DCPR RRC Pulse Shaping to Increase Capacity

1
DCPR RRC Pulse Shaping to Increase Capacity

• Ryan Shoup, John Taylor, Bob Wezalis, Josh
  Model
• Aug 2004

2
Rationale / Overview

• Root Raised Cosine (RRC) Review
• Spectrally efficient waveform
• Implementation
• General RRC considerations wrt DCPR
• Envelope variation
• Performance considerations
• Timing recovery
• Matched filter demodulation
• General considerations when increasing DCPR
  capacity
• Laboratory demonstration of the Current
  modulation scheme with Root Raised Cosine (RRC)
  Filtering
• Concept
• Laboratory demonstration

3
GOES Data Collection System (Today)

• Communications link designed to relay information
  gathered from data collection platforms (DCPs)
  located throughout Western Hemisphere
• 400 KHz bandwidth allocated for the the GOES DCS
  communications link
• Multiple Access system
• 200 FDMA channels _at_ 1500 Hz
• 33 FDMA channels _at_ 3000 Hz
• Three data rates supported per channel
• 100 bps BPSK modulation _at_ 1500 Hz
• 300 bps 8-PSK TCM modulation _at_ 50 dBm (max) and
  1500 Hz
• 1200 bps 8-PSK TCM modulation _at_ 53 dBm (max) and
  3000 Hz
• Channels alternate between two satellites
• DCP Messages comprise of a header information
• Desirable to modify system to support growth in
  DCPs during the GOES-R era

4
DCPR 8-PSK Performance
5
RRC

• Root Raised Cosine (RRC) Filtering implemented in
  software/firmware
• Spectrally efficient pulse shape
• Pulse shape well suited to accommodate modest
  amount of growth to system
• Capacity increase of 2x
• Implementation in transmitter relatively easy via
  digital filtering
• FPGA, D/A, and LPF
• Components probably in many transmitters already
  anyhow
• Commercial ICs also available to perform RRC
  filtering
• Pulse shape parameters
• Rolloff coefficient (?)
• 0 ? ? ? 1
• Bandwidth required ? as ? ?
• BER performance sensitivity ? as ? ?
• Number of coefficients (N)
• Bandwidth required ? as N ?
• BER performance ? as N ?
• Wide use of RRC
• Cellular telephony
• Satellite

6
Notional RRC Transmitter
Notional DCPR transmitter
Baseband RRC Waveform
DCPR Message Formatter
Sensor
TCM
D/A
Upconverter
FIR Filter
LPF
HPA

• RRC filtering would be performed on data
  collected from sensor
• FIR Filter easily performed on small low cost
  CPLD/FPGA or software via microprocessor
• Only low speed D/A ( 15 KHz) needed
• Analog LPF removes D/A sampling images created
  at the sample frequency

7
Notional RRC Receiver
Notional DCPR 8-PSK Receiver
Trellis Decoder
Down Sample
Phase Compensation
Downconverter
LPF
A/D
FIR Filter
Peak Matched Filter Output Detector
Digital PLL

• Rx employs RRC matched filter detection for best
  performance
• Rx employs software/circuitry to detect/track the
  timing of peak outputs of the matched filter
• A digital PLL can be used with an easily
  software configurable bandwidth
• Trellis decoder remains same as used today

8
RRC Implementation Tradeoffs

• Practical baseband spectrum approaches
  theoretical as Coeffs ?
• 100 Coefficients results in near ideal baseband
  spectrum

9
RRC Implementation Tradeoffs

• Practical baseband spectrum more closely
  resembles theoretical as ? ?

10
RRC Considerations Envelope Variation

• Although average power same, instantaneous power
  varies more than that of a waveform that occupies
  more bandwidth
• Effect measured or quantified by the peak to
  average ratio
• Ideally, transmit HPA will be linear over full
  range of instantaneous power
• If effect not mitigated, may potentially require
  larger transmitter power amplifiers
• Effect of envelope variation mitigated by
• Use of higher value of roll off coefficient
• Use of coding to reduce required EB/N0
• High Power Amplifier linearization techniques

11
8-PSK Peak to Average
Legend
1
Standard-compliant filter
RRC, alpha 1.0
0.8
RRC, alpha 0.1
Probability
0.6
0.4
0.2
0
0
1
2
3
4
5
6
7
8
9
10
Instantaneous Peak Power to Average Power Ratio
3 to 4 dB
6 dB
Instantaneous RRC 8-PSK power can be upto 2-4 dB
higher than notional standards-compliant filter
7.25 dB
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RRC Considerations Matched Filtering

• Optimal implementation requires that CDA employ
  matched filter demodulation
• Filter matched to transmitted RRC waveform
• Without matched filter, loss can be on the order
  of 1 dB

DCPR 8-PSK TCM Asymptotic BER
-2
10
Integrate and Dump
-3
10
-4
10
Probability of Bit Error
-5
10
-6
10
-7
10
Ideal
-8
10
3
4
5
6
7
8
9
10
SNR per bit (EB/N0)
No matched filter at receiver degrades BER
13
RRC Considerations Matched Filtering
DCPR 8-PSK TCM Asymptotic BER

• With matched filter at the receiver, only
  relatively few coefficients needed for near ideal
  BER

-2
10
Few Coeffs
-3
10
-4
10
Probability of Bit Error
-5
10
-6
10
-7
10
Ideal
-8
10
3
4
5
6
7
8
9
10
SNR per bit (EB/N0)
14
RRC Considerations Timing Recovery

• Ideal BER performance requires matched filter
  output sampled at appropriate time
• Error results in BER degradation due to ISI and
  SNR loss
• Performance degradation function of timing error
  and ?

Loss due to ISI Incurred from timing error
Example RRC sensitivity to timing error
0.2 dB
Performance Loss
1.2 dB
2.2 dB
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DCPR Capacity Considerations

• DCP Transmit Power Levels
• Keep at current levels to minimize changes to
  DCPs
• Desirable to avoid need for new antennas, larger
  power amplifiers, etc.
• Ideally DCP power levels even reduced to avoid
  issues with instantaneous power variation
  associated with RRC waveforms
• Satellite Power Levels
• Desire to keep signal power levels through
  satellite only moderately greater than that
  associated with GOES NOP series
• Power limitations on AGC circuitry and amplifier
• Transmit Power Levels
• Need to ensure compliance with PFD requirements
• Neg 154 dBW per m2 per 4 KHz
• Current levels 10 dB lower when channels fully
  loaded
• Frequency tolerance
• Need to consider frequency tolerance
  specification when adding additional FDMA
  channels

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8-PSK TCM with RRC Filtering

• Apply RRC filtering to the current modulation
• Roll off coefficient ? 1.0
• Theoretical Bandwidth required 300 Hz
• Subdivide the current 1500 Hz DCPR channel into
  two channels doubling the number of 300 bps
  channels available
• Allocate 750 Hz per channel
• More than necessary to accommodate actual
  (non-ideal) waveform, frequency tolerance
  specification, and some guard band
• Additional channels decrease power per channel
• If AGC limits power levels, then EB/N0 at the
  ground receiver decreases
• Capability for additional channels function of
  specified minimum G/T at ground site
• Increasing min ground station G/T by 3 dB would
  accommodate a doubling of capacity while
  maintaining satellite power levels to those seen
  today
• Only necessary for DRGS sites, as Wallops G/T far
  exceeds specified min DCPR min receive G/T
• Draft version of GOES-R IRD calls for G/T
  increase as well as satellite downlink EIRP
  increase

17
RRC 8-PSK TCM Demo

• Benchtop demonstration to demonstrate 8-PSK TCM
  RRC BER performance
• Demonstration details
• Data sequence (PRN) generation and Trellis
  encoding performed in a Xilinx FPGA
• RRC filtering and upconversion performed by RS
  Signal Generator (SMIQ)
• Noise added at RF via Carrier to Noise Generator
  (CNG)
• Downconversion done using mini-circuits RF
  component mixer and HP signal generator (LO)
• Labjack used to A/D baseband I/Q signals
• PC microprocessor receiver performs timing
  recovery, phase recovery, matched filter
  demodulation, trellis decoding, and measures BER
• Frequency recovery not needed as HP LO frequency
  locked to SMIQ
• Observed BER performance 1 dB from theoretical
• Caveat Transmit amplifiers operated in linear
  region

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RRC 8-PSK TCM Demo Block Diagram
Frequency Reference
HP Sig Gen
CNG
70 MHz
Xilinx FPGA
RS SMIQ
Mixer
Data (300 bps) Clock
Interface Board
Baseband I/Q
IQ A/D samples
Labjack
PC
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RRC 8-PSK TCM Demo Photo
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RRC 8-PSK TCM Demo Photo
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RRC 8-PSK TCM Demo Spectrum
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RRC 8-PSK TCM Demo BER
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Summary

• RRC filtering effective means to increase
  capacity of DCPR system to accommodate future
  growth
• RRC filtering introduces issues
• Power levels
• Transmit filtering
• Timing recovery
• Need to perform Matched Filtering
• At DCPR data rates, transmit filtering, matched
  filtering, phase/frequency recovery easily
  implemented in low cost FPGAs, or microprocessors