# Presentation: 173883

## DCPR RRC Pulse Shaping to Increase Capacity

- Ryan Shoup, John Taylor, Bob Wezalis, & Josh Model
- Aug 2004

## 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

**Notes:**
Briefing addresses applying RRC filtering to the DCPR system
Two RRC modulation approaches are discussed.  First is applying RRC to the current modulation to reduce the per-channel bandwidth from 1500 Hz to 500 Hz using the current TCM modulation.  The next considers turbo coding and OQPSK.

## 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

## 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/A

- DCPR
- Message
- Formatter

- Upconverter

- FIR Filter

- Baseband
- RRC 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_

- Trellis
- Decoder

- Downconverter

- LPF

- A/D

- FIR Filter

- Peak Matched Filter
- Output Detector

- Down
- Sample

- Phase
- Compensation

- 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

## 8-PSK Peak to Average

- 0

- 1

- 2

- 3

- 4

- 5

- 6

- 7

- 8

- 9

- 10

- 0

- 0.2

- 0.4

- 0.6

- 0.8

- 1

- Instantaneous Peak Power
- to Average Power Ratio

- Probability

- Standard-compliant filter

- RRC, alpha = 1.0

- RRC, alpha = 0.1

- Legend

- ~ 3 to 4 dB

- ~ 6 dB

- ~ 7.25 dB

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

## 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

- 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

- Probability of Bit Error

- 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

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

- 3

- 4

- 5

- 6

- 7

- 8

- 9

- 10

- 10

- -8

- 10

- -7

- 10

- -6

- 10

- -5

- 10

- -4

- 10

- -3

- 10

- -2

- Probability of Bit Error

- 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 dB

- 0.2 dB

- 2.2 dB

- Example: RRC sensitivity
- to timing error

- Loss due to ISI
- Incurred from
- timing error

- 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

- Xilinx
- FPGA

- R&S
- SMIQ

- Data (300 bps)
- Clock

- CNG

- 70 MHz

- Mixer

- HP
- Sig Gen

- Interface
- Board

- Frequency
- Reference

- 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

## RRC 8-PSK TCM Demo BER

## 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