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10-IEEE and WiMax

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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 current developments, 4G technology will be supporting both data and voice on the same network and the standard IEEE (WiMax) and Long Term Evolution (LTE) are the candidates 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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IEEE Standard IEEE ( ):http://www.ieee802.org/16/ Part 16: Air Interface for Fixed Broadband Wireless Access Systems From the Abstract: It specifies air interface for fixed Broadband Wireless Access (BWA) systems supporting multimedia services; MAC supports point to multipoint with optional mesh topology; multiple physical layer (PHY) each suited to a particular operational environment:

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IEEE Standard WirelessMAN-SC, Single Carrier (SC), Line of Sight (LOS), 10-66GHz, TDD/FDD WirelessMAN-SCa, SC, 2-11GHz licensed bands,TDD/FDD WirelessMAN OFDM, 2-11GHZ licensed bands,TDD/FDD WirelessMAN-OFDMA, 2-11GHz licensed bands,TDD/FDD WirelessHUMAN 2-11GHz, unlicensed,TDD MAN: Metropolitan Area Network HUMAN: High Speed Unlicensed MAN Table 1 (Section 1.3.4) Air Interface Nomenclature:

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IEEE e 2005: Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems Amendment 2: Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands and Corrigendum 1 Scope (Section 1.1): it enhances IEEE to support mobility at vehicular speed, for combined fixed and mobile Broadband Wireless Access; higher level handover between base stations; licensed bands below 6GHz.

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IEEE : Reference Model (Section 1.4), Figure 1 By Layers: Service Specific Convergence Sublayer (CS) CS-SAP SAP=Service Access Point MAC Common Part Convergence Sublayer (CS) MAC-SAP Security Sublayer Physical Layer PHY-SAP MAC PHY Section 5 Section 6 Section 7 Section 8 External Data

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Parameters for IEEE (OFDM only) e-2005 Frequency Band2GHz-11GHz2GHz-11GHz fixed 2GHz-6GHz mobile OFDM carriersOFDM: 256 OFDMA: 2048 OFDM: 256 OFDMA: 128, 256, 512,1024, 2048 ModulationQPSK, 16QAM, 64QAM Transmission Rate1Mbps-75Mbps DuplexingTDD or FDD Channel Bandwidth(1,2,4,8)x1.75MHz (1,4,8,12)x1.25MHz 8.75MHz (1,2,4,8)x1.75MHz (1,4,8,12)x1.25MHz 8.75MHz

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randomization data Error Correction Coding TX IEEE Structure M-QAM mod OFDM mod De-rand. data Error Correction Decoding RX M-QAM dem OFDM dem Coding rates 1/2 2/3 3/4 5/6 M-QAM OFDM carriers Choices: Channel B/width 1.25 MHz 5 MHz 10 MHz …

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OFDM and OFDMA (Orthogonal Frequency Division Multiple Access) Mobile WiMax is based on OFDMA; OFDMA allows for subchannellization of data in both uplink and downlink; Subchannels are just subsets of the OFDM carriers: they can use contiguous or randomly allocated frequencies; FUSC: Full Use of Subcarriers. Each subchannel has up to 48 subcarriers evenly distributed through the entire band; PUSC: Partial Use of Subcarriers. Each subchannel has subcarriers randomly allocated within clusters (14 subcarriers per cluster).

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Section 8.3.2: OFDM Symbol Parameters and Transmitted Signal OFDM Symbol dataguard (CP)

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An OFDM Symbol is made of Data Carriers: data Pilot Carriers: synchronization and estimation Null Carriers: guard frequency bands and DC (at the modulating carrier) channel frequency pilots data Guard band

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FFT size N_used N_nulls N_pilots N_data OFDM Subcarrier Parameters: Fixed WiMax Fixed and Mobile WiMax

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IEEE , with N= IFFT Data (192) Pilots (8) Nulls (56)

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IEEE Implementation In addition to OFDM Modulator/Demodulator and Coding we need Time Synchronization: to detect when the packet begins Channel Estimation: needed in OFDM demodulator Channel Tracking: to track the time varying channel (for mobile only) In addition we need Frequency Offset Estimation: to compensate for phase errors and noise in the oscillators Offset tracking: to track synchronization errors

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Basic Structure of the Receiver WiMax Demodulator Demodulated Data Received Signal Time Synchronization: detect the beginning of the packet and OFDM symbol Channel Estimation: estimate the frequency response of the channel

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In IEEE (256 carriers, 64 CP) Time and Frequency Synchronization are performed by the Preamble. Long Preamble: composed of 2 OFDM Symbols Short Preamble: only the Second OFDM Symbol First OFDM SymbolSecond OFDM Symbol 320 samples 4 repetitions of a short pulse+CP 64 2 repetitions of a long pulse + CP Time Synchronization

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The standard specifies the Down Link preamble as QPSK for subcarriers between -100 and +100: Using the periodicity of the FFT:

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64 Short Preamble, to obtain the 4 repetitions, choose only subcarriers multiple of 4: Add Cyclic Prefix: 64

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Long Preamble: to obtain the 2 repetitions, choose only subcarriers multiple of 2 : 128 Add Cyclic Prefix: CP

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Several combinations for Up Link, Down Link and Multiple Antennas. We can generate a number of preambles as follows: With 2 Transmitting Antennas: With 4 Transmitting Antennas:

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Time Synchronization from Long Preamble preambleOFDM Symbols Received signal: xcorr Compute Crosscorrelation Coefficient: 1. Coarse Time Synchronization using Signal Autocorrelation

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MAX when Effect of Periodicity on Autocorrelation (no Multi Path). Let L=64. Max starts at …. Same signal data

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MAX when Effect of Periodicity on Autocorrelation (no Multi Path): … and ends at Same signal data

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MAX when Effect of Periodicity on Autocorrelation (with Multi Path of max length ): Max starts at …. Same signal data

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MAX when Effect of Periodicity on Autocorrelation (with Multi Path of max length ): and ends at Same signal data

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With Noise: Then, at the maximum:

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Information from Crosscorrelation coefficient: Estimate of Beginning of Data Estimate of Channel Length Estimate of SNR

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2. Fine Time Synchronization using Cross Correlation with Preamble xcorr Since the preamble is random (almost like white noise), it has a short autocorrelation: 64128

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… with dispersive channel xcorr Since the preamble is random, almost white, recall that the crosscorrelation yields the impulse response of the channel 64128

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However this expression is non causal. It can be written as (change index ): Which van be computed as the output of an FIR Filter with impulse response:

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Taking the time delay into account we obtain: Since the preamble is random, almost white, recall that the crosscorrelation yields the impulse response of the channel 64128

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Compare the two (non dispersive channel): Autocorrelation of received data Crosscorrelation with preamble

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Synchronization with Dispersive Channel Channel impulse response Autocorrelation of received data Crosscorrelation with preamble Start of Data

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Synchronization with Dispersive Channel Let be the length of the channel impulse response Channel impulse response

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In order to determine the starting point, compute the energy on a sliding window and choose the point of maximum energy xcorr Maximum energy L=max length of channel = length of CP

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xcorr Impulse response of channel Example

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Auto correlation Cross correlation max

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OFDM TX OFDM RX m-th data block Channel Estimation Recall that, at the receiver, we need the frequency response of the channel: Transmitted: Received: channel freq. response

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From the Preamble: at the beginning of the received packet. The transmitted signal in the preamble is known at the receiver: after time synchronization, we take the FFT of the received preamble Received Preamble: Estimated initial time 256 samples FFT

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Solve for using a Wiener Filter (due to noise): noise covariance Problem: when we cannot compute the corresponding frequency response Fact: by definition, if otherwise (ie DC, odd values, frequency guards)

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Two solutions: 1. Compute the channel estimate only for the frequencies k such that and interpolate for the other frequencies. This might not yield good results and the channel estimate might be unreliable; known interpolate

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2.Recall the FFT and use the fact that we know the maximum length L of the channel impulse response Since the preamble is such that either or for the indices where we can write: for so that we have 100 equations and L=64 unknowns.

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This can be written in matrix form: where

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Write it in matrix form:

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Least Squares solution this is ill conditioned. eigenvalues

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1. Generate matrix kF=[2,4,6,…,100, 156, …, 254]’; non-null frequencies (data and pilots) n=[0,…,63]; time index for channel impulse response V=exp(-j*(2*pi/256)*kF*n); M=inv(V’*V+0.001*eye(64))*V’; Channel Frequency Response Estimation: 2. Generate vector z from received data y[n]: Let n0 be the estimated beginning of the data, from time synchronization. Then y0=y(n0-256:n0-1); received preamble Y0=fft(y0); decoded preamble z=Y0(kF+1).*conj(Xp256(kF+1))/2; multiply by transmitted preamble h=M*z; channel impulse response 3. Channel Frequency Response: H=fft(h, 256);

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Data in Trigger when preamble is detected Channel Estimate out Simulink Implementation

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Example: Spectrum of Received Signal Estimated Frequency Response of Channel NOT TO SCALE As expected, it does not match in the Frequency Guards

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WiMax-2004 Demodulator WiMax256.mdl data Ch. Start after processing preamble Standard OFDM Demod (256 carriers) Error Correction Decoding

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Channel Tracking In mobile applications, the channel changes and we need to track it. IEEE tracks the channel by embedding pilots within the data. In the FUSC (Full Use of Sub Carriers) scheme, the pilots subcarriers are chosen within the non-null subcarriers as with

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nulls DC (null) pilots data nulls OFDM Symbol m subcarrier k

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