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1 Presentation on Phase I Work
Robust Ultra High Frequency (UHF) Satellite Communications Protocol for UUVs SBIR Topic #: N02-019 Contract #: N C-4577 Deliverable Item 0001AC Presentation on Phase I Work Wavix, Incorporated 27 January 2003

2 Phase-I Objectives Understand the maritime noise environment
Characterize RF (UHF) communication impediments Identify mitigating (low-level) protocol techniques Recommend development paths to follow

3 The Challenge

4 Maritime Noise Concerns Original List
Signal fading from wave motions rocking UUV High seas coating antenna or changing local RF propagation characteristics Shadowing by waves Surface wave reflections causing multi-path interference Changing atmospheric conditions in LOS Nature 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.

5 Maritime Noise Concerns Non-Obstacles I
Antenna wash-over Choose appropriate non-conductive housing Water has small attenuation at UHF Ground-plane issues Appropriate choice of antenna: Quadrifilar Helix has small sensitivity to ground plane Helical antenna is very sensitive to ground plane This 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).

6 Maritime Noise Concerns Non-Obstacles II
Water aerosols Water has little attenuation in UHF band (Atmospheric water effects become serious above ~2-4 GHz) UUV rocking Motions are too small to disrupt signals Motions are too slow for Doppler effects Considering 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.

7 Characteristic Length Scales
Wave Heights: 2 m RF Wavelength: 1 m 300 MHz) Surface Roughness: 0.1 m This 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.

8 Characteristic Time Scales
Wave times: ~5-10 s SATCOM frames: 8.96 s, made of 1024 blocks [5 kHz waveform] s, made of ms time chips [25 kHz waveform] Geo propagation time: 500 ms SATCOM blocks: 8.75 ms blocks in 5 kHz waveform; variable number/channel 0.052 ms “time chips” in 25 kHz waveform; channel format varies but order of 100 might be typical ATM packet size: ~2.5 ms IP packets are 4 to 10 times larger, but variable Symbol 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.

9 Maritime Noise Concerns Obstacles
Small look angle to satellite  Wave obscuring Ocean-surface roughness Nature of the RF channel From 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.

10 Satellite Elevation vs. Latitude
Latitudes = Elevations (for G = 0!) = 82 = 71 = 56 50* = 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.

11 Waves Obscuring LOS Fading by attenuation at small look angles; some multipath at large look angles Operations in northern latitudes: near-horizon viewing of SATCOM satellites Operations in sea-state four: wave heights of 1.25 to 2.5 meters (average = 1 PI) Depth of fades: ~7 1 GHz Wave period: 5 to 10 seconds Time scale of fades: ~1-3 seconds The 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.

12 Ocean Surface Roughness
Fading effects from multi-path interference Length scales of ~0.1 meter mean less interaction with UHF signals Most serious at very small look angles

13 Nature 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 utility Memory channels are much more difficult to simulate (Markov chains) Much less research exists for fading channels Memoryless 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 Noise BER = Bit-Error Rate

14 Protocol Strategy Look 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.

15 Illustration 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

16 BER vs. S/N BER 1 2 3 4 5 6 7 8 9 10 11 12 13 14 QPSK(BPSK)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 -6 -5 -4 -3 -2 -1 BER QPSK(BPSK) Non-coherent FSK E b /N o (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, and The 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.

17 Tradeoffs Discussed UUV System Configuration Satellite Access Methods
Physical Layer: Modulation Physical Layer: Coding Link-Layer Directions & Recommendations This slide outlines how we will organize our low-level protocol tradeoff discussions, plus a few other topics.

18 UUV System Configuration
Antenna Type of antenna Housing for antenna Antenna on mast Antenna diversity Power A system issue that affects RF performance There 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.

19 Satellite Access Methods
Single-User Channels Frequency Division: FDMA Code Division: CDMA Spread-spectrum approach would be challenging in satellite environment because of dynamic signal balancing  Time Division: TDMA [& GSM] Provides adequate multi-use capabilities Mature technology Compatible with SATCOM, although different specifications may be desirable We 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.

20 System-Level Schematic
Bit-Stream To Packet Channel Encoding Mod. & Xmit Link Layer Physical Layer Air Link We 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 to Bit-Stream Channel Decoding Rec. & De-Mod.

21 Bits rate vs. Symbol rate
Bi-state: 1 bit/symbol 4-state: 2 bit/symbol To 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)

22 Physical Layer: Modulation
Q Q BPSK QPSK  10 11 I 00 1 01 Q “M-ary” PSK Trellis-coded These 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 keying QPSK= quadrature phase shift keying I I

23 information rate/bandwidth
Modulation Tradeoff The result from information theory limits information 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.

24 Channel 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.

25 FEC Techniques & Tradeoffs
Block Codes, which operate on blocks of symbols, are generally better on block errors Hamming Codes BCH codes Reed-Solomon Coding  Convolutional Codes, which operate on the stream of bits, are generally better on bit errors Convolutional Codes  Viterbi Decoding  Turbo Codes (Turbo anything is very hot) Turbo Product Codes Coding gains are generally 2 – 3 dB This 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.

26 Reed-Solomon Coding I RS 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 symbols t = (n – k)/2 symbols can be corrected The degree of error correction depends on the number of added bits (2t). Adding bits increases bandwidth overhead.

27 Reed-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 bits In 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 polynomial O Bits are added to a block that make the total block into a polynomial that divides the generating polynomial

28 Block Code Performance I
Graph by K. Azadet IEEE High-Speed Study Group Plenary meeting, Montreal July 1999

29 Block Code Performance II
Graph by K. Azadet IEEE High-Speed Study Group Plenary meeting, Montreal July 1999

30 Convolutional Encoding Concept
+ Xmit a 1 2 3 4 5 n Xmit b Convolutional 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.

31 Convolutional 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

32 Viterbi Decoding (with Soft-Decoding Information)
11 10 01 etc. 00 Rec. bitstream As 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 Estimation In practice: Algorithms available in firmware  Soft-decoding uses signal strength to assist in decision making, for a gain of ~2 dB.

33 Concatenation 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.

34 The 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 interleaving The tradeoff in choosing the size of the interleaver is between time scale of bit dispersal and tolerable delay times in transmission. x1 r1’ x2 r2’ x3 r3’ x4 r4’ x1’

35 Recommended Concatenation
 Reed-Solomon Outer Code RS(255, 239)  Large-Order Interleaving Scale to be determined by communication constraints, but scaled to be effective against fading from waves  Convolutional Inner Code Constraint length 7 Rate ½ to ¾ Viterbi decoder with soft-decision Coding gain ~5 dB => BER reduction from ~10-3 to 10-9

36 Physical Layer Diagram
Transmit Receive (Link-Layer Packets) Reed-Solomon Encoding, RS(255,238) Large-Scale Bit Interleaving Convolutional Encoding L=7, R=1/2 or 3/4 NRZI Encoding QPSK modulation (Link-Layer Packets) Reed-Solomon Decoding, RS(255,238) Large-Scale Bit De-Interleaving Viterbi Decoding with Soft-Decision NRZI Decoding Quadrature Demodulation Antenna Diversity NRZI encoding may not have been mentioned before.

37 Link Layer Recommendations
Minimize Automatic Retransmit Request (ARQ) Reserve it for higher-level protocols Minimize handshaking Recognize transmission delay times Build a delay-tolerant network We 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.

38 Metaframing A concept we introduced in the proposal, but didn’t develop in Phase I Not SATCOM compatible, but similar in framework. Requires: Good receiver timing and clock synchronization Reconsideration of guard times Data backcapture to achieve gains

39 Hierarchy of Recommendations
Easy Implement within UHF SATCOM TDMA/DAMA capability Medium Protocols beyond SATCOM capabilities Possibly implemented over dedicated SATCOM channels May require new radios May affect UUV system design Hard Not compatible with existing SATCOM specifications These categories are all “roughly speaking”, not precisely defined.

40 Easy Recommendations Use Quadrifilar Helix Antenna
Use existing TDMA/DAMA satellite access Use existing QPSK Modulation Use existing convolutional inner code length 7, rate ½ or ¾ Add additional interleaving and RS(255,239) outer code if possible SATCOM 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.

41 Medium Recommendations
Implement antenna diversity Implement 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 ¾ Viterbi decoder with soft-decoding Define delay-tolerant link-layer through transport-layer protocols. Possibly redesign TDMA specifications (over dedicated SATCOM channel)

42 Hard Recommendation Build 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.


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