DCPR-1 2/26/2004 MIT Lincoln Laboratory DCPR RRC Pulse Shaping to Increase Capacity Ryan Shoup, John Taylor, Bob Wezalis, & Josh Model Aug 2004

DCPR-12/26/2004

MIT Lincoln Laboratory

DCPR RRC Pulse Shaping to Increase Capacity

Ryan Shoup, John Taylor, Bob Wezalis, & Josh Model

Aug 2004

MIT Lincoln Laboratory2

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


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 @ 1500 Hz• 33 FDMA channels @ 3000 Hz

– Three data rates supported per channel• 100 bps BPSK modulation @ 1500 Hz• 300 bps 8-PSK TCM modulation @ 50 dBm (max) and 1500 Hz• 1200 bps 8-PSK TCM modulation @ 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


DCPR 8-PSK Performance

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

2 3 4 5 6 7 8 9 10 11 12

SNR per Bit (EB/N0)

BE

R

Trellis Encoded DCPR 8-PSK BER (Theoretical/Asymptotic)

Trellis Encoded DCPR 8-PSK BER (Simulated)

Uncoded QPSK BER (Theoretical)


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


Notional RRC Transmitter

Sensor

Notional DCPR transmitter

TCM D/ADCPR MessageFormatter

UpconverterFIR Filter

BasebandRRC Waveform

LPF

• 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

HPA


Notional RRC Receiver

Notional DCPR 8-PSK Receiver

TrellisDecoder

Downconverter LPF A/D FIR Filter

Peak Matched FilterOutput Detector

DownSample

PhaseCompensation

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


RRC Implementation Tradeoffs

• Practical baseband spectrum approaches theoretical as Coeffs

• 100 Coefficients results in near ideal baseband spectrum


RRC Implementation Tradeoffs

• Practical baseband spectrum more closely resembles theoretical as


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


0 1 2 3 4 5 6 7 8 9 100

0.2

0.4

0.6

0.8

1

Instantaneous Peak Powerto Average Power Ratio

Pro

babi

lity

Standard-compliant filter

RRC, alpha = 1.0

RRC, alpha = 0.1

Legend

~ 3 to 4 dB

~ 6 dB~ 7.25 dB

8-PSK Peak to Average

Instantaneous RRC 8-PSK power can be upto 2-4 dB higherthan notional standards-compliant filter


DCPR 8-PSK TCM Asymptotic BER• 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

10 -8

10 -7

10 -6

10 -5

10 -4

10 -3

10 -2

Pro

bab

ility

of

Bit

Err

or

3 4 5 6 7 8 9 10

SNR per bit (EB/N0)

Ideal

Integrate and Dump

RRC Considerations: Matched Filtering

No matched filter at receiver degrades BER


3 4 5 6 7 8 9 1010 -8

10 -7

10 -6

10 -5

10 -4

10 -3

10 -2

Pro

bab

ility

of

Bit

Err

or

DCPR 8-PSK TCM Asymptotic BER

Ideal

Few Coeffs

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

RRC Considerations: Matched Filtering

SNR per bit (EB/N0)


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

1.2 dB0.2 dB

2.2 dB

Example: RRC sensitivityto timing error

Loss due to ISIIncurred fromtiming error1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

9 10 11 12 13 14 15 16 17

SNR per Bit

BE

R

'Uncoded 8-PSK (Theoretical) RRC, a=0.1, Timing Error = 5% (Simulated)

RRC, a=0.99, Timing Error = 5% (Simulated) RRC, a=0.35, Timing Error = 5% (Simulated)

Performance Loss:


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


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


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 R&S 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


RRC 8-PSK TCM Demo Block Diagram

XilinxFPGA

R&SSMIQ

Data (300 bps)Clock

CNG

+70 MHz

Mixer

HPSig Gen

InterfaceBoard

FrequencyReference

Labjack PC

Baseband I/Q

I&Q A/D samples


RRC 8-PSK TCM Demo Photo


RRC 8-PSK TCM Demo Photo


RRC 8-PSK TCM Demo Spectrum

300 BPS 8-PSK Spectra (Measured)

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-500 -450 -400 -350 -300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 400 450 500

Frequency Relative to Center (Hz)

Mag

nit

ud

e (d

B)


RRC 8-PSK TCM Demo BER

RRC 8-PSK Trellis Coding

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

1.00E-01

2 3 4 5 6 7 8 9 10

SNR Per Bit (EB/N0)

Pro

bab

ilit

y o

f B

it E

rro

r

DCPR TCM (Simulated)

S/W Rx TCM 8-PSKPerformance (Measured)


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

Documents

DCPR-1 2/26/2004 MIT Lincoln Laboratory DCPR RRC Pulse Shaping to Increase Capacity Ryan Shoup, John Taylor, Bob Wezalis, & Josh Model Aug 2004