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10-IEEE802.16 and WiMax

10-IEEE802.16 and WiMax. Applications: various Area Networks. According to the applications, we define three “Area Networks”:

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10-IEEE802.16 and WiMax

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  1. 10-IEEE802.16 and WiMax

  2. 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 IEEE802.11 (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 IEEE802.16 (WiMax) and Long Term Evolution (LTE) are the candidates

  3. More Applications • 1. WLAN (Wireless Local Area Network) standards and WiFi. In particular: • IEEE 802.11a 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 802.16 • 3. 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 802.20 (to come) for high mobility communications (cars, trains …)

  4. IEEE 802.16 Standard • IEEE 802.16 2004( 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:

  5. IEEE 802.16-2004 Standard Table 1 (Section 1.3.4) Air Interface Nomenclature: • 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

  6. IEEE 802.16e 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 802.16-2004 to support mobility at vehicular speed, for combined fixed and mobile Broadband Wireless Access; • higher level handover between base stations; • licensed bands below 6GHz.

  7. IEEE 802.16-2004: Reference Model (Section 1.4), Figure 1 External Data SAP=Service Access Point By Layers: CS-SAP Service Specific Convergence Sublayer (CS) Section 5 MAC-SAP MAC MAC Common Part Convergence Sublayer (CS) Section 6 Security Sublayer Section 7 PHY-SAP Physical Layer PHY Section 8

  8. Parameters for IEEE 802.16 (OFDM only)

  9. IEEE802.16 Structure data Error CorrectionCoding M-QAM mod OFDM mod TX randomization data Error CorrectionDecoding M-QAM dem OFDM dem RX De-rand. Choices:

  10. 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) .

  11. Section 8.3.2: OFDM Symbol Parameters and Transmitted Signal OFDM Symbol guard (CP) data

  12. 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) pilots data frequency Guard band channel Guard band

  13. OFDM Subcarrier Parameters: Fixed and Mobile WiMax Fixed WiMax

  14. IEEE 802.16, with N=256 Data (192) 0 0 Pilots (8) 12 13 Nulls (56) 24 38 24 63 24 88 12 100 101 IFFT 155 156 12 168 24 193 24 218 24 243 12 255 255

  15. IEEE802.16 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

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

  17. Time Synchronization In IEEE802.16 (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 Symbol Second OFDM Symbol 320 samples 320 samples 2 repetitions of a long pulse + CP 4 repetitions of a short pulse+CP 64 64 64 64 64 128 128 64

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

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

  20. Long Preamble: to obtain the 2 repetitions, choose only subcarriers multiple of 2 : 128 128 Add Cyclic Prefix: 64 128 128 CP

  21. 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:

  22. Time Synchronization from Long Preamble 1. Coarse Time Synchronization using Signal Autocorrelation Received signal: preamble OFDM Symbols 64 128 128 Compute Crosscorrelation Coefficient: xcorr

  23. Effect of Periodicity on Autocorrelation (no Multi Path). Let L=64. Max starts at …. Same signal data 64 128 128 data 64 128 128 MAX when

  24. Effect of Periodicity on Autocorrelation (no Multi Path): … and ends at Same signal data 64 128 128 data 64 128 128 MAX when

  25. Effect of Periodicity on Autocorrelation (with Multi Path of max length ): Max starts at …. Same signal 64 data 128 128 64 data 128 128 MAX when

  26. Effect of Periodicity on Autocorrelation (with Multi Path of max length ): and ends at Same signal 64 data 128 128 64 data 128 128 MAX when

  27. With Noise: Then, at the maximum:

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

  29. 2. Fine Time Synchronization using Cross Correlation with Preamble xcorr Since the preamble is random (almost like white noise), it has a short autocorrelation: 64 128 128 128

  30. … with dispersive channel xcorr Since the preamble is random, almost white, recall that the crosscorrelation yields the impulse response of the channel 64 128 128 128

  31. 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:

  32. 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 64 128 128 128

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

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

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

  36. 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

  37. Example xcorr Impulse response of channel

  38. Auto correlation max Cross correlation

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

  40. 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 Estimated initial time Received Preamble: 64 128 128 256 samples FFT

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

  42. 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

  43. 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.

  44. This can be written in matrix form: where

  45. Write it in matrix form:

  46. Least Squares solution this is ill conditioned. eigenvalues

  47. Channel Frequency Response Estimation: 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’; 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);

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

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

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

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