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

This submission discusses the design trade-offs and considerations for the SUN network, including the use of DSSS, FHSS, and OFDM modulation schemes. It also highlights the advantages and disadvantages of each scheme and explores the co-location problem of DSSS.

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

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

  2. Design Considerations for the SUN Network Presented by: Jeritt E. Kent Senior RF Specialist

  3. Terms… • Both DSSS and FHSS use the term “spread spectrum” • DSSS increases the modulation rate using a spreading code to “spread” the signal band spectrum • FHSS pseudorandomly hops from narrowband to narrowband within a wider band using each narrowband for a specific time period

  4. Let’s talk DSSS first… • Can have higher capacity than FHSS at the expense of bandwidth • Influenced by environmental factors • Reflections and interferers… • Good for point-to-multipoint over short distance or point-to-point long distance

  5. What about OFDM? • Orthogonal frequency-division multiplexing (OFDM) is a frequency-division multiplexing (FDM) scheme utilized as a digital multi-carrier modulation method • A large number of closely-spaced orthogonal sub-carriers are used to carry data • The data is divided into several parallel data streams or channels, one for each sub-carrier • Each sub-carrier is modulated with a conventional modulation scheme (such as QAM or PSK) at a low symbol rate • Total data rates are similar to conventional single-carrier modulation schemes in the same bandwidth

  6. Summary of advantages Can adapt to noisy channel conditions Robust against narrow-band co-channel interference Good spectral efficiency => support high data rates Narrow channels => long symbol time => low ISI Efficient implementation using FFT Summary of disadvantages Requirement 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 And regarding OFDM…

  7. Expounding on OFDM… • OFDM is higher complexity than FSK • 256 to 1024FFTs are not free • The multiplier for deployment is greater than 100Munits • Addition of link improvement via means like diversity are costlier • So is unused bandwidth • System 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 budget • OFDM cannot match FSK’s +30dBm peak • It is peak power that gets through barriers like walls

  8. And now FHSS… • Robust technology • Little influence from noise, reflections, other radios, or environmental factors • High immunity to narrowband interference and frequency-selective fading • Number of simultaneously active collocated systems in a geography can be far higher than for DSSS • No need for antenna diversity

  9. 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 specs • This is where FHSS plays out the aces!

  10. 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

  11. And… • If more than 3 systems are collocated, their channels will overlap, forcing users to share the channels • Actual system behavior and interference is a function of the overlapping size and signal strength • When 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…

  12. And regarding resilience… • While the processing gain and jamming margin of the original DSSS data signal can be increased by lengthening the spreading code • This increases bandwidth and even further reduces collocation • It requires higher inband linearity • As the power of a narrowband interferer increases, the radio will eventually fail completely without precursor • In an industrial zone, this would be a showstopper… SUN applications MUST be agnostic as to their deployment

  13. So… what about resilience? • Two types of interference • Wideband 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 higher • This 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.”

  14. 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

  15. 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 other • And 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

  16. 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 overwhelming • Looking at 99% power bandwidth: For a BT = 0.5 GFSK, it is 0.69 For MSK it is 0.78 • An advantage for GFSK? • Sure….

  17. But… • A fourth order filter will only have about 27dB adjacent channel rejection at 1x IF away • Take a 100-200KHz LIF receiver, for example • If you have another channel within this region, you don’t have a lot of rejection You have lost on channel utilization

  18. And… • At 2x IF away we are at the 50-60dB rejection region • This 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 closer • And, it will have an negative impact on the overall robustness of the network • You increase the probability of interference as the density of the network increases • You 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?

  19. Here is why not… • GFSK requires additional filtering • Side lobes do carry information • You do not get the clean spectral mask for nothing • COST • Limits the types of modulators you can use • GFSK will have ISI • It 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

  20. 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…

  21. 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 PA • Simple, power efficient design • LOW COST • And… it can be received with a non-linear receiver • A simple discriminator or correlator • LOW COST

  22. And what is MSK? • Minimum frequency-shift keying or minimum-shift keying (MSK) is a particularly spectrally efficient form of coherent FSK • MSK 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 0.5 • MSK does not intentionally introduce ISI • MSK delivers a constant envelope, so does GMSK, by the way

  23. 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

  24. Implementing FSK or GFSK on silicon… • Low-bandwidth FSK systems should theoretically be more expensive to implement than DSSS • First, the channel filter must have a lower bandwidth • This occupies more silicon area for a given noise performance • But, 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 necessaryperformance for FSK at an attractive cost-point

  25. Summary – a case for FSK • Given: • Ease of silicon implementation • Compressed PA • Non-linear receiver • Sufficient performance and range • Low Cost • Robustness to Interferers • Supports lots of channels – collocation • Network sharing options / clusters • Coexistence with WiFi FSK is the recommended modulation option for SUN

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