Wireless Physical Layer Concepts: Part IIIjain/cse574-10/ftp/j_5phy.pdf · Wireless Physical Layer Concepts: Part III Raj Jain Professor of CSE Washington University in Saint Louis
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Based on measured data in the field1. Hata Model2. COST 231 Extension to Hata Model3. COST 231-Walfish-Ikegami Model4. Erceg Model5. Stanford University Interim (SUI) Models6. ITU Path Loss Models
Based on 1968 measurement in Tokyo by OkumuraClosed form expression by Hata in 1980 fc = carrier frequency, ht = height of the transmitting (base station) antenna, hr = height of the receiving (mobile) antennaa() = correction factor for the mobile antenna height based on the size of the coverage areaDesigned for 150-1500 MHz
, 10( ) = 69.55 26.16 ( )logL urban cP d dB f+
10 10 1013.82 ( ) ( ) (44.9 6.55 ( )) ( )log log logt r th a h h d− − + −
ITU Path Loss ModelsITU Path Loss ModelsIndoor office, outdoor-to-indoor pedestrian, and vehicular. Low delay spread (A), medium delay spread (B)Pedestrian:
User multiple receive antennaSelection combining: Select antenna with highest SNRThreshold combining: Select the first antenna with SNR above a thresholdMaximal Ratio Combining: Phase is adjusted so that all signals have the same phase. Then weighted sum is used to maximize SNR
Use multiple antennas to transmit the signalAmple space, power, and processing capacity at the transmitter (but not at the receiver).If the channel is known, phase each component and weight it before transmission so that they arrive in phase at the receiverand maximize SNRIf the channel is not known, use space time block codes
Phased Antenna Arrays: Receive the same signal using multiple antennasBy phase-shifting various received signals and then summing ⇒ Focus on a narrow directional beamDigital Signal Processing (DSP) is used for signal processing ⇒ Self-aligning
Space Time Block Codes (STBC)Space Time Block Codes (STBC)Invented 1998 by Vahid Tarokh.Transmit multiple redundant copies from multiple antennasPrecisely coordinate distribution of symbols in space and time. Receiver combines multiple copies of the received signals optimally to overcome multipath.Example: Two antennas:
S1 S2-S2* S1*
Space
TimeAntenna 1 Antenna 2
Slot 1Slot 2
S1* is complex conjugate of S1 ⇒ columns are orthogonal
Most WiMAX deployments will use TDD.Allows more flexible sharing of DL/UL data rateDoes not require paired spectrumEasy channel estimation ⇒ Simpler transceiver designCon: All neighboring BS should time synchronize
OFDMOFDMOrthogonal Frequency Division MultiplexingTen 100 kHz channels are better than one 1 MHz Channel ⇒ Multi-carrier modulation
Frequency band is divided into 256 or more sub-bands. Orthogonal ⇒ Peak of one at null of othersEach carrier is modulated with a BPSK, QPSK, 16-QAM, 64-QAM etc depending on the noise (Frequency selective fading)Used in 802.11a/g, 802.16, Digital Video Broadcast handheld (DVB-H) Easy to implement using FFT/IFFT
Advantages of OFDMAdvantages of OFDMEasy to implement using FFT/IFFTComputational complexity = O(B log BT) compared to previous O(B2T) for Equalization. Here B is the bandwidth and T is the delay spread.Graceful degradation if excess delayRobustness against frequency selective burst errorsAllows adaptive modulation and coding of subcarriersRobust against narrowband interference (affecting only some subcarriers)Allows pilot subcarriers for channel estimation
OFDM: Design considerationsOFDM: Design considerationsLarge number of carriers ⇒ Smaller data rate per carrier⇒ Larger symbol duration ⇒ Less inter-symbol interferenceReduced subcarrier spacing ⇒ Increased inter-carrier interference due to Doppler spread in mobile applicationsEasily implemented as Inverse Discrete Fourier Transform (IDFT) of data symbol blockFast Fourier Transform (FFT) is a computationally efficient way of computing DFT
Orthogonal Frequency Division Multiple AccessEach user has a subset of subcarriers for a few slotsOFDM systems use TDMAOFDMA allows Time+Freq DMA ⇒ 2D Scheduling
In a scalable OFDMA system, the number of carriers for 10 MHz channel is 1024. How many carriers will be used if the channel was 1.25 MHz, 5 MHz, or 8.75 MHz.
OFDMA, http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiple_accessHistory of OFDMA and How it Works, http://www.webopedia.com/DidYouKnow/Computer_Science/2005/OFDMA.asp