Presentation on theme: "Presentation on Phase I Work"— Presentation transcript:
1Presentation on Phase I Work Robust Ultra High Frequency (UHF) Satellite Communications Protocol for UUVsSBIR Topic #: N02-019Contract #: N C-4577Deliverable Item 0001ACPresentation on Phase I WorkWavix, Incorporated27 January 2003
2Phase-I Objectives Understand the maritime noise environment Characterize RF (UHF) communication impedimentsIdentify mitigating (low-level) protocol techniquesRecommend development paths to follow
4Maritime Noise Concerns Original List Signal fading from wave motions rocking UUVHigh seas coating antenna or changing local RF propagation characteristicsShadowing by wavesSurface wave reflections causing multi-path interferenceChanging atmospheric conditions in LOSNature of channel noise: high BER?This is a list of concerns from our original Phase-I proposal, conditions which represent difficulties expected by UUVs trying to communicate in harsh maritime environments. These are sorted in the next two slides into those concerns that are not major problems, and those that are addressed in the remainder of the presentation.
5Maritime Noise Concerns Non-Obstacles I Antenna wash-overChoose appropriate non-conductive housingWater has small attenuation at UHFGround-plane issuesAppropriate choice of antenna:Quadrifilar Helix has small sensitivity to ground planeHelical antenna is very sensitive to ground planeThis slide and the next slide list four concerns that, although they could be issues, we believe are easily dealt with using straightforward system, mechanical, or RF design, or else they are not relevant to the operation of the proposed system (e.g., not a problem in the UFH band).
6Maritime Noise Concerns Non-Obstacles II Water aerosolsWater has little attenuation in UHF band(Atmospheric water effects become serious above ~2-4 GHz)UUV rockingMotions are too small to disrupt signalsMotions are too slow for Doppler effectsConsidering in particular the example of UUV motions, comparing orders of magnitude of, say, size of motions and the wavelength of the UHF waves tells us that interaction will be a small effect.
7Characteristic Length Scales Wave Heights: 2 mRF Wavelength: 1 m 300 MHz)Surface Roughness: 0.1 mThis is a physicist’s approach!When the length scales of two possibly interacting phenomena are widely dissimilar, the phenomena are unlikely to interact; the possibility for strong interaction increases when the length scales are similar.
8Characteristic Time Scales Wave times: ~5-10 sSATCOM frames:8.96 s, made of 1024 blocks [5 kHz waveform]s, made of ms time chips [25 kHz waveform]Geo propagation time: 500 msSATCOM blocks:8.75 ms blocks in 5 kHz waveform; variable number/channel0.052 ms “time chips” in 25 kHz waveform; channel format varies but order of 100 might be typicalATM packet size: ~2.5 msIP packets are 4 to 10 times larger, but variableSymbol size: ~0.05 ms 19.6 bits/second)As with length scales, when time scales differ widely, interaction tends to be minimized; when time scales are similar, interaction tends to be maximized.
9Maritime Noise Concerns Obstacles Small look angle to satellite Wave obscuringOcean-surface roughnessNature of the RF channelFrom the original list of concerns, these are the ones that we believe require the most attention in communication protocols, although the low-angle question is also a bigger concern about using geostationary SATCOM satellites.The following slides takes a first look at each of these in turn.
10Satellite Elevation vs. Latitude Latitudes = Elevations(for G = 0!)= 82= 71= 5650* = 40= 25= 12(* US/Canadian border,English Channel, etc.)The point is this: communication with any satellite becomes difficult when the elevation falls below 20 to 40 degrees, but even in the best situation (I.e., the satellite and user are at the same longitude), a geostationary satellite is below 40 degrees elevation at north Atlantic latitudes.
11Waves Obscuring LOSFading by attenuation at small look angles; some multipath at large look anglesOperations in northern latitudes: near-horizon viewing of SATCOM satellitesOperations in sea-state four: wave heights of 1.25 to 2.5 meters (average = 1 PI)Depth of fades: ~7 1 GHzWave period: 5 to 10 secondsTime scale of fades: ~1-3 secondsThe combination of waves big enough to obscure the line-of-sight to the satellite, and high latitudes that place a geo satellite low on the horizon is probably the most difficult challenge to reliable, global SATCOM use by UUVs.Also, the depth of the fades and the long time scale are a serious challenge.Dispersion relation for deep-sea gravity waves is omega^2 = g k (or, more generally, omega^2 = g k * tanh(kd) ), where the wavenumber is k = (2 * pi)/L and the angular frequency is omega = (2 * pi)/T, L is the wavelength and T is the period.
12Ocean Surface Roughness Fading effects from multi-path interferenceLength scales of ~0.1 meter mean less interaction with UHF signalsMost serious at very small look angles
13Nature of the RF Channel The RF channel is principally a fading channel (Rayleigh channel, or channel with memory), and not a noisy channel (AWGN channel, or memoryless channel)BER has limited utilityMemory channels are much more difficult to simulate (Markov chains)Much less research exists for fading channelsMemoryless means that the statistical mechanisms for corrupting bits are independent in time; in a channel with memory, lost bits are strongly correlated.AWGN = Additive White Gaussian NoiseBER = Bit-Error Rate
14Protocol StrategyLook at low-level protocol strategies that will have the greatest utility in mitigating the predominantly fading maritime-communications channel.We now look in much greater depth at low-level protocol strategies that will contribute most in overcoming fading in the RF maritime comms channel.
15Illustration courtesy Catherine Werst This is an overabundant schematic of the OSI 7-layer model for network communications, put here to emphasize that we are looking at fundamental communications problems and that our solution will emphasize the lowest levels of the model.Illustration courtesy Catherine Werst
16BER vs. S/N BER 1 2 3 4 5 6 7 8 9 10 11 12 13 14 QPSK(BPSK) 1234567891011121314-6-5-4-3-2-1BERQPSK(BPSK)Non-coherent FSKEb/No(dB)This slide is to introduce a couple of important concepts that go along with talking about Bit-Error Rates:The presumed relation between BER and power, andThe idea of coding gain AT some BER.The x-axis is energy/bit per noise-energy/unit bandwidth, a precise way of quantifying signal/noise.
17Tradeoffs Discussed UUV System Configuration Satellite Access Methods Physical Layer: ModulationPhysical Layer: CodingLink-Layer Directions & RecommendationsThis slide outlines how we will organize our low-level protocol tradeoff discussions, plus a few other topics.
18UUV System Configuration AntennaType of antennaHousing for antennaAntenna on mastAntenna diversityPowerA system issue that affects RF performanceThere are system issues that have nothing really to do with protocols, and are outside our scope of consideration, but can affect the performance of the UHF communications system.
19Satellite Access Methods Single-User ChannelsFrequency Division: FDMACode Division: CDMASpread-spectrum approach would be challenging in satellite environment because of dynamic signal balancing Time Division: TDMA [& GSM]Provides adequate multi-use capabilitiesMature technologyCompatible with SATCOM, although different specifications may be desirableWe will recommend using TDMA as an access technique, since it provides plenty of capability and flexibility, possible compatibility with SATCOM, and basic compatibility if we were to implement our metaframing concepts.
20System-Level Schematic Bit-StreamTo PacketChannelEncodingMod.& XmitLinkLayerPhysicalLayerAir LinkWe have touched on issues relating to the radio system and air link (nature of noise in the channel).In the remainder we will be looking first (and briefly) at modulation techniques before moving on to a more detailed look at channel coding, then ending with some remarks about packets and higher-level protocols.Packet toBit-StreamChannelDecodingRec. &De-Mod.
21Bits rate vs. Symbol rate Bi-state:1 bit/symbol4-state:2 bit/symbolTo help clarify the concepts, and to make it clearer that advanced coded-modulation schemes don’t provide a useful gain in our system scenario.Despite the baud rate, which represents symbol rate in the channel and we might equate with carrier frequency, bit rate can be larger if we use a method to code more symbols by the state of the carrier.Symbol Transmissions (baud)
22Physical Layer: Modulation QQBPSKQPSK 1011I00101Q“M-ary” PSKTrellis-codedThese are in-phase [I] and quadrature [Q] phase diagrams for a carrier modulated using different symbol encoding methods, i.e. different modulation schemes.BPSK= binary phase shift keyingQPSK= quadrature phase shift keyingII
23information rate/bandwidth Modulation TradeoffThe result from information theory limitsinformation rate/bandwidth(i.e., baud rate), but not bits/second, which can be increased with a compensating increase in transmit power.Higher-order modulation techniques are useful for bandwidth-limited applications, but we recommend QPSK .The increase in power is necessary to offset the increase in noise susceptibility that comes from effectively packing more bits into a symbol transmission.In particular, telephone lines are bandwidth limited but not particularly power limited; higher throughput with modems has come about through the use of higher-order modulation techniques (plus error-correcting techniques).The current application is not particularly bandwidth limited, and power and noise can be an important issue.We recommend QPSK for its simplicity, it’s relative efficiency, and its familiarity, and it’s SATCOM compatibility.
24Channel Coding & Forward Error Correction Channel coding describes the process by which a logical bit stream gets turned into a modulated signal suitable for transmission of the desired information.Forward Error Correction (FEC) describes coding techniques that encode the bit stream so that errors in the received bit stream can be corrected.Coding & FEC all take place in the lowest layer of the protocol stack, transforming a bit stream into its most desirable form for modulating the carrier for transmission.The benefit of an FEC technique is described as its coding gain.FEC is not the same as error detection (e.g., CRC bits).Not all channel coding is FEC (e.g., NRZI).Channel coding is not a simple process, and it has benefited remarkably from theoretical advances in just the last few decades.
25FEC Techniques & Tradeoffs Block Codes, which operate on blocks of symbols, are generally better on block errorsHamming CodesBCH codesReed-Solomon Coding Convolutional Codes, which operate on the stream of bits, are generally better on bit errorsConvolutional Codes Viterbi Decoding Turbo Codes (Turbo anything is very hot)Turbo Product CodesCoding gains are generally 2 – 3 dBThis slide mentions names of most types of FEC techniques, so that we can discuss some tradeoffs, without going into any detail on many of them, but look more closely at the ones we recommend. Those in bold will be discussed further.
26Reed-Solomon Coding IRS coding operates on a block of symbols, not modifying it but adding additional bits to the stream that can correct errors in the block.RS(n,k)n encoded symbols; k message symbolst = (n – k)/2 symbols can be correctedThe degree of error correction depends on the number of added bits (2t).Adding bits increases bandwidth overhead.
27Reed-Solomon Coding II In principal: Modern theoretical treatments of Reed-Solomon coding use Galois group theory [GF(28)]!In brief: combinations of correcting bits can efficiently identify erroneous combinations of information bitsIn practice: The algorithms are widely available as firmware.NASA specifies RS(255,239) or RS(255, 223) for deep-space missions. We recommend RS(255,239) for its generally good performance and easy availability.In brief:O The group that describes a particular code has a generating polynomialO Bits are added to a block that make the total block into a polynomial that divides the generating polynomial
28Block Code Performance I Graph by K. AzadetIEEE High-Speed Study GroupPlenary meeting, MontrealJuly 1999
29Block Code Performance II Graph by K. AzadetIEEE High-Speed Study GroupPlenary meeting, MontrealJuly 1999
30Convolutional Encoding Concept +Xmit a12345…nXmit bConvolutional coding operates on the passing bitstream, in effect spreading information about each bit into the stream that comes behind it, allowing some bits to be lost and yet retain the ability to recover the original bitstream.+Constraint length (n); rate (1/2); puncturing.
31Convolutional Encoding Characteristics Operational choices:Constraint length (length of shift register)Generating polynomials (useful constraint lengths are generally known to have optimal choices)Symbol rate (determined by number of adders)Puncturing (to increase the symbol rate at the cost of decoding difficulty)Our recommendation, again, is based on simplicity, easy availability and, in this case, SATCOM compatibility.We recommend: Constraint length 7 Rate to be determined, but ½ and ¾ are common
32Viterbi Decoding (with Soft-Decoding Information) 111001etc.00Rec. bitstreamAs with Reed-Solomon coding, decoding the convolutionally encoded signal is much harder than encoding it in the first place.This device is a schematic of a decoding trellis.Viterbi’s contribution was creating an algorithm that could estimate the maximum-likelihood path by examining a relatively short number of nodal transitions.In principal: Maximum-Likelihood EstimationIn practice: Algorithms available in firmware Soft-decoding uses signal strength to assist in decision making, for a gain of ~2 dB.
33Concatenation Concept If one coding scheme is good,wouldn’t two be better?In fact, Reed-Solomon and Convolutional Encoding are complementary and commonly used together.We are thinking specifically of serial concatenation in which the bitstream is fed sequentially to two different encoders; parallel concatenation is also used, e.g. in Turbo Coding schemes.There is a practical limit to concatenation of course, and the one we recommend is practically as far as it usually goes.One of the more common implementations of the concatenation idea is in CD players.
34The Importance of Interleaving …Interleaving effectively transforms block errors into bit errors.Interleaving is neither error-correcting or error-detecting; it is an error avoidance technique.There is no coding gain associated with interleavingThe tradeoff in choosing the size of the interleaver is between time scale of bit dispersal and tolerable delay times in transmission.x1r1’x2r2’x3r3’x4r4’x1’…
35Recommended Concatenation Reed-Solomon Outer CodeRS(255, 239) Large-Order InterleavingScale to be determined by communication constraints, but scaled to be effective against fading from waves Convolutional Inner CodeConstraint length 7Rate ½ to ¾Viterbi decoder with soft-decisionCoding gain ~5 dB =>BER reduction from ~10-3 to 10-9
36Physical Layer Diagram TransmitReceive(Link-Layer Packets)Reed-Solomon Encoding, RS(255,238)Large-Scale Bit InterleavingConvolutional EncodingL=7, R=1/2 or 3/4NRZI EncodingQPSK modulation(Link-Layer Packets)Reed-Solomon Decoding, RS(255,238)Large-Scale Bit De-InterleavingViterbi Decodingwith Soft-DecisionNRZI DecodingQuadrature DemodulationAntenna DiversityNRZI encoding may not have been mentioned before.
37Link Layer Recommendations Minimize Automatic Retransmit Request (ARQ)Reserve it for higher-level protocolsMinimize handshakingRecognize transmission delay timesBuild a delay-tolerant networkWe found that more effort was required in the physical layer to understand and choose details correctly, to build a solid foundation for the higher protocol layers.
38MetaframingA concept we introduced in the proposal, but didn’t develop in Phase INot SATCOM compatible, but similar in framework.Requires:Good receiver timing and clock synchronizationReconsideration of guard timesData backcapture to achieve gains
39Hierarchy of Recommendations EasyImplement within UHF SATCOM TDMA/DAMA capabilityMediumProtocols beyond SATCOM capabilitiesPossibly implemented over dedicated SATCOM channelsMay require new radiosMay affect UUV system designHardNot compatible with existing SATCOM specificationsThese categories are all “roughly speaking”, not precisely defined.
40Easy Recommendations Use Quadrifilar Helix Antenna Use existing TDMA/DAMA satellite accessUse existing QPSK ModulationUse existing convolutional inner codelength 7, rate ½ or ¾Add additional interleaving and RS(255,239) outer code if possibleSATCOM offers this convolutional encoding; we don’t know whether existing radios implement Viterbi decoding (probably) with soft-decision processing (maybe).In addition, interleaving and RS encoding are not specified by SATCOM; it is not clear whether one could implement these usefully outside of existing radio systems, or whether they would need to be implemented “in the guts”, i.e., require a new radio design to be concatenated effectively.
41Medium Recommendations Implement antenna diversityImplement in custom-designed radio:Full concatenated channel coding:Reed-Solomon outer code: RS(255, 238)Interleaving (depth to be determined)Convolutional inner code: length 7, rate ½ or ¾Viterbi decoder with soft-decodingDefine delay-tolerant link-layer through transport-layer protocols.Possibly redesign TDMA specifications (over dedicated SATCOM channel)
42Hard RecommendationBuild a new, non-geostationary satellite system that will give significantly better coverage over the oceans and ease the RF communication channel’s susceptibility to fading because of high seas and small look angles.