Adaptive Lattice Filters for CDMA Overlay

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Adaptive Lattice Filtersfor CDMA Overlay

• DSP 2 Project Presentation
• By
• Rajat Kapur
• AdityaKiran Jagannatham

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CDMA Technology

• CDMA is a Multiple Access wireless technique.
• Uses the idea of Spread Spectrum
• Benefits of CDMA
• 1.Capacity increases of 8 to 10 times that of an
  AMPS analog system
• 2. Improved call quality,
• 3.Simplified system planning through frequency
  reuse.
• 4.Enhanced privacy and Bandwidth on demand
• 5. Possibility of fewer cell sites

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(No Transcript)
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The Overlay Concept

• Motivation Increasing demand for BW in Mobile
  Comm.
• Establishment of PCN in the 1.85-1.99 GHz Band
  (92)
• Previously occupied by Narrow-band Microwave
  Signals
• This situation of Spectrum Sharing
  CDMA Overlay.
• Initial experiments in Houston, Orlando, San
  Diego using
• Broad-Band CDMA
• Goals
• 1. Overlay would not excessively interfere with
  N-Band
• 2. PCN users can operate efficiently in the
  overlay environment
• 3. Conform to PCN philosophy (100?W in 183m cell
  radius)

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Adaptive Filtering in CDMA Overlay

• LMS Filtering employed in N-Band interference
  rejection (96)
• Lattice Filtering suggested as an alternative by
  J.Wang and V.Prahatheesan (2K)
• Lattice shown to outperform LMS

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Overlay Receiver
CDMA Receiver
Nband Filter
Channel
Hard Decision
Lattice Filter
BP Filter
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• P represents Signal Power
• f0 is the CDMA Carrier Frequency
• bk kth user binary information
• N Tb/Tc Processing Gain
• ?k Rayleigh Fading Parameter ( E(?k2) 2? )
• ?k Random Phase 0,2?
• ?k Path Delay 0, Tb (Rayleigh Flat Fading
  Channel)
• Bc 2/Tc - CDMA Signal Bandwidth
• j(t) Narrow-Band Interference Signal
• n(t) Band-Limited AWGN (PSD N0/2)

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• jc(t), js(t) Inphase and Quadrature Narrow-Band
  Components
• ? frequency offset from CDMA Carrier
• p Bj/Bc
• q ?Tc

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Lattice Filter Structure
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• Lattice Recursive Equation

• Cleaned CDMA Signal

.which is the final stage lattice output
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Analysis

• Almost EXACT analysis !!!
• Reflection Coefficient Update Equation is given
  as
• Ta - Update Interval
• Ta ? 2/Bj, 2/Bj Input Correlation Time
• ? - Step Size
• Signal Sampled _at_ Tc (Chip Time)
• Input Signal independent at update intervals
• No need to ASSUME Independence !
• Central Limit Principle applied

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Analysis Contd

• Input at sample time intervals is given as

• Correlation of input samples

• It can be derived.

• Observation Correlation at Tc , 2Tc exclusively
  from N-Band Signal
• Tc ? Correlation Time of CDMA Signal
• Hence is analogous to White Noise

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Analysis Contd

• Reflection Coefficient at jth iteration

• The Product term indicates dependence on past
  data
• For a large number of co-channel users ( K 30
  or gt) , the term

• Can be simplified as

using CLP

• To yield

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Analysis Contd

• Which in the limit yields

• clearly showing ER1 depends on step size ? !!!
• Observe If ? 0

• the optimal Wiener Filter Coefficient
• Similarly, it can shown that

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Analysis Contd

• Where A is given by

pretty complicated !!!
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Analysis Contd

• SNR Calculation The Despreader O/P is given as

• bi(?) - ?th bit of ith user
• J NBand interference, N Interference from
  Noise
• I Co-Channel User Interference

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Analysis Contd

• where

• this FINALLY concludes our analysis !!!
• Precise Details can be found in references

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Simulations

• System Specs.
• K 30 (No. of Co-Channel Users), ? 0.1 (-7 dB
  Fading)
• p Bj/Bc 5 (0.05) ? Ta 20 Tc
• q ?Tc 0 (?0)
• N Tb/Tc Processing Gain 750
• J/S 17,20,23 dB
• b 32 Kbps, BPSK Signal
• Link Specs.
• f0 1.884 GHz (B-M), 1.956 GHz (M-B)
• Chip Rate 24 Mchips/sec ? Tc 1/24E6
• 48 MHz BW for each DS Waveform
• N-Band Interference - 64 QAM _at_ 45 Mbps
• Specs. taken from On the Feasibility of a CDMA
  Overlay for PCN (92)

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Simulation Results Convergence behavior of R1
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Results Contd Convergence behavior of R1
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Results Contd LMS Vs Lattice SNR Performance
From Adaptive Lattice Filters for CDMA Overlay
(Trans. Comm., 2K)
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Sim. Log.

• Simulations done in Base-Band
• Iterations of the order 750 X 30 X 30 750 X 30
  X 40
• Random Binary Sequences used as PN Sequences
• ? 1 for user ? no attenuation on Direct Path
• White Noise used

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Conclusions

• CDMA Overlay effective for frequency re-use
• Each stage of the Lattice Converges independent
  of others
• Lattice Filter provides faster rate of
  convergence compared to LMS Filter
• Lattice Filter has good capability of Narrow Band
  
• Interference Suppression

References

• Adaptive Lattice Filters for CDMA Overlay-
  Trans. Comm., 2K
• Adaptive LMS Filters for Cellular CDMA Overlay-
  Select Areas in Comm., 96
• On the Feasibility of CDMA Overlay for PCN-
  Select Areas in Comm.,92
• Cellular CDMA Overlay Systems- IEE Proc. Comm.,
  96