Doc #P802-15-09-0131-00-004g

Project: IEEE P802.15 Working Group for Wireless Personal Area Networks Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)(WPANs)

Submission Title: [Design Considerations for the SUN Network]Date Submitted: [11 March 2009]Source: [Jeritt E. Kent] Company [Analog Devices]

Address [] Voice[]E-Mail: [Jeritt.Kent @ analog.com]

Re: []

Abstract: Considerations and design trade-offs appropriate to the SUN applicaiton

Purpose: Contribution to TG4g PHY proposal evaluation

Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15.

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Design Considerations for the SUN Network

Presented by: Jeritt E. KentSenior RF Specialist

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Terms…

Both DSSS and FHSS use the term “spread spectrum”DSSS increases the modulation rate using a spreading code

to “spread” the signal band spectrumFHSS pseudorandomly hops from narrowband to narrowband

within a wider band using each narrowband for a specific time period

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Let’s talk DSSS first…

Can have higher capacity than FHSS at the expense of bandwidth

Influenced by environmental factorsReflections and interferers…

Good for point-to-multipoint over short distance or point-to-point long distance

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What about OFDM?

Orthogonal frequency-division multiplexing (OFDM) is a frequency-division multiplexing (FDM) scheme utilized as a digital multi-carrier modulation methodA large number of closely-spaced orthogonal sub-carriers are

used to carry dataThe data is divided into several parallel data streams or channels, one

for each sub-carrierEach sub-carrier is modulated with a conventional modulation scheme (such

as QAM or PSK) at a low symbol rateTotal data rates are similar to conventional single-carrier modulation

schemes in the same bandwidth

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And regarding OFDM…

Summary of advantagesCan adapt to noisy channel

conditionsRobust against narrow-band co-

channel interferenceGood spectral efficiency =>

support high data rates Narrow channels => long symbol

time => low ISIEfficient implementation using

FFT

Summary of disadvantagesRequirement that total bandwidth

> coherence bandwidth => total bandwidth requirement of at least 1 MHz => suitable for few users with high data rates instead of many users with low data rates

High PAPR => needs linear power amplifiers => higher cost and complexity

Relies on high precision clocks and filters => higher cost and complexity

Requirement for DSP/FFT processing => higher cost

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Expounding on OFDM…

OFDM is higher complexity than FSK256 to 1024FFTs are not freeThe multiplier for deployment is greater than 100MunitsAddition of link improvement via means like diversity are costlier

So is unused bandwidthSystem cost is the consideration, not just silicon cost

Tx for OFDM is more complicated – PA, etc.

If one considers a real world point-to-point model for SUN, peak power will be important in overall link budgetOFDM cannot match FSK’s +30dBm peak

It is peak power that gets through barriers like walls

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And now FHSS…

Robust technologyLittle influence from noise, reflections, other radios, or

environmental factorsHigh immunity to narrowband interference and frequency-

selective fadingNumber of simultaneously active collocated systems in a

geography can be far higher than for DSSSNo need for antenna diversity

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It is important to understand the application…

Is OFDM a plausible modulation format for SUN? Well…It requires a lot of processing… a lot…And wideband OFDM does not play well in sharing the spectrum, an

example is 802.11n And SUNs (Smart Utility Networks) do not need a lot of throughput

(250kbps – maybe up to 1Mbps… no more)

Instead COLOCATION and RESILIENCE are the driving specsThis is where FHSS plays out the aces!

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DSSS’ Collocation Problem…

FCC requires that DSSS provide a minimum processing gain of 10dB Since processing gain is 10log(rc/rb), then rc/rb must be at least a ratio of 10

The smallest Barker code that meets this requirement for a 1Mbps datarate is eleven (11) to give a processing gain of 10.41dB

With a rule of thumb that null-to-null bandwidth of DSSS is 2 times the chip rate, we have a channel bandwidth of 22MHz

This means that only three DSSS channels can coexist in the 2.4GHz band (83.5MHz), without overlap

In order to get this level of collocation performance on DSSS We need tight control on output power which can get complicated

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And…

If more than 3 systems are collocated, their channels will overlap, forcing users to share the channelsActual system behavior and interference is a function of the

overlapping size and signal strengthWhen the strength of an interfering signal exceeds that of the

original data signal by some the jamming margin Errors occur repeatedly, and data throughput of the DSSS radio

ceases

This is not the best model for SUN applications…

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And regarding resilience…

While the processing gain and jamming margin of the original DSSS data signal can be increased by lengthening the spreading codeThis increases bandwidth and even further reduces collocationIt requires higher inband linearity

As the power of a narrowband interferer increases, the radio will eventually fail completely without precursorIn an industrial zone, this would be a showstopper…

SUN applications MUST be agnostic as to their deployment

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So… what about resilience?

Two types of interferenceWideband and Narrowband

Facts given wideband interference:1) DSSS can operate with a lower SNR than FHSS (6dB or more)2) DSSS has greater range than FHSS for same Tx power

Probability of narrowband interferers is higherThis could render DSSS unworkable while only affecting FHSS as

a capacity hit – but it would still function!

The story is similar for narrowband interference, and per 15.247’s rules for FHSS, “the incorporation of intelligence… is permitted.”

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Conclusion…

FHSS is the better choice for the SUN applications for which 802.15.4g was defined

“For installations (like SUN applications) requiring big coverage and multiple collocated cells, it would

be much easier to use FHSS.”- Sorin Schwartz

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And of the FHSS modulation options, the best choice for SUN is FSK…

For FHSS you want a lot of channels, in fact as many as you can afford, which ideally do not influence each otherAnd that's where FSK has advantages

Yes, Eb/No can be worse than for DSSS Yes, co-channel rejection may not be the best

But, FSK offers a very compact spectrum This is good news for FHSS

It allows us to pack many narrow channels into a given bandwidth with minimal co-influence among them

But…

There are actually modulation formats that are more compact

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Isn’t there a version of FSK called GFSK that is more spectrally efficient? Isn’t is “cleaner”?…

What is the advantage of GFSK over FSK? One thing – NARROWER BANDWIDTH

It turns out the advantage may not be overwhelmingLooking at 99% power bandwidth:

For a BT = 0.5 GFSK, it is 0.69For MSK it is 0.78

An advantage for GFSK?

Sure….

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But…

A fourth order filter will only have about 27dB adjacent channel rejection at 1x IF away

Take a 100-200KHz LIF receiver, for exampleIf you have another channel within this region, you don’t have a lot

of rejection

You have lost on channel utilization

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And…

At 2x IF away we are at the 50-60dB rejection regionThis makes the probability of interference a lot lower

But, add to that the image rejection…Then, there are no real advantages in packing the channels closerAnd, it will have an negative impact on the overall robustness of

the networkYou increase the probability of interference as the density of the

network increasesYou increase the number of retries which increases interference due

to more traffic on the network.....etc.

So then, why might one want to use GFSK?

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Here is why not…

GFSK requires additional filteringSide lobes do carry information

You do not get the clean spectral mask for nothingCOSTLimits the types of modulators you can use

GFSK will have ISIIt is not significant

At a 0.5 BT it is about 0.3dB It can be difficult for a hopping synthesizer to maintain phase

coherence over a wide bandwidth 83.5MHz for the 2.4GHz band

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And here is why…

GFSK offers the option of substantially increasing capacity!Perhaps, at the expense of a few dBs of sensitivity…

But…

Perhaps not…

Some silicon vendors show little difference in receiver sensitivity between FSK and GFSK…

Some guarantee phase continuity at bit boundaries…

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So, then, why FSK???

A key benefit that makes FSK nice for applications like SUN:

FSK has a constant envelope

So… it can be transmitted with a compressed PASimple, power efficient designLOW COST

And… it can be received with a non-linear receiverA simple discriminator or correlatorLOW COST

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And what is MSK?

Minimum frequency-shift keying or minimum-shift keying (MSK) is a particularly spectrally efficient form of coherent FSKMSK is a continuous phase modulation scheme where the

modulated carrier contains no phase discontinuities and frequency changes occur at the carrier zero crossings

MSK is unique due to the relationship between the frequency of a logical zero and one; the difference between the higher and lower frequency is identical to half the bit rate

Thus, the modulation index is 0.5MSK does not intentionally introduce ISIMSK delivers a constant envelope, so does GMSK, by the way

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So…

It really comes down to the application

Silicon vendors that can offer both FSK and GFSK with minor cost overhead provide designers with options for optimization

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Implementing FSK or GFSK on silicon…

Low-bandwidth FSK systems should theoretically be more expensive to implement than DSSSFirst, the channel filter must have a lower bandwidth

This occupies more silicon area for a given noise performanceBut, the contribution to overall chip area is typically very small

Second, the synthesizer must have better noise performance to achieve low adjacent channel power

But…

Some silicon manufacturers have the right technology to deliver the necessary performance for FSK at an attractive cost-point

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Summary – a case for FSK

Given:Ease of silicon implementation

Compressed PANon-linear receiver

Sufficient performance and rangeLow CostRobustness to InterferersSupports lots of channels – collocationNetwork sharing options / clustersCoexistence with WiFi

FSK is the recommended modulation option for SUN