LOW LATENCY MICROWAVE

Fact, Fiction & Future

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MICROWAVE IS FASTER THAN FIBER AND IS QUICKLY BECOMING DEFACTO STANDARD FOR ULTRA LOW LATENCY TRANSPORT

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Air Latency is the dominant Factor Angular deviation comes at a high cost

RegulatoryAvailability of licensed

spectrum

Infrastructure

Equipment Operating ModesRepeaters and Regenerators

Number of HopsModem and RF Performance

Equipment Latency

LOW LATENCY MICROWAVE FACTS

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Air Latency is the dominant Factor Angular deviation comes at a high cost

RegulatoryAvailability of licensed

spectrum

Infrastructure

Equipment Operating ModesRepeaters and Regenerators

Number of HopsModem and RF Performance

Equipment Latency

Angular Deviation

IncreasedDistance

IncreasedLatency

Percentage

5 Degrees 2.8 Miles 15.0 µs 0.38%

10 Degrees 11.3 Miles 60.5 µs 1.54%

15 Degrees 25.9 Miles 138.4 µs 3.53%

20 Degrees 47.2 Miles 251.8µs 6.42%

Even with microwave working at the speed of light a less than ideal pathwill cause a significant latency penalty

735 MilesAIR LATENCY

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REGULATORY

• We are all bound in our microwave designs and planningby the availability of licensed spectrum

- Optimal frequency bands are scarce on primary routes

- Channel sizes (bandwidth) and efficiency (bits per Hz) are regulated

- Limitations to the total carrying capacity of the pipeline

- Emission Masks and Interference Regulations

- Power Control

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REDUCE NUMBER AND COMPLEXITY OF THE HOPS

INTRODUCTION OF THE REPEATER CONCEPT

- Latency in Nx10 ns- RF Repeater

- Limited RF Options- Mask Limitations

- IF Repeater- Frequency Agile- Mask Limitations

- Digital Repeater- Frequency Agile- Less Susceptible to Mask Limitations

RegeneratorNode

RegeneratorNode

RepeaterNode

RepeaterNode

30% reduction

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RegeneratorNode

RepeaterNode

RepeaterNode

RegeneratorNode

RepeaterNode

RepeaterNode

RegeneratorNode

RepeaterNode

RepeaterNode

RegeneratorNode

RepeaterNode

RegeneratorNode

ADAPTIVE REPEATER TECHNOLOGY

Ultra Low End-End Latency

Low End-End Latency (still better than fiber)

99% of time

1% of time

Continuous monitoring of the received signal quality

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RF AND MODEM PERFORMANCE

WHERE DOES IT HELP?

• Antenna Size

• Number of Repeaters

- FEC Coding

- Rx Threshold

- Higher Quality Oscillators

Ultimately all revolves around having better System Gain !!!

• Link Distance

• Number of Hops

• Link Availability

TECHNIQUES INCLUDE:

- Tx Power

- Rx Sensitivity

- Diversity

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MODEM TECHNOLOGY

Forward Error Correction

Modulations and Bandwidths

Diversity ProtectionImproves link availability and hence low latency availability

Adaptive Modulation and Adaptive Coding Latency Increase

Phase noise reduction

FEC Method

Linear and Iterative

Convolutional Codes

Algorithm

LPDC, Turbo Codes

Viterbi

Latency

High

Medium

Block Codes

No FEC

Reed Solomon

- - -

Low*

Lowest

SG Imp.

High

Good*

None

Good

BW30 MHz

40 MHz

Mod

16 Q

AM

32 Q

AM

50 MHz

128

QA

M

64 Q

AM

256

QA

M

QP

SK

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RF TECHNOLOGY

IF Preprocessing

Adaptive Predistortion

Higher Output Power

Clean LOs

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QUANTIFY RF / MODEM PERFORMANCE

• Example: Chicago rain zone, 99.999 Availability, 6 Ft Antenna, 64QAM

Frequency Band

6 GHz

11 GHz

Std System Gain

51.6 Miles

35.1 Miles

+3dB

55.8 Miles

38.5 Miles

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EQUIPMENT LATENCY

Payload InterfaceInterface Alternatives (Ethernet, L1 Interfaces, Proprietary)

Switch Forwarding (Store and Forward, Early Packet Forwarding)

Switch Buffering

Serialization DelayInterface speed differentials force store and forward stages and buffering

Modem Technology (Traditional)Scrambling / Descrambling

Encoding / Decoding

Interleaving / Deinterleaving

Modulation / Demodulation

Analog Front End

Reduce or Eliminate

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FICTION, MISCONCEPTIONS AND MYTHS

Equipment latency is most important

Focus in In and Out of the box

latency

Reduced Latency = Reduced

Reliability

Link Reliability

Equipment Reliability

High site count is OK if path is optimal

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EQUIPMENT LATENCY-THE MOST IMPORTANT FACTOR

Equipment latency is important but not the most important factor

Route design

Path and Frequency planning

Maximizing the number of repeaters

RF / Modem performance

Reduce the number of hops

Improve link availability

Equipment Latency

End to end latency is the goalOptimal blend of above features Lowest end to end latency + Highest availability

~3-20 µs

RegeneratorNode

RepeaterNode

~100-500 ns

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LOW LATENCY = LOW LINK RELIABILITY

To achieve low latency multiple modem aspects are sacrificed

Coding gain

Interleaving

Reduced equalization

Additional signal processing techniquesAdaptive repeaters

IF combining – Space Diversity

Adaptive pre-distortion

Combined Signal

Main - Distorted Diversity -DistortedMultipath

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LOW LATENCY = LOW EQUIPMENT RELIABILITY

Low latency can be built on a field proven platform with very high MTBF

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HIGH SITE COUNT IS OK IF PATH IS OPTIMAL

Can be offset by having more repeaters

Noise AccumulationNoise accumulates over a higher number of nodes

Points of FailureHave more active elements in the network

Operational ComplexityMore equipment to manage, troubleshoot (link and network) and

maintain

CAPEX + OPEXInitial spending, cost of leasing towers and maintenance

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LOW LATENCY MICROWAVE FUTURE

Lower Latency

Increase number of repeaters

Specialized Modems

All outdoor radio repeaters

Increased Capacity

Multi-Carrier

XPIC

Channel Bonding

Higher Modulations

Beyond Microwave

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LOWER LATENCY

Increase number of repeatersThrough Signal Processing techniques enhance the repeater operation

Able to meet regulatory requirements over a larger number of repeaters

Specialized modemsOptimally combine ASIC and FPGA technologies

Dual/Quad header modems to create repeaters

Minimize transfer delay in the pipeline

All outdoor radio repeatersEliminate cable latency

Place modem and RF on the tower

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INCREASED CAPACITY

Multi-carrierOutdoor branching for up to 8 channels in a single antenna

Use dual or quad header modems

XPIC + DiversityCross polar cancelling for lowest possible latency

Regulatory requirements internationally – Efficient channel utilization (H & V)

Channel bondingCreate wider carriers by bonding 2 to 4 channels together

Single RF for concatenated wide channel

Higher ModulationsHigher bits per Hz efficiencies using up to 1024 QAM

Better signal processing in the modem and cleaner RF

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BEYOND MICROWAVE

FSOHigher capacity than microwave, minimizes serialization delay

Limited to short distances due to physical and atmospheric disturbances

Must reduce cost and increase usability for wide scaled adoption

MMWHigher capacity than microwave, minimizes serialization delay

Limited to short distances due to atmospheric disturbances

Prone to interference in heavily congested areas

Further ahead than FSO in adoption but requires tighter regulation and

more efficient use of the bandwidth

W W W . A V I A T N E T W O R K S . C O M