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Multi Carrier Modulation and OFDM

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Transmission of Data Through Frequency Selective Time Varying Channels We have seen a wireless channel is characterized by time spread and frequency spread. Time Spread Frequency Spread

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if symbol duration >> time spread then there is almost no Inter Symbol Interference (ISI). 10 time channel 10 phase still recognizable Problem with this: Low Data Rate!!! Single Carrier Modulation in Flat Fading Channels

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this corresponds to Flat Fading Frequency channel Flat Freq. Response Frequency … in the Frequency Domain

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if symbol duration ~ time spread then there is considerable Inter Symbol Interference (ISI). 10 time channel ?? phase not recognizable Single Carrier Modulation in Frequency Selective Channels

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One Solution: we need equalization channelequalizer 10 time 10 Channel and Equalizer Problems with equalization: it might require training data (thus loss of bandwidth) if blind, it can be expensive in terms computational effort always a problem when the channel is time varying

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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) 1 0 time channel time “symbol” The Multi Carrier Approach

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Compare Single Carrier and Multi Carrier Modulation Frequency channel Block of symbols subcarriers Each subcarrier sees a Flat Fading Channel: Easy Demod MC Frequency 1 One symbol Frequency Flat Fading Channel: Easy Demod SC

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In MC modulation each “MC symbol” is defined on a time interval and it contains a block of data data intervalguard interval OFDM Symbol data MAX channel time spread with Structure of Multi Carrier Modulation

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the “guard time” is long enough, so the multipath in one block does not affect the next block Data Block TXRX We leave a “guard time” between blocks to allow multipath Guard Time data+guard Guard Time TX RX NO Inter Block Interference!

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Baseband Complex Signal: MC Signal Transmitted Signal:

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“Orthogonal” Subcarriers and OFDM data intervalguard interval Choose: Orthogonality:

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still orthogonal at the receiver!!! Orthogonality at the Receiver transient response Transmitted subcarrier Channel (LTI) Received subcarrier steady state response

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Let be the sampling frequency; be the number of data samples in each symbol; the subcarriers spacing Then: with the guard time. OFDM symbols in discrete time

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Summary OFDM Symbol Sampling Interval guarddata TIME: Freq spacing FREQUENCY: # samples # subcarriers

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OFDM Symbol and FFT Where: positive subcarriers negative subcarriers unused subcarriers

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Guard Time with Cyclic Prefix (CP) CP from the periodicity IFFT{ X }CP

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OFDM Demodulator with See each block: No Inter Block Interference

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Overall Structure of OFDM Comms System IFFT+CP P/S FFT-CP S/P

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To recover the transmitted signal you need a very simple one gain equalization: received transm.noise channel Use simple Wiener Filter: Simple One Gain Equalization

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OFDM as Parallel Flat Fading Channels Significance: a Freq. Selective Channel becomes N Flat Fading Channels OFDM Mod OFDM Demod Frequency Selective channel N Flat Fading Channels

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OFDM Parameters Summarize basic OFDM Parameters: sampling rate in Hz N length of Data Field in number of samples L length of Cyclic Prefix in number of samples total number of Data Subcarriers data time data frequency guard

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IEEE a: Frequency Bands: GHz and GHz (12 channels) Modulation OFDM Range: 100m IEEE g Frequency Bands: GHz Modulation: OFDM Range: 300m

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Channel Parameters: FCC Example: the Unlicensed Band 5GHz U-NII (Unlicensed National Information Infrastructure) 4 channels in the range GHz 8 channels in the range GHz

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Channel Parameters: Example IEEE In terms of a Transmitter Spectrum Mask (Sec in IEEE Std a-1999) Typical Signal Spectrum Typical BW~16 MHz

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In either case: Sampling frequency FFT size Cyclic Prefix DATACP

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Sub-carriers: (48 data + 4 pilots) + (12 nulls) = 64 Pilots at: -21, -7, 7, 21 FrequencyTime IFFT

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DATA Frequencies: Subcarriers index

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Time Block:

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Overall Implementation (IEEE a with 16QAM). 1. Map encoded data into blocks of 192 bits and 48 symbols: data Encode Interleave … … Buffer (192 bits) … x48=192 bits Map to 16QAM … +1+j3 -1+j +3-j3 … +1-j

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Overall Implementation (IEEE a with 16QAM). 2. Map each block of 48 symbols into 64 samples +1+j3 … -3-j +3-j3 … +1-j IFFT time domain frequency domain null 24 data 2 pilots

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Constraints on OFDM Symbol Duration: to minimize CP overhead roughly!!! Frequency Spread Time Spread for channel Time Invariant Channel Parameters: Physical

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Summary of OFDM and Channel Parameters Channel: 1.Max Time Spread sec 2.Doppler Spread Hz 3.Bandwidth Hz 4.Channel Spacing Hz OFDM (design parameters): 1.Sampling Frequency 2.Cyclic Prefix 3.FFT size (power of 2) 4.Number of Carriers

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Channel: 1.Max Time Spread 2.Doppler Spread 3.Bandwidth 4.Channel Spacing OFDM (design parameters): 1.Sampling Frequency 2.Cyclic Prefix 3.FFT size (power of 2) 4.Number of Carriers Example: IEEE802.11a

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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 Applications: various Area Networks

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More Applications 1. WLAN (Wireless Local Area Network) standards and WiFi. In particular: IEEE a in Europe and North America HiperLAN /2 (High Performance LAN type 2) in Europe and North America MMAC (Mobile Multimedia Access Communication) in Japan 2. WMAN (Wireless Metropolitan Network) and WiMax IEEE Digital Broadcasting Digital Audio and Video Broadcasting (DAB, DVB) in Europe 4. Ultra Wide Band (UWB) Modulation a very large bandwidth for a very short time. 5. Proposed for IEEE (to come) for high mobility communications (cars, trains …)

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