channelequalizertimetime11Channel and EqualizerProblems with equalization:it might require training data (thus loss of bandwidth)if blind, it can be expensive in terms computational effortalways a problem when the channel is time varying
7 The Multi Carrier Approach let symbol duration >> time spread so there is almost no Inter Symbol Interference (ISI);send a block of data using a number of carriers (Multi Carrier)1timechannel“symbol”
8 Compare Single Carrier and Multi Carrier Modulation FrequencychannelBlock of symbolssubcarriersEach subcarrier sees a Flat Fading Channel: Easy DemodMC1One symbolFlat Fading Channel: Easy DemodSC
9 Structure of Multi Carrier Modulation In MC modulation each “MC symbol” is defined on a time interval and it contains a block of dataOFDM Symboldatadatadatadatadataguard intervaldata intervalwithMAX channel time spread
10 NO Inter Block Interference! Guard TimeWe leave a “guard time” between blocks to allow multipathTXRXGuard Timethe “guard time” is long enough, so the multipath in one block does not affect the next blockData BlockData Blockdata+guardRXTXNO Inter Block Interference!
11 MC SignalTransmitted Signal:Baseband Complex Signal:
12 “Orthogonal” Subcarriers and OFDM guard intervaldata intervalChoose:Orthogonality:
13 Orthogonality at the Receiver transient responseTransmitted subcarrierChannel (LTI)Received subcarriersteady state responsestill orthogonal at the receiver!!!
14 OFDM symbols in discrete time Letbe the sampling frequency;be the number of data samples in each symbol;the subcarriers spacingThen:with the guard time.
30 Overall Implementation (IEEE 802.11a with 16QAM). 1. Map encoded data into blocks of 192 bits and 48 symbols:dataEncode InterleaveBuffer (192 bits)Map to 16QAM… ……1101+1+j3-1+j+3-j3…+1-j…4x48=192 bits
31 Overall Implementation (IEEE 802.11a with 16QAM). 2. Map each block of 48 symbols into 64 samplestime domainfrequency domainnull+1+j3…-3-j+3-j3+1-j24 data 2 pilotsnull24 data 2 pilotsIFFT
32 Channel Parameters: Physical Frequency SpreadTime SpreadConstraints on OFDM Symbol Duration:roughly!!!to minimize CP overheadfor channel Time Invariant
33 Summary of OFDM and Channel Parameters Max Time Spread secDoppler Spread HzBandwidth HzChannel Spacing HzOFDM (design parameters):Sampling FrequencyCyclic PrefixFFT size (power of 2)Number of Carriers
34 Example: IEEE802.11aChannel:Max Time SpreadDoppler SpreadBandwidthChannel SpacingOFDM (design parameters):Sampling FrequencyCyclic PrefixFFT size (power of 2)Number of Carriers
35 Applications: various Area Networks According to the applications, we define three “Area Networks”:Personal Area Network (PAN), for communications within a few meters. This is the typical Bluetooth or Zigbee application between between personal devices such as your cell phone, desktop, earpiece and so on;Local Area Network (LAN), for communications up 300 meters. Access points at the airport, coffee shops, wireless networking at home. Typical standard is IEEE (WiFi) or HyperLan in Europe. It is implemented by access points, but it does not support mobility;Wide Area Network (WAN), for cellular communications, implemented by towers. Mobility is fully supported, so you can move from one cell to the next without interruption. Currently it is implemented by Spread Spectrum Technology via CDMA, CDMA-2000, TD-SCDMA, EDGE and so on. The current technology, 3G, supports voice and data on separate networks. For (not so) future developments, 4G technology will be supporting both data and voice on the same network and the standard IEEE (WiMax) seems to be very likely
36 More Applications1. WLAN (Wireless Local Area Network) standards and WiFi. In particular:IEEE a in Europe and North AmericaHiperLAN /2 (High Performance LAN type 2) in Europe and North AmericaMMAC (Mobile Multimedia Access Communication) in Japan2. WMAN (Wireless Metropolitan Network) and WiMaxIEEE3. Digital BroadcastingDigital Audio and Video Broadcasting (DAB, DVB) in Europe4. Ultra Wide Band (UWB) Modulationa very large bandwidth for a very short time.5. Proposed for IEEE (to come) for high mobility communications (cars, trains …)