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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: Chirp Spread Spectrum

Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: Chirp Spread Spectrum (CSS) PHY Presentation for 802.15.4a Date Submitted: January 04, 2005 Source: John Lampe Company: Nanotron Technologies Address: Alt-Moabit 61, 10555 Berlin, Germany

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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: Chirp Spread Spectrum

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  1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: Chirp Spread Spectrum (CSS) PHY Presentation for 802.15.4a Date Submitted: January 04, 2005 Source: John Lampe Company: Nanotron Technologies Address: Alt-Moabit 61, 10555 Berlin, Germany Voice: +49 30 399 954 135, FAX: +49 30 399 954 188, E-Mail: j.lampe@nanotron.com Re: This is in response to the TG4a Call for Proposals, 04/0380r2 Abstract: The Nanotron Technologies Chirp Spread Spectrum is described and the detailed response to the Selection Criteria document is provided Purpose: Submitted as the candidate proposal for TG4a Alt-PHY 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. J. Lampe, R. Hach, L. Menzer, Nanotron

  2. Chirp Spread Spectrum (CSS)PHY Presentationfor 802.15.4a by John Lampe, Rainer Hach, & Lars Menzer Nanotron Technologies GmbHBerlin, Germany www.nanotron.com J. Lampe, R. Hach, L. Menzer, Nanotron

  3. General Properties of Chirp Signals • Simplicity • Basically a 2 ary baseband transmission • The ‘windowed chirp’ is a linear frequency sweep with a total duration of 1 us J. Lampe, R. Hach, L. Menzer, Nanotron

  4. Chirp Properties (cont.) • Real part of a windowed up-chirp signal with a total duration of 1us J. Lampe, R. Hach, L. Menzer, Nanotron

  5. Chirp Properties (cont.) • Figure shows the autocorrelation function (acf) and cross-correlation function (ccf) of the signal described above • Note that the ccf has a constant low value (compared to DS sequences). J. Lampe, R. Hach, L. Menzer, Nanotron

  6. Chirp Properties (cont.) • This figure shows the PSD of the signal with a power of 10 dBm • By padding the signal with zeros the “frequency resolution” has been set to 100 kHz so that the plot is similar to what a spectrum analyzer would measure. • At 12 MHz offset from the center is below -30dBm (which is the ETSI requirement for out of band emissions) J. Lampe, R. Hach, L. Menzer, Nanotron

  7. Chirp Properties (cont.) • Further processing of the signals Sig A and Sig B for symbol detection could be done in coherent (real part processing) or non coherent manner (envelope filtering). • Since the analytical results are well known for AWGN channels we will mention these • Simulations over other channels will all refer to the non coherent system as drawn below. J. Lampe, R. Hach, L. Menzer, Nanotron

  8. Chirp Properties (cont.) • This figure shows the analytical BER values for 2 ary orthogonal coherent and non coherent detection and the corresponding simulation results (1E5 symbols) for up down chirp (using the chirp signals defined above) • The performance loss due to the non-orthogonality of up and down chirp is very small. J. Lampe, R. Hach, L. Menzer, Nanotron

  9. Signal Robustness • The proposed CSS PHY is designed to operate in a hostile environment • Multipath • Narrow and broadband intentional and unintentional interferers • Since a chirp transverses a relatively wide bandwidth it has an inherent immunity to narrow band interferers • Multipath is mitigated with the natural frequency diversity of the waveform • Broadband interferer effects are reduced by the receiver’s correlator • Forward Error Correction (FEC) can further reduce interference and multipath effects. • Three non-overlapping frequency channels in the 2.4 GHz ISM band • This channelization allows this proposal to coexist with other wireless systems such as 802.11 b, g and even Bluetooth (v1.2 has adaptive hopping) via DFS • CSS proposal utilizes CCA mechanisms of Energy Detection (ED) and Carrier Detection • These CCA mechanisms are similar to those used in IEEE 802.15.4-2003 • In addition to the low duty cycle for the applications served by this standard sufficient arguments were made to convince the IEEE 802 sponsor ballot community that coexistence was not an issue. J. Lampe, R. Hach, L. Menzer, Nanotron

  10. Support for Interference Ingress • Example (w/o FEC): • Bandwidth B of the chirp = 20 MHz • Duration time T of the chirp = 1 µs • Center frequency of the chirp (ISM band) = 2.442 GHz • Processing gain, BT product of the chirp = 13 dB • Eb/N0 at detector input (BER=10-4) = 12 dB • In-band carrier to interferer ratio (C/I @ BER=10-4)= 12 - 13 = -1 dB • Implementation Loss = 2 dB J. Lampe, R. Hach, L. Menzer, Nanotron

  11. Support for Interference Egress • Low interference egress • IEEE 802.11b receiver • More than 30 dB of protection in an adjacent channel • Almost 60 dB in the alternate channel • these numbers are similar for the 802.11g receiver J. Lampe, R. Hach, L. Menzer, Nanotron

  12. Regulatory • Devices manufactured in compliance with the CSS proposal can be operated under existing regulations in all significant regions of the world • including but not limited to North and South America, Europe, Japan, China, Korea, and most other areas • There are no known limitation to this proposal as to indoors or outdoors • The CSS proposal would adhere to the following worldwide regulations: • United States Part 15.247 or 15.249 • Canada DOC RSS-210 • Europe ETS 300-328 • Japan ARIB STD T-66 J. Lampe, R. Hach, L. Menzer, Nanotron

  13. Scalability – Data Rate • Mandatory rate = 1 Mb/s • Optional rate = 267 Kb/s • Other possible data rates include 2 Mb/s to allow better performance in a burst type, interference limited environment or a very low energy consumption application • Lower data rates achieved by using interleaved FEC • Lower chirp rates would yield better performance • longer range, less retries, etc. in an AWGN environment or a multipath limited environment • It should be noted that these data rates are only discussed here to show scalability, if these rates are to be included in the draft standard the group must revisit the PHY header such as the SFD. J. Lampe, R. Hach, L. Menzer, Nanotron

  14. Scalability – Frequency Bands • The proposer is confident that the CSS proposal would also work well in other frequency bands • Including the 5975 to 7250 MHz band • Mentioned in the new FCC operating rules “SECOND REPORT AND ORDER AND SECOND MEMORANDUM OPINION AND ORDER” released December 16, 2004 J. Lampe, R. Hach, L. Menzer, Nanotron

  15. Scalability – Data Whitener • Additionally, the group may consider the use of a data whitener, similar to those used by Bluetooth and IEEE 802.11 to produce a more “noise-like” spectrum and allow better performance in synchronization and ranging. J. Lampe, R. Hach, L. Menzer, Nanotron

  16. Scalability – Power Levels • For extremely long ranges the transmit power may be allowed to rise to each country’s regulatory limit • For example the US would allow 30 dBm of output power with up to a 6 dB gain antenna • The European ETS limits would specify 20 dBm of output power with a 0 dB gain antenna • Note that even though higher transmit requires significantly higher current it doesn’t significantly degrade battery life since the transmitter has a much lower duty cycle than the receiver, typically 10% or less of the receive duty cycle • In this manner the averaged transmitter current drain will be less than the averaged receiver current drain. J. Lampe, R. Hach, L. Menzer, Nanotron

  17. Scalability – Backward Compatibility • Due to some of the similarities with DSSS it is possible to implement this proposal in a manner that will allow backward compatibility with the 802.15.4 2.4 GHz standard • The transmitter changes are relatively straightforward • Changes to the receiver would include either dual correlators or a superset of CSS and DSSS correlators • It is anticipated that this backward compatibility could be achieved via mode switching versus a dynamic change on-the-fly technique • left up to the implementer • This backward compatibility would be a significant advantage to the marketplace by allowing these devices to communicate with existing 802.15.4 infrastructure and eliminating customer confusion J. Lampe, R. Hach, L. Menzer, Nanotron

  18. Communication No system inherent restrictions are seen for this proposal The processing gain of chirp signals is extremely robust against frequency offsets such as those caused by the Doppler effect due to high relative speed vrel between two devices Such situations also occur when one device is mounted on a rotating machine The limits will be determined by other, general processing modules (AGC, symbol synchronization,...) Ranging The ranging scheme proposed in this document relies on the exchange of two hardware acknowledged data packets One for each direction between two nodes We assume that the longest time in this procedure is the turnaround time tturn between the two nodes which will be determined by the respective uC performance. During this time the change of distance should stay below the accuracy da required by the application. For da =1m tturn=10 ms this yields vrel << 100m/s Mobility Values J. Lampe, R. Hach, L. Menzer, Nanotron

  19. MAC Enhancements and Modifications • There are no anticipated changes to the 15.4 MAC to support the proposed Alt-PHY. Three channels are called for with this proposal and it is recommended that the mechanism of channel bands from the proposed methods of TG4b be used to support the new channels. There will be an addition to the PHY-SAP primitive to include the choice of data rate to be used for the next packet. This is a new field. • Ranging calls for new PHY-PIB primitives that are expected to be developed by the Ranging subcommittee. J. Lampe, R. Hach, L. Menzer, Nanotron

  20. Channel Models • Since this proposal refers to the 2.4GHz ISM band, only channel models with complete parameter set covering this frequency range can be considered • At the time being these are LOS Residential (CM1) and NLOS Residential (CM2). • The 100 realizations for each channel model were bandpass filtered with +-15MHz around 2.437GHz which corresponds to the second of the three sub-bands proposed. • The filtered impulse responses were down converted to complex baseband. • The magnitudes over time are shown in the following plots • Furthermore some graphs of the function H_tilde as described and required in the SCD are shown • For now we assume that the neighbor sub-bands will not differ significantly from the center sub-band and that we restrict simulations on the center sub-band • The SCD requirements on the payload size to be simulated seem to be somewhat inconsistent. At some point 10 packets with 32 bytes are mentioned which would be a total of 2560 bits. On the other hand a PER of 1% is required which mean simulating more than 100 packets or 25600 bits. • Since the delay spread and thus the time in which subsequent symbols can influence each other of all given channel impulse responses are well below the symbol duration of 1us suggested by this proposal we believe that we get the best results when we simulate a large number of independent transmissions of symbols. • Assuming an equal probability of error for all bits of a packet we can give the relationship between the BER and PER by With N being the number of payload bits. • Thus we can calculate the BER which is required for any PER: • For PER=1% and N=256 we get BER=3.9258E-5 J. Lampe, R. Hach, L. Menzer, Nanotron

  21. Channel Model 1 J. Lampe, R. Hach, L. Menzer, Nanotron

  22. Channel Model 2 J. Lampe, R. Hach, L. Menzer, Nanotron

  23. Size and Form Factor • The implementation of the CSS proposal will be much less than SD Memory at the onset • following the form factors of Bluetooth and IEEE 802.15.4/ZigBee • The implementation of this device into a single chip is relatively straightforward • As evidenced in the “Unit Manufacturing Complexity” slides J. Lampe, R. Hach, L. Menzer, Nanotron

  24. PHY SAP Payload Bit Rate& Data Throughput • The PPDU is composed of several components as shown in the figure below • The following figure shows in greater detail, the component parts of each PPDU. J. Lampe, R. Hach, L. Menzer, Nanotron

  25. PHY SAP Payload Bit Rate& Data Throughput (cont) • The SFD structure has different values for, and determines, the effective data rate for PHR and PSDU • The Preamble is 32 bits in duration (a bit time is 1 us) • In this proposal, the PHR field is used to describe the length of the PSDU that may be up to 256 octets in length • In addition to the structure of each frame, the following shows the structure and values for frames including overhead not in the information carrying frame J. Lampe, R. Hach, L. Menzer, Nanotron

  26. PHY SAP Payload Bit Rate& Data Throughput (cont) • The figures show the structure, as defined in the IEEE Standard 802.15.4 and the SCD • cases of acknowledged transmissions as used in section 3.2.1 values and for unacknowledged transmissions. • For this proposal, the value of Tack and SIFS are retained from the IEEE Standard 802.15.4 and are each 192 microseconds • The value of LIFS is also shown as 192 microseconds. • Additional revisions of this proposal may show a different value as the authors discuss the need for longer LIFS values with members of TG4b • The values of SIFS and LIFS have a MAC dependency above the value of 192uS required for PHY turn around • SIFS has a value of 192us (12 symbols) in the current standard and LIFS has a value of 40 symbols. J. Lampe, R. Hach, L. Menzer, Nanotron

  27. Simultaneous Operating Piconets • Separating Piconets by frequency division • This CSS proposal includes a mechanism for FDMA by including the three frequency bands used by 802.11 b, g and also 802.15.3 • It is believed that the use of these bands will provide sufficient orthogonality • The chirp signal defined earlier has a rolloff factor of 0.25 which in conjunction with the space between the adjacent frequency bands allows filtering out of band emissions easily and inexpensively. J. Lampe, R. Hach, L. Menzer, Nanotron

  28. Signal Acquisition • The signal acquisition is basically determined by the structure and duration of the preamble • In contrast to ‘always on’ systems like DECT or GSM • Low duty rate systems must be able to acquire a signal without any prior knowledge about that signal’s level or timing • While the IEEE 802.15.4-2003 uses a preamble duration of 32 symbols (128 µs at 2.4 GHz) other commercially available transceiver chips (e.g. nanoNET TRX from Nanotron) use 30 symbols at 1MS/s (i.e. 30µs). • For consistency with IEEE 802.15.4-2003 this CSS proposal is based upon a preamble of 32 symbols which at 1MS/s turns out as 32 µs • Existing implementations demonstrate that modules, which might be required to be adjusted for reception (Gain Control, Frequency Control, Peak Value Estimation, etc.), can be setup in such a time duration • The probability of missing a packet is then simply determined by the probability that the SFD is received correctly • As shown before, the BER required for a PER of 1%, is BER=3.9258E-5 J. Lampe, R. Hach, L. Menzer, Nanotron

  29. Clear Channel Assessment • A combination of symbol detection (SD) and energy detection (ED) has proven to be useful in practice. The duration of the preamble can be used as upper bound for the duration for both detection mechanism. By providing access to the threshold for ED the system allows the application to adjust its behavior (false alarm vs. miss probability) according to its needs. J. Lampe, R. Hach, L. Menzer, Nanotron

  30. System Performance • Simulation over 100 channel impulse responses (as required in the SCD) were performed for channel model 1 and channel model 2. • No bit errors could be observed on channel model 1 (simulated range was 10 to 2000m). This is not really surprising because this model has a very moderate increase of attenuation over range (n=1.79) • The results for channel model 2 are displayed below. The parameter n=4.48 indicates a very high attenuation for higher ranges. The results were interpreted as BER and PER respectively and for convenience were plotted twice (linear and log y scale). J. Lampe, R. Hach, L. Menzer, Nanotron

  31. System Performance J. Lampe, R. Hach, L. Menzer, Nanotron

  32. System Performance J. Lampe, R. Hach, L. Menzer, Nanotron

  33. System Performance J. Lampe, R. Hach, L. Menzer, Nanotron

  34. We approximate the impact of σu by the simple formula: The assume the rising speed of a the signal being proportional to the signal bandwidth: Since the power σ2 of band-filtered AWGN is proportional to the bandwidth we know that: Which leads to: TOA Estimation for Ranging Noise and Jitter of Band-Limited Pulse Given a band-limited pulse with noise σuwe want to estimate how the jitter (timing error) σt with which the passing of the rising edge of the pulse through a given threshold can be detected is effected by the bandwidth B. J. Lampe, R. Hach, L. Menzer, Nanotron

  35. TroundA TreplyA Node A t Tprop Tprop Tprop Tprop Node B t TreplyB TroundB Symmetrical Double-Sided Two-Way Ranging (SDS TWR) Tround... round trip time Treply... reply time Tprpp... propagation of pulse Double-Sided: Each node executes a round trip measurement. Symmetrical: Reply Times of both nodes are identical (TreplyA =TreplyB). Results of both round trip measurements are used to calculate the distance. J. Lampe, R. Hach, L. Menzer, Nanotron

  36. final measurement error Ranging - Effect of Time Base Offset Errors Assuming offset errors eA, eB of the timebases of node A and B we get: On the condition that the two nodes have almost equal behavior, we can assume: This has the effect that timebase offsets are canceled out: Calculations show that for 40ppm crystals and 20 us max difference between TroundA and TroundB and between TreplyA and TreplyB an accuracy below 1 ns can easily be reached! J. Lampe, R. Hach, L. Menzer, Nanotron

  37. Influence of Symmetry Error: Calculation of an SDS TWR Example System • Example System EtA = ±40 ppm, EtB = ±40 ppm (worst case combination selected): • Conclusion: Even 20 µs Symmetry Error allows excellent accuracy of distance ! Symmetry Error below 2 µs can be guaranteed in real implementations ! J. Lampe, R. Hach, L. Menzer, Nanotron

  38. pulse to transmit PHY packet to transmit Ti Ti transmitted pulse PHY packet transmitted t ΔTi1 ΔTi2 ΔTi3 ΔTin t ΔTiRX1 ΔTiRX2 ΔTiRX3 ΔTiRXn Ti Ti Ti Ti tToA’ Principle of Dithering and Averaging • Dithering: • Averaging: J. Lampe, R. Hach, L. Menzer, Nanotron

  39. Simulation of a SDS TWR System Example system: Simulates SDS TWR + Dithering & Averaging Crystal Errors ±40 ppm Single shot measurements @ 1 MBit/s data rate (DATA-ACK) Transmit Jitter = ± 4 ns (systematic/pseudo RN-Sequence) Pulse detection resolution = 4 ns Pulses averaged per packet = 32 Symmetry error = 4 µs (average) Distance = 100 m Results of Distance Error ∆d: |∆dWC| < 50 cm |∆dRMS| < 20 cm (ideal channel without noise) J. Lampe, R. Hach, L. Menzer, Nanotron

  40. Link Budget footnotes [1] Rx noise figure: in addition the proposer can select other values for special purpose (e.g. 15 dB for lower cost lower performance system) [2] The minimum Rx sensitivity level is defined as the minimum required average Rx power for a received symbol in AWGN, and should include effects of code rate and modulation. J. Lampe, R. Hach, L. Menzer, Nanotron

  41. Sensitivity • The sensitivity to which this CSS proposal refers is based upon non-coherent detection • It is understood that coherent detection will allow 2 - 3 dB better sensitivity but at the cost of higher complexity (higher cost?) and poorer performance in some multipath limited environments • The sensitivity for the 1 Mb/s mandatory data rate is -92 dBm for a 1% PER in an AWGN environment with a front end NF of 7 dB • The sensitivity for the optional 267 kb/s data rate is -97 dBm for a 1% PER in an AWGN environment with a front end NF of 7 dB J. Lampe, R. Hach, L. Menzer, Nanotron

  42. Power Management Modes • Power management aspects of this proposal are consistent with the modes identified in the IEEE 802.15.4: 2003 standard • There are no modes lacking nor added • Once again, attention is called to the 1 Mbit/s basic rate of this proposal and resulting shorter “on” times for operation J. Lampe, R. Hach, L. Menzer, Nanotron

  43. Power Consumption The typical DSSS receivers, used by 802.15.4, are very similar to the envisioned CSS receiver • The two major differences are the modulator and demodulator • The modulator of the CSS is much simpler than the DSSS however since the major power consumption is the transmitter and the difference is negligible • The power consumption for a 10 dBm transmitter should be 198 mW or less • The receiver for the CSS is remarkably similar to that of the DSSS with the major difference being the correlator • The correlator for the CSS uses a frequency dispersive mechanism while the DSSS uses a chip additive correlator • The difference in power consumptions between these correlators is negligible so the power consumption for a 6 dB NF receiver should be 40 mW or less • The power consumed during the CCA is basically similar to the receiver power consumption • All of the receiver circuits are being used during the CCA (correlator is used for the carrier detect function) J. Lampe, R. Hach, L. Menzer, Nanotron

  44. Power Consumption (cont.) • Power save mode is used most of the time for this device and has the lowest power consumption • Typical power consumptions for 802.15.4 devices are 3 µW or less • Energy per bit is the power consumption divided by the bit rate • The energy per bit for the 10 dBm transmitter is less than 0.2 µJ • The energy per bit for the receiver is 40 nJ • As an example, the energy consumed during an exchange of a 32 octet PDU between two devices (including the transmission of the PDU, the tack, and the ack) would be 70.6 µJ for the sender device while the receiving device consumed 33.2 µJ • As a reference point it should be noted that according to the Duracell web site, a Duracell AA alkaline cell contains more than 12,000 Joules of energy. J. Lampe, R. Hach, L. Menzer, Nanotron

  45. Antenna Practicality • The antenna for this CSS proposal is a standard 2.4 GHz antenna such as widely used for 802.11b,g devices and Bluetooth devices. These antennas are very well characterized, widely available, and extremely low cost. Additionally there are a multitude of antennas appropriate for widely different applications. The size for these antennae is consistent with the SCD requirement. J. Lampe, R. Hach, L. Menzer, Nanotron

  46. Time To Market • No regulatory hurdles • No research barriers – no unknown blocks, CSS chips are available in the market • Normal design and product cycles will apply • Can be manufactured in CMOS J. Lampe, R. Hach, L. Menzer, Nanotron

  47. Unit Manufacturing Complexity • Target process:RF-CMOS, 0.18 µm feature size J. Lampe, R. Hach, L. Menzer, Nanotron

  48. Unit Manufacturing Complexity • Target process:RF-CMOS, 0.13 µm feature size J. Lampe, R. Hach, L. Menzer, Nanotron

  49. Summary Chirp Spread Spectrum (CSS) is simple, elegant, efficient • Combines DSSS and UWB strengths • Adds precise location-awareness • Robustness – multipath, interferers, correlation, FEC, 3 channels, CCA • Can be implemented with today’s technologies • Low-complexity • Low-cost • Low power consumption • Globally certifiable • Scalability with many options for the future • Backward compatible with 802.15.4-2003 • Mobility enhanced • No MAC changes (minimal for ranging) • Size and Form Factor – meets or exceeds requirements • Excellent throughput • SOPs – FD channels • Signal Acquisition – excellent • Link Budget and Sensitivity – excellent • Power Management and Consumption - meets or exceeds requirements • Antenna – many good choices J. Lampe, R. Hach, L. Menzer, Nanotron

  50. Summary Chirp Spread Spectrum (CSS) Proposal Meets the PAR and 5C: • Precision ranging capability accurate to one meter or better • Extended range over 802.15.4-2003 • Enhanced robustness over 802.15.4-2003 • Enhanced mobility over 802.15.4-2003 • International standard • Ultra low complexity (comparable to the goals for 802.15.4-2003) • Ultra low cost (comparable to the goals for 802.15.4-2003) • Ultra low power consumption (comparable to the goals for 802.15.4-2003) • Support coexisting networks of sensors, controllers, logistic and peripheral devices in multiple compliant co-located systems. J. Lampe, R. Hach, L. Menzer, Nanotron

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