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    • EE 552/452, Spring, 2008 Wireless Communications (and Networks) Zhu Han Department of Electrical and Computer Engineering Class 16 Mar. 13 th , 2008                                                             
    • Outline
      • Review
        • Nyquist 3 laws
      • Spread Spectrum
        • CDMA
        • Frequency hopping
      • Project1
      • Homework
      • Exam
    • Baseband binary data transmission system.
      • ISI arises when the channel is dispersive
      • Frequency limited -> time unlimited -> ISI
      • Time limited -> bandwidth unlimited -> bandpass channel -> time unlimited -> ISI
      p(t)
    • ISI Example 5 T 0 t Sequence of three pulses (1, 0, 1) sent at a rate 1/ T sequence sent 1 0 1 sequence received 1 1 (!) 1 Signal received Threshold 4 T 3 T 2 T T 0 - T -2 T -3 T
    • ISI
      • Nyquist three criteria
        • Pulse amplitudes can be detected correctly despite pulse spreading or overlapping, if there is no ISI at the decision-making instants
          • 1: At sampling points, no ISI
          • 2: At threshold, no ISI
          • 3: Areas within symbol period is zero, then no ISI
        • At least 14 points in the finals
          • 4 point for questions
          • 10 point like the homework
    • 1st Nyquist Criterion: Time domain p(t) : impulse response of a transmission system (infinite length) t 0 1 p(t) -1  shaping function Equally spaced zeros, interval no ISI !
    • 1st Nyquist Criterion: Time domain Suppose 1/T is the sample rate The necessary and sufficient condition for p(t) to satisfy Is that its Fourier transform P ( f ) satisfy
    • Raised cosine shaping 2w
      • Tradeoff: higher r, higher bandwidth, but smoother in time.
      W ω P ( ω ) r=0 r = 0.25 r = 0.50 r = 0.75 r = 1.00 0 t 0 p ( t )
    • spread-spectrum transmission
      • Three advantages over fixed spectrum
        • Spread-spectrum signals are highly resistant to noise and interference . The process of re-collecting a spread signal spreads out noise and interference, causing them to recede into the background.
        • Spread-spectrum signals are difficult to intercept . A Frequency-Hop spread-spectrum signal sounds like a momentary noise burst or simply an increase in the background noise for short Frequency-Hop codes on any narrowband receiver except a Frequency-Hop spread-spectrum receiver using the exact same channel sequence as was used by the transmitter.
        • Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference . The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently.
    • PN Sequence Generator
      • Pseudorandom sequence
        • Randomness and noise properties
        • Walsh, M-sequence, Gold, Kasami, Z4
        • Provide signal privacy
    • Direct Sequence (DS)-CDMA
      • It phase-modulates a sine wave pseudo-randomly with a continuous string of pseudo-noise code symbols called " chips ", each of which has a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate.
      • It uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal.
    • Direct Sequence Spread Spectrum
      • Unique code to differentiate all users
      • Sequence used for spreading have low cross-correlations
      • Allow many users to occupy all the frequency/bandwidth allocations at that same time
      • Processing gain is the system capacity
        • How many users the system can support
    • Spreading & Despreading
      • Spreading
        • Source signal is multiplied by a PN signal: 6.134, 6.135
      • Processing Gain:
      • Despreading
        • Spread signal is multiplied by the spreading code
      • Polar {±1} signal representation
    • Direct Sequence Spreading
    • Spreading & Despreading
    • CDMA – Multiple Users
      • One user’s information is the other’s interferences
      • If the interference structure can be explored, multiuser detection
        • Match filter
        • Decorrelator
        • MMSE decodor
        • Successive cancellation
        • Decision feedback
    • Performance of CDMA
      • All interference users can be viewed as noises, can be approximated by Gaussian noise (but not in reality)
      • Signal power 6.140
      • Noise power 6.142
      • Interference power is shown in 6.144
      • Small noise approximation 6.145
      • Near-far problem
    • Frequency Hopping Spread Spectrum
      • Frequency-hopping spread spectrum ( FHSS ) is a spread-spectrum method of transmitting radio signals by rapidly switching a carrier among many frequency channels , using a pseudorandom sequence known to both transmitter and receiver .
      • Military, bluetooth
    • Performance of Frequency Hopping
      • No collision 6.146
      • With collision 6.147
      • Probability of collision
        • Slotted FH, 6.148
        • Unslotted FH, 6.151
      • Probability of Error
        • Slotted FH, 6.149
        • Noise is zero, 6.150
        • Unslotted FH, 6.152
    • Road Map 1XRTT/3XRTT cdma2000 1999 2000 2001 2002 IS-95B 2G 2.5G 3G Phase 1 3G Phase 2 CDMA (IS 95 A) IS 95 B GSM TDMA EDGE UWC-136 GPRS W-CDMA 3X No 3X cdmaOne IS-95A 1X
    • 2G: IS-95A (1995)
      • Known as CDMAOne
      • Chip rate at 1.25Mbps
      • Convolutional codes, Viterbi Decoding
      • Downlink (Base station to mobile):
        • Walsh code 64-bit for channel separation
        • M-sequence 2 15 for cell separation
      • Uplink (Mobile to base station):
        • M-sequence 2 41 for channel and user separation
      8~13 kbps Speech Rate Up to 14.4 kbps Data Rate 1.25 Mbps (chip rate) Channel Bit Rate 600 mW Maximum Tx Power 64 Number of Channel BPSK/QPSK Modulation Scheme 1.25 MHz Channel Separation 824-849 MHz Downlink Frequency 869-894 MHz Uplink Frequency CDMA Multiple Access IS-95, ANSI J-STD-008 Standard
    • 2.5G: IS-95B (1998)
      • Increased data rate for internet applications
        • Up to 115 kbps (8 times that of 2G)
      • Support web browser format language
        • Wireless Application Protocol (WAP)
    • 3G Technology
      • Ability to receive live music, interactive web sessions, voice and data with multimedia features
      • Global Standard IMT-2000
        • CDMA 2000, proposed by TIA
        • W-CDMA, proposed by ARIB/ETSI
      • Issued by ITU (International Telecommunication Union)
      • Excellent voice quality
      • Data rate
        • 144 kbps in high mobility
        • 384 kbps in limited mobility
        • 2 Mbps in door
      • Frequency Band 1885-2025 MHz
      • Convolutional Codes
      • Turbo Codes for high data rates
    • 3G: CDMA2000 (2000)
      • CDMA 1xEV-DO
        • peak data rate 2.4 Mbps
        • supports mp3 transfer and video conferencing
      • CDMA 1xEV-DV
        • Integrated voice and high-speed data multimedia service up to 3.1 Mbps
      • Channel Bandwidth:
        • 1.25, 5, 10, 15 or 20 MHz
      • Chip rate at 3.6864 Mbps
      • Modulation Scheme
        • QPSK in downlink
        • BPSK in uplink
    • 3G: CDMA2000 Spreading Codes
      • Downlink
        • Variable length orthogonal Walsh sequences for channel separation
        • M-sequences 3x2 15 for cell separation (different phase shifts)
      • Uplink
        • Variable length orthogonal Walsh sequences for channel separation
        • M-sequences 2 41 for user separation (different phase shifts)
    • 3G: W-CDMA (2000)
      • Stands for “wideband” CDMA
      • Channel Bandwidth:
        • 5, 10 or 20 MHz
      • Chip rate at 4.096 Mbps
      • Modulation Scheme
        • QPSK in downlink
        • BPSK in uplink
      • Downlink
        • Variable length orthogonal sequences for channel separation
        • Gold sequences 2 18 for cell separation
      • Uplink
        • Variable length orthogonal sequences for channel separation
        • Gold sequences 2 41 for user separation