Simulcast Systems for Public Safety Rick Taylor Senior Scientist, - - PowerPoint PPT Presentation

Presentation1 29-Nov-11

Simulcast Systems for Public Safety

Rick Taylor Senior Scientist, PSPC Lynchburg, Va 1 Dec 2011

Presentation2

Public Safety Mission Critical Communications Systems

• Secure Wide-area Voice and Data

Networking

• Interoperability and Reliability are Key
• Fragmented Frequency Bands
• Rapid Access - Sub-second Across Network
• Mostly Group Calls (“One to Many”)
• Near Ubiquitous Coverage

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Near Ubiquitous Coverage

• Coverage Reliability Requirement is Typically 95-98%
• Service Areas: From Small Towns Through Statewide
• 3 Watts Portable Output Power

Portable In- Building Portable In-Basement Mobile Towers Typically Spaced 2- 20 miles 30 dB Building Loss Typical 100 W Base Station Output Power + Ant Gain Typical

Presentation4 June 8, 2007

P25I P Trunked Sites

P25 IP Trunked Multisite System

I P Consoles I P WAN Network Switching Center Network Management Network Administration I nteroperability Gateways

Presentation5 June 8, 2007

P25I P Simulcast Sites

P25 IP Simulcast Systems

I P Consoles I P WAN Network Switching Center Network Management Network Administration I nteroperability Gateways Control Point & Voters

• Used Since the Late 80’s

Primarily in Urban Areas

Presentation6 29-Nov-11

Simulcast System Transmit Overlap

Delay Spread = Abs(tb – ta)

Too Much Causes Communications Loss!

tb

propagation delay

Both Towers Transmit the Same Signal at Essentially the Same Time Typically > 3 miles Site Separation

ta

propagation delay

Note: Typical Harris simulcast systems have more than two sites

(Delay Spread Only Significantly Effects the Sites’ TX Signal)

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Delay Spread from a P25 (C4FM) 3-Site Simulcast System

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Pink Signal Overlap Areas Have Excessive Delay Spread to Communicate Less Pink = Better Coverage

What Can Be Done with the Modulation to Improve the Tolerance to Delay Spread?

Presentation8 29-Nov-11

Backround: Review of Eye Patterns

• An Overlay of Time Segments of the Demodulated Digital Signal, with

Each Segment an Integer Multiple of the Bit (or Symbol) Period

EDACS Demodulated Signal

S y stem V iew 10 .4 5e

• 3

10 .4 5e

• 3

10 .6 5e

• 3

10 .6 5e

• 3

10 .8 5e

• 3

10 .8 5e

• 3

11 .0 5e

• 3

11 .0 5e

• 3

11 .2 5e

• 3

11 .2 5e

• 3

1 50 0e-3

• 50

0e-3

• 1

A m p l i t u d e Ti m e in S econd s E x tra cted from w 0 (S am ples 1 00 0 to 1 100 )

2-LEVEL EDACS DEMODULATED SIGNAL

• 500e-3
• 1

Eye Pattern Showing Two Bit Periods

Presentation9 29-Nov-11

Background: Recovering the Information from a 2-Level FSK Modulation An “Open” Eye Reduces Errors in Data Recovery (i.e. Lowers Bit Error Rate)

• 500e-3
• 1

Eye

Threshold

Declare “1” Declare “0”

Desired Sample Point

Presentation10 29-Nov-11

Eye Pattern of P25 Phase 1 4-Level C4FM Modulation

• 2
• 4

Amplitude Time in Seconds Sliced w3 (No Repeat, Start = 200, Length = 60)

Desired Sample Points

0,1 0,0 1,0 1,1

3 Thresholds

Di-Bits Declared: (Figure shows three symbol periods)

Presentation11 29-Nov-11

Deterioration of P25 Eye Due to Simulcast Delay Spread Strong Signal, no Fading

• 2
• 4

• 2
• 4

Strong Signal, 25 usec Delay Spread Fading Strong Signal, 50 usec Delay Spread Fading

• 2
• 4

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Model of Delay Spread’s Effect On the Eye

Wider is Better!

1st Path Eye

Delay Spread

2nd Path Eye Path1+Path2 Usable Eye

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Eye Patterns of Different Modulations

SystemView 100e-6 100e-6 200e-6 200e-6 300e-6 300e-6 400e-6 400e-6 500e-6 500e-6 600 600 3 2 1

• 1
• 2
• 3

Amplitude

Time in Seconds Sliced w4 (No Repeat, Start = 1,000, Length = 30) SystemView 100e-6 100e-6 200e-6 200e-6 300e-6 300e-6 400e-6 400e-6 500e-6 500e-6 600 600 3 2 1

• 1
• 2
• 3

Amplitude

Time in Seconds Sliced w8 (No Repeat, Start = 1,000, Length = 30) SystemView 100e-6 100e-6 200e-6 200e-6 300e-6 300e-6 400e-6 400e-6 500e-6 500e-6 600 600 3 2 1

• 1
• 2
• 3

Amplitude

Time in Seconds Sliced w16 (No Repeat, Start = 1,000, Length = 30) SystemView 100e-6 100e-6 200e-6 200e-6 300e-6 300e-6 400e-6 400e-6 500e-6 500e-6 600e 600e 2 1

• 1
• 2

Amplitude

Time in Seconds Sliced w20 (No Repeat, Start = 1,000, Length = 30)

C4FM, α =0.2 (Present P25) CQPSK, α = 0.2 C4FM, α =1 CQPSK, α =1

SystemView 100e-6 100e-6 200e-6 200e-6 300e-6 300e-6 400e-6 400e-6 500e-6 500e-6 600e-6 600e-6 2 1

• 1
• 2

Amplitude Time in Seconds Sliced w3 (No Repeat, Start = 200, Length = 30)

Preferred Wide CQPSK (Denoted WCQPSK)

Note: α is the filter rolloff factor

Presentation14 29-Nov-11

WCQPSK π/4 Differential Phase Modulation

• 500e-3
• 1
• 1.5

Variable Envelope Modulated Signal To Linearized PA

Modulated Time Domain Sig

Raised Cosine Filter (α) Raised Cosine Filter(α)

Table Lookup I Q I Q

“Constant Envelope” Signal To PA

I, Q Modulator

4 Level Symbols In

• 3,-1,1,3

I

α is Raised Cosine Filter Rolloff Factor

CQPSK I, Q Constellation

• 40
• 40
• 20
• 20

• 20
• 40
• 60

I Q

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Symbol Period Symbol Period

Our Patented WCQPSK Modulation Filter ‘s 2 nd Zero Crossing Impulse Response Enables a Wider Eye

2nd Zero Crossing

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Discernible WCQPSK Eyes Remain Even for 120 usec Delay Spread Faded, Delay Spread=0

• 20e+3

Faded, Delay Spread=40 usec

• 20e+3

Static, Strong Signal

• 10e+3
• 20e+3
• 30e+3

Faded, Delay Spread=120 usec

• 20e+3

Presentation17 29-Nov-11

BER Versus Delay Spread Comparison

2.0% BER for DAQ 3.4 Voice Quality

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 10 20 30 40 50 60 70 80 90 100 BER % Delay Spread (usec)

BER Versus Delay Spread Measurements

P25 C4FM WCQPSK 6 KHz RX Filter

Approx 2X Improvement Over C4FM

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WCQPSK Spectrum Out Of Our Base Station’s Linearized PA Meets the Stringent 210d NB Mask

• Peak Hold

Measurement

• 100 W Avg

Power

Presentation19 29-Nov-11

Summary of WCQPSK Conceptual Design

• Simulcast Delay Spread BER Can Be Greatly

Improved with PSPC’s WCQPSK Linear Modulation that Has “Wider” Eyes  > 2X Delay Spread Improvement Over P25 C4FM  Meets Required FCC Masks and has Low TX ACP  Has Slightly Better Sensitivity than P25 Phase 1 C4FM Systems

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Questions?

29-Nov-11

Contact Info: richard.taylor@harris.com