A turbo-coded hybrid ARQ for low earth orbit microsatellite communications

*Correspondence to: Yusep Rosmansyah, Centre for Communication Systems Research (CCSR), University of Surrey,Guildford GU2 5XH, U.K. E-mail: [email protected]

Contract/grant sponsor: Institut Teknologi Bandung (ITB), Indonesia

CCC 0737}2884/99/060367}15$17.50 Received November 1998Copyright ( 1999 John Wiley & Sons, Ltd. Accepted June 1999

INTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS

Int. J. Satell. Commun. 17, 367}381 (1999)

A turbo-coded hybrid ARQ for low earth orbitmicrosatellite communications

Yusep Rosmansyah1*, Peter Sweeney1 and Martin N. Sweeting21 Centre for Communication Systems Research (CCSR), University of Surrey, Guildford GU2 5XH, U.K.

2 Surrey Space Centre (SSC), University of Surrey, Guildford GU2 5XH, U.K.

SUMMARY

A proposed turbo-coded hybrid ARQ designed for improving the throughput of UoSAT/SSTL LEOmicrosatellite downlink communications is presented. The #exibility and robust error correcting capabilityof turbo codes are incorporated adaptively into the existing ARQ protocol. The proposed scheme optimizesthe increment of coding parity and schedules the decoding iterations to take place during intervals betweenmicrosatellite passes. Simulations show a remarkable improvement over the existing protocol, giving morethan 84% gross throughput or 69% net throughput at channel bit error rate of 5]10~3, whereas with thecurrent protocol the throughput would be close to zero. Copyright ( 1999 John Wiley & Sons, Ltd.

KEY WORDS: turbo codes; hybrid ARQ; LEO; microsatellite

1. Introduction

This work assesses the hybridization of turbo codes (TCs) with the PACSAT broadcast automaticretransmission request (PB-ARQ) currently in use by Surrey satellite technology limited(UoSAT/SSTL) low earth orbit (LEO) microsatellites. It is an improvement to a similar schemebased on convolutional codes.1,2 Performance, compatibility, simplicity, and customizability areimportant design criteria. Therefore, the description is oriented towards optimization of practicalconstraints and requirements. Computer search is the main design tool, and evaluation is carriedout by way of simulations. The ultimate goal is to implement a high throughput hybrid ARQprotocol suitable for LEO microsatellite downlink communication subsystems.

Several studies concerning the hybridization of TCs to ARQ have appeared in publications.3~7

However, all these are general treatises, and some of them are too complex to be implemented ina LEO microsatellite environment.

Although the proposed scheme is intended speci"cally for a downlink LEO microsatellitecommunication subsystem, the basic concept would also be applicable to related systems, as inEarth observation satellites. Based on the experience of operating LEO microsatellite communi-cation systems,8 the downlink turns out to be more crucial than the uplink. Besides facing the

problems su!ered by uplink, the downlink has its additional limitations and requirements,including:

f The onboard transmitter resources*power, antennas, and ground plane*are very limited.8f As the satellite takes Earth images it creates several megabytes of "les which need to be

regularly down-loaded.f The link represents a broadcast link, point-to-multipoint communication, with the micro-

satellite acting as a critical central node. The link protection is even more important for datadissemination to portable power-limited receive-only terminals.

The rest of the paper are organized as follows: Section 2 outlines the existing system, includingthe observed channel characteristics. In Section 3, the design of the proposed scheme is described.Simulation results along with brief analysis are presented in Section 4. The conclusions of thepaper are found in Section 5.

2. System description

2.1. Dexnitions

In this paper, the throughput e.ciency (simply throughput for brevity) g is de"ned as the ratio ofthe amount of &information' successfully received to the amount of &raw data' received in attemptsof transferring the &information'. Nevertheless, there are two possible de"nitions that may beadopted. First, the gross throughput g

'3044is the measure commonly used by other authors in

assessing theoretical ARQ schemes.3,4,7,9~11 Apart from parity bits for error correction anddetection, all other data items are classi"ed as &information'. Also, data items allocated for framesynchronizations are not taken into account. Thus,

g'3044

"

+M&information'N transferred correctly

M+ M&information'N#+ MparityNN transferred(1)

Second, net throughput g/%5

is a user-oriented throughput measure, in which &information' referssolely to the user's "le being transferred. The &raw data' refers to the total accumulated data of allkinds, that is, user's "le, frame synchronizations bits, and parity bits. It can be expressed as

g/%5

"

+ Muser's fileN transferred correctly

+ Mdata of all kindsN transferred. (2)

2.2. The existing protocol

The currently operational UoSAT/SSTL LEO microsatellites employ an uncoded &hole-"lling'ARQ for their downlink communications. This can be thought of as a special type of selectiverepeat (SR-) ARQ. During a "le downloading, the receiving ground station maintains a list of thepackets that are received erroneously. These represent &holes' in the received "le, hence the name.The retransmission is not requested until the "le transfer is complete.

Figure 1 shows the frame allocation.12 Notice the two nested CRC-16s used for error detection.With uncoded communication, it can be easily deduced that the throughput may drop rapidlywith the increasing channel bit error rate (BER).

368 Y. ROSMANSYAH, P. SWEENEY AND M. N. SWEETING

Copyright ( 1999 John Wiley & Sons, Ltd. Int. J. Satell. Commun. 17, 367}381 (1999)

Figure 1. The current frame allocation. (a) PACSAT broadcast frame. (b) Amateur radio version of X.25 protocol:AX.25 frame

2.3. The scrambled link

A scrambler is used at the physical layer in favour of timing recovery at the receiver since itguarantees that the scrambled bit sequence avoid long consecutive sequence of &1's or &0's.Scrambling is carried out by regarding a complete AX.25 frame as an input data polynomial andthen dividing it by the scrambling}descrambling polynomial G(X). The descrambler performsjust the opposite: multiplying the scrambled data polynomial by the same G(X). It is from thisdescrambling mechanism an undesired e!ect arises, that is, BER multiplication. Given

G(X)"X17#X12#1, (3)

it can be reasoned that a single bit error in the scrambled data will generate another two bit errorsupon descrambling, each located 12 and 17 bits away from the original one. Figure 2 might clarifysuch an e!ect. Thus, the multiplicative factor is 3, although the actual factor will reduce whenBER is higher. While this has no bad consequences for the uncoded link, for a single bit erroris as harmful as hundreds, it will obviously hinder the error correction capability of the codedscheme.

Modulation and demodulation scheme employ a type of continuous phase binary frequencyshift keying (CPBFSK) derived from frequency modulation (FM) scheme. A concept of FM signaldiscrimination is applied for demodulation.13

2.4. The observed channel characteristics

Referring to References 8 and 13, it can be summarized that the errors in the LEO downlinkmicrosatellite channel are predominantly hostile due to a number of factors which becomes worsewhen they occur simultaneously. Furthermore, since at least 80% of time a microsatellite is visibleat less than 203 elevation angle, the characteristic is aggravated by terrestrial channel impair-ments. Despite these qualitative characteristics, no analytical nor empirical channel model isavailable yet, so that the development of a coded scheme still has to make channel modelassumptions.

2.5. The LEO delayed-communication link

Unlike those of the common geostationary satellites, the footprint of a LEO microsatellite ismoving with respect to a particular position on the Earth. Viewed from a "xed ground stationlocated at a high latitude communicating with a polar orbiting microsatellite, a pass*a state

A TURBO-CODED HYBRID ARQ 369


Figure 2. The implementation of the scrambler and descrambler de"ned by equation (3)

where a ground station is within the footprint of the microsatellite*lasts about 12}20 min andthe interval between passes is within 4}6 h. For a typical UoSAT/SSTL microsatellite that isavailable via NASA's Rapid Spacecraft Acquisition Program with 80 MB RAMdisk capacity and38)4 kbps transmission bit rate,14 it is obvious that less than 7)2% of the whole RAMdiskcontents can be downloaded in one pass under the most optimistic conditions.

3. Basic design

3.1. Choice of codes

TCs are taken into consideration because of the following reasons:

1. Their powerful error correcting capability would be a demanded feature for such anunknown channel, and even more, knowing that the microsatellite is power-limited. Punc-turing and code combining can make their rates, hence their error correcting capability,vary, from approximately 1)0 to as small as feasible. By employing TCs, the resulting hybridARQ scheme can practically operate in either uncoded, recursive systematic convolutional(RSC)-coded, or turbo-coded mode.

2. The availability of a wide range of code parameters allows customization. For a particularencoder, there is #exibility in the choice of decoder and its parameters. This feature enablescustomization to meet the environmental requirements. For instance, for a main groundstation located in a noisy environment, one may select the most powerful (therefore the mostdemanding) decoder along with the most powerful parameter selection (e.g. large number ofiterations), as opposed to those for a portable terminal operated in a quiet environment.

3. The simplicity of their encoding process is well suited to the high reliability, low computingpower and low memory requirements of the on-board microsatellite system, regardless ofwhether implementation is in hardware or software.

4. Their systematic nature allows code hybridization with minimal changes (that is, thecompatibility requirement) to the existing uncoded scheme which has been proven tooperate reasonably well.



Figure 3. A turbo encoder

5. Their coded (parity) bit sequences emanating from the RSC constituent encoders posses thecharacteristic of the scrambled bit sequence, since both RSC encoders and scrambler arebased on recursive shift register operations. The di!erences are that the RSC encoderemploy shorter memory length and have additional feed forward connections. Taking theadvantages from this characteristic, the scrambler can be bypassed when transmitting thesesequences, so that the BER multiplication e!ect is avoided.

6. They inherently encourage the use of frequency or link diversity and code combining iffurther improvement is considered necessary.

7. In one-way image transmission, where BER causes gradual degradation, their ability ofimproving the post-decoding BER through iterations is yet another interesting feature.

Decoding delay and complexity are the main disadvantages that need to be addressed.Fortunately, the decoding is performed on the ground, and therefore the complexity is nota critical issue. Likewise, for a non-real-time communication system such as that of a LEOmicrosatellite downlink communication subsystem, decoding delay caused by iterations anddeinterleaving processes is hardly a problem. In fact, it is only a tiny fraction of the microsatellite-pass interval.

3.2. Turbo code

The TC used as a mother code in the proposed scheme is that based on parallel concatenationof two RSC constituent encoders, an interleaver, and two constituent soft-input soft-output(SISO) decoders for performing iteration decoding. This is similar to that in Reference 15. Theencoder and decoder are exempli"ed in Figures 3 and 4, respectively. The determination of theparameters is to be found in Section 4.1.

3.3. Protocol

Given the characteristics of TCs, the constraints imposed by the microsatellite, and nature ofLEO downlink communication subsystem, it is possible to hybridize a TC into the existing ARQ



Figure 4. A turbo decoder

Figure 5. The proposed scheme. The turbo coded hybrid PB-ARQ is just an extension of the capability of the existingPB-ARQ scheme

such that it allows uncoded}scrambled mode for transmission session and coded}unscrambledmode for retransmission session. A top level architecture of this scheme is illustrated in Figure 5.Basically, there are two modes of operation:

1. Conventional uncoded ARQ mode for transmission session (0th retransmission): Apart frombu!ering the erroneously received packets, the protocol acts as it originally is, as if therewere no added-in capability. This implies the use of uncoded-and-scrambled link anddesignated as THE EXISTING PB-ARQ in Figure 5. This mechanism is made possiblethanks to the systematic nature of TCs.

2. ¹urbo-coded ARQ mode for retransmission session (1st, 2nd, 2nth retransmissions): Themode of ARQ operation is switched to this mode by the retransmission request from theground station. In Figure 5 it is designated as TURBO CODED HYBRID PB-ARQ. In thisscheme, considering that a rate-1

3mother TC is powerful enough, thus only parity bits that

will be sent during retransmission session. Therefore, as mentioned above, there will be noscrambling}descrambling processes involved. Filling the &holes' now is no longer retransmit-ting the corresponding packets, but rather, retransmitting a portion of the parity corre-sponding to those packets.

Having devised the modes of operation, the following matters need to be considered:

1. ;tilization of quantizer at the demodulator: The need of soft-decision demodulated bitsmeans the need to upgrade the current demodulator to be able to produce quantized values,rather than simply producing hard-decision bits.



Figure 6. The proposed frame allocation

2. Soft-decision descrambling: With a TC capability in place, the erroneously received packetsare now not wasted. Rather, their soft-decision values are bu!ered to be used later by thedecoder in case retransmissions are requested.

Unfortunately, utilization of the scrambler}descrambler circuitry (even only for theuncoded packets, i.e., the systematic bit sequence), not only causes BER multiplication e!ect,but also gives rise to the problem as how to perform soft-decision descrambling at thereceiver.

The hard-descrambling process is practically performed by XOR-ing (or adding modu-lo-2) the current bit with the delayed ones selected according to G(X) coe$cients. See againFigure 2. Now, the problem is a question of how to perform soft-XOR operation. Denoting5 as soft-XOR operator, the principle of log-likelihood ratio (LLR) summation16,17 may beadopted. For practical purposes with simplicity in mind, and taking into account that thedecision process in the demodulator does not use maximum a posteriori probability (MAP)criteria, an approximation turns out to give acceptable results:

a5 b+!sign(a) sign(b)min(DaD, DbD) (4)

where a and b are any soft-decision bits. This expression indicates that the soft-XORoperation tends to reduce the con"dence values of the resulting soft-decision bits. In view ofsoft-decision decoding, this is another unfavored e!ect from the use of scrambling}de-scrambling.

As additional information, the contents of the soft-descrambler shift register cells areinitialized to &!1' values. The hard-XOR operator = in Figure 2 is accordingly replaced byits soft version 5.

3. ;tilization of the existing frame allocation for the uncoded packets: Referring to the opensystem interconnection (OSI) layering terminology, the design focuses on the data-link layer.The existing frame allocation is used for the uncoded packets, whereas a simple frameallocation containing parity and frame synchronization #ags is devised for the coded ones,as seen in Figure 6. There are no changes to the contents of the headers belonging to otherlayers.



4. Re5ning the proposed scheme

4.1. TC parameters

The determination of important parameters of the chosen TC are outlined as follows:

f RSC constituent encoders: A TC with memory length of 4 is chosen as a trade-o! among theerror correcting capability, decoding complexity, and the ease of handling within a byte-oriented protocol such as the PB-ARQ. For similar reason, two identical RSC constituentencoders are adopted. Many publications con"rmed by our simulations suggest that RSC(1,35/23, 35/23) performs better for the problem at hand, and hence applied. Such an encoder isportrayed in Figure 3.

f SISO decoding: Soft-output Viterbi algorithm (SOVA) and MAP decoding algorithm are theprevailing decoding choices for use as turbo constituent decoders. Both may be equallyapplicable for our purpose as part of customization depending on ground station environ-ment. However, as performance of MAP is better, this was chosen in this work. Simpli"edMAP (SMAP) decoding algorithm derived in Reference 18 is implemented and appropriate-ly modi"ed.

f Channel noise variance estimation: Generally, TCs are insensitive to channel noise varianceerror.19,20 Computer search indicated that assigning a constant value of 0)25 resulted inunnoticeable degradation compared to that with exact knowledge. Therefore, this strategy isapplied.

f Interleaver and trellis termination: As suggested in References 21, 22, for the scheme inconsideration, pseudorandom interleaver and full trellis termination strategies are betterchoice, hence adopted. Any further enhancements such as trellis self-terminating interleaver,list decoding, and code combining have not been considered and deferred until the detailedchannel characteristic is obtained.

f Iteration termination: Since CRCs are readily available in the existing protocol beinghybridized and continue to be used, iteration termination is decided by computing the innerCRC.

f Maximum number of iterations: Assignment of maximum number of iterations on eachretransmission is related to optimization of retransmission scheduling and is the subject ofthe subsection that follows.

4.2. Retransmission scheduling

In order to obtain a high throughput but relatively simple ARQ, a speci"c retransmissionscheme must be formulated such that it retransmits only a certain amount of redundancy justenough for correcting the erroneous bits in a particular received packet. This can be broken downto the problems of

f minimizing the total number of transmitted parity bits,f minimizing the number of retransmissions,f managing the decoder to be able to exploit its maximum error correcting capability.

Essentially, this can be regarded as a problem of "nding optimum puncturing matrices. Therest involve accommodating the consequences of the resulting matrices so that the overall designworks properly. The lack of proper channel model results in a simplistic design and evaluation



Table I. Scheme parameters for highest achievable gross throughput at channel BER)0)1(legend: [G3] TC, Max

}Rtx"7, Q"256, unscrambled)

Order of retransmission 0 1 2 3 4 5 6 7

Max. no. iterations 0 0)5 15 15 15 15 15 15Code rates (approx.) 1 8/9 4/5 8/11 2/3 8/13 4/7 8/15

Puncturing matrices AFF0000 B A

FF8000 B A

FF8080 B A

FF8180 B A

FF8181 B A

FFA181 B A

FFA1A1 B A

FFA5A1 B

procedure when taking the assumption for the channel characteristic (see Section 5.1). A moreappropriate design is awaiting results of the on-going channel measurement campaign at SSC.

Moreover, other parameters such as number of iterations in every decoding process can only beset empirically. This is due to immature concept of TCs in general. The improvement per iterationin TCs still cannot be explained analytically.23 However, the time availability in each microsatel-lite-pass interval (4}6 h) renders the problem immaterial.

Starting with the encoding process, Figure 6a shows the original packet excluding thesynchronization #ags. Inputting this packet, by "rst excluding its outer CRC, to a mother turboencoder as in Figure 3 results in 4 new blocks of parity denoted by CODED

}FRAME

}1,

CODED}FRAME

}2, TAIL

}1, and TAIL

}2, as depicted in Figure 6b. It is these blocks that are

subject to selection by the puncturing matrices during the retransmission session. In general,information block of Figure 6a and parity blocks of Figure 6b can be regarded as 3 output blocksof the mother turbo encoder, hence the 3-row puncturing matrices. The number of columns waschosen to be 8. This is considered a good compromise between the granularity of the decrementalcode rates and the ease of representation in byte-oriented protocol implementation. As seen inTable I, it is even more concise to represent them in hexadecimal notation.

The puncturing matrices listed in Table I resulted from the fact that increasing the redundancyby small steps will eventually satisfy the required redundancy by nearly an exact amount, henceavoiding oversupply. In this case, in every retransmission, only the smallest additional redun-dancy possible is sent, that is, 1

24part of the total blocks. In terms of gross throughput, this scheme

is expected to give the highest performance.The second column of Table I corresponds to the transmission session. There are no decoding

nor iterations. If the redundancy bits used for CRCs were not taken into account, the overall coderate would be 1)0. Likewise, the associated puncturing matrix may be interpreted as &transmit theentire "rst block of the encoder outputs, and keep the rest'.

The third column represents the "rst retransmission session. Note that we select and retransmitonly 1

8part from the CODED

}FRAME

}1 plus TAIL

}1. This means that the turbo code is treated

as a RSC code, so that only 12

iteration is required. Total code rate is approximately 89. It is

obvious that the "rst and second retransmission requests can be generated in real-time, providedthe "rst constituent SISO decoder is able to keep pace with the operational bit rate.

Columns 4}7 of the table indicate that the protocol operates in full TC modes. Therefore,TAIL

}2 is now required. If the turbo decoder incorporating several iterations could not cope with

the operational bit rate, the decoding process could be performed during microsatellite-passintervals.



Table II. Scheme parameters for practical implementation over un-scrambled link (legends: [G4] and [N2] TC, Max

}Rtx"3, Q"16,

unscrambled)

Order of retransmission 0 1 2 3

Max. no. iterations 0 0)5 6 6Code rates (approx.) 1 8/9 8/11 8/15


FF8000 B A

FF8180 B A

FF85A1 B

E!ective for BER) 0 1)10~3 5)10~2 1)10~1

Table III. Scheme parameters for practical implementation(legend: [N3] TC, Max

}Rtx"4, Q"16, scrambled)

Order of retransmission 0 1 2 3 4

Max. no. iterations 0 0)5 9 12 5Code rates (approx.) 1 8/9 8/10 8/15 8/22


FF8000 B A

FF8080 B A

FFA5A1 B A

FFF7F7 B

E!ective for BER) 0 1)10~4 1)10~2 5)10~2 1)10~1

As far as real implementation is concerned, maximizing the net throughput has to be compro-mised with the maximum number of retransmissions and levels of quantization. Analysis andcomputer search resulted in the scheme parameters listed in Tables II and III are believed to givegood performances over unscrambled and scrambled channel, respectively. Notice that themaximum number of retransmissions has been reduced to 3 (4 for scrambled link) and themodulator output has been quantized to 16 levels. Additionally, for the sake of implementationsimplicity, TAIL

}2 is sent along with TAIL

}1 at the "rst retransmission, so that a minor penalty

in performance will be observed.

5. Simulation results and discussions

5.1. Assumptions

In assessing the performance, the following assumptions are taken:

1. ;tilization of A=GN channel model: The reasons for using additive white Gaussian noise(AWGN) as channel model are twofolds: "rst, the unavailability of proper channel model oran approximation thereof prompts the choice to a simple and conventional one. Second, theAWGN has been widely used by other authors to assess their schemes. Therefore, it ismandatory to adopt the same approach if performance comparison is to be carried out



fairly. If, instead of BER as in our case, Eb/N

0was used for channel characterization,

the equivalent value could be calculated correspondingly given the applied modulationscheme.

2. Negligibility of return link problems: Most of the time, there is no signi"cant communicationproblem for uplink.13 This is justi"ed by the quick responses in uploading "les to theUoSAT/SSTL microsatellites.

3. Negligibility of physical layer problems: It is assumed that synchronizations are alwayssuccessful regardless of the integrity of synchronization bit #ags. Again, this is for the sake ofcomparability with the published schemes. In practice, this assumption would not be validparticularly when channel BER is very high. The e!ect would eventually be the degradationof the throughput.

4. Negligibility of propagation time e+ects: The maximum propagation time of an 800 km orbitmicrosatellite is approximately 17 ms, that is, when the elevation angle is 03. This representshalf of the waiting time for a microsatellite in order to receive a retransmission request fromthe ground station since it transmitted the last packet consituting a downloaded "le. Thee!ect of this applies to any form of hole-"lling ARQ, no matter whether it is uncoded orcoded. However, with a coded scheme, the number of retransmissions will be minimized, andthe overall throughput is improved. Nonetheless, this e!ect is not taken into account in thispaper.

5. Dismissal of the non-maximum packet length cases: The case for packet length other thanthe maximum is not discussed. Consider for instance the minimum packet length, that is,when the packet contains only one-byte user's "le segment. In the error-free channelalone, the net throughput is only 1

34, for the 33 bytes being overhead. Moreover, non-

maximum packet length case is an unavoidable consequence of packetized communicationsanyway.

5.2. Gross throughput performance

The performance curves of the schemes represented in Figure 7 are compatible with thethroughput measure commonly used in other publications. Scrambler}descrambler are notemployed in these simulations. From the practical point of view, this omission provides perfor-mance estimation when scrambled physical layer is removed. Referring to the legends startingfrom top, the "rst two curves partially denoted by [G1] and [G2] are taken from Reference 1 forcomparison. These correspond, respectively, to the gross throughput of the uncoded ARQ andthe hybrid convolutional-coded ARQ with Viterbi decoding and code combining. The remainingtwo curves are variants of the proposed scheme: the third one, [G3], corresponds to a set ofparameters listed in Table I; and the last one, [G4], is that listed in Table II with the number ofquantization levels of demodulator output being 16.

It can be seen that [G3] gives maximum gross throughput. The minor dip on the curve at BER1]10~3 results from oversupply of redundancy primarily because of TAIL

}2 which is sent

unnecessarily. It may also be due to the coarse decremental rate from the use of only 8-columnpuncturing matrices. While this scheme gives maximum gross throughput, allowing such largenumber of possible retransmission might not be feasible in real application. Decreasing both themaximum number of retransmissions to 3 and quantization levels to 16 as in the schemerepresented by [G4], the resulting performance degrades only slightly. This scheme is found to bepractically optimum for the unscrambled link.



Figure 7. Gross throughput performances of various PB-ARQ schemes simulated over AWGN channels

Both variations of turbo coded schemes outperform the rest remarkably. While gross through-put of the uncoded scheme starts dropping close to 0% at BER 5]10~3, those based on TC stillgive more than 84%.

Similarly, these schemes also give better throughput performance compared to the schemesbased on convolutional codes9,24 and indirectly,10,11,25 block codes,26 or product codes.27Comparisons with similar schemes based on TCs requires careful analysis, since the authors mayuse di!erent TC parameters as well as measure of merits. Assuming binary phase shift keying(BPSK) modulation schemes are used and plotting the curve againts E

b/N

0, it can be shown that

the proposed schemes give better throughput performance than those in References 3 and 4. Asregards to the similar schemes in References 5 and 6, comparison is not straightforward, as theyuse frame error rate (FER) for assessing the performance. The performance of the proposedschemes look slightly better than that in Reference 7, but the TC parameters are di!erent.

5.3. Net throughput performance

The net throughput simulations are intended for assessing the e$ciency measure from thepoint of view of users. Based on these performance curves, given the transmission bit rate andBER estimation of the channel, users can estimate an actual transfer rate when they download"les. The results are summarized in Figure 8. Note that for an uncoded scheme [N1], the netthroughput performance over a scrambled link is identical to that of the unscrambled one, for thereason described in Section 2.3. The second one, the [N2], is associated with the [G4] in Figure 7.This can be regarded as an assessment of performance if scrambler}descrambler were not present.



Figure 8. The net throughput performances of various PB-ARQ schemes simulated over AWGN channels andscrambled links

The last one, designated by [N3], is the performance of the scheme of which parameters are givenin Table III. Both of the last two schemes employ 16 levels of demodulator output quantization.

Comparing the performances of the last scheme [N3] with that of the second one [N2],a reasonable degradation due to the use of scrambler}descrambler can be noticed. An extraretransmission, sums up the maximum number to 4, is required to cope with the prescrambledBER of more than 1]10~1. However, in general, owing to the robustness of TC, for prescram-bled channel BER of less than 1]10~2, a rate-4

5TC is shown to be su$cient. In other words, the

average number of retransmissions will be not more than 2, unless the channel is extremelyerroneous. Again, this scheme still gives a remarkable improvement over the existing one [N1].

5.4. Decoding speed

As an illustration of the decoding speed, using a modest 80486DX2-66 PC, the time taken fordecoding a packet using unoptimized SMAP software decoder is around 1)8 s, corresponding toapproximately 1227 bps decoding speed. This result indicates that for a professional implementa-tion, either a powerful processor or hardware decoder, whether it is based on digital signalprocessors (DSPs) or "eld programmable gate array (FPGA), is required.

6. Conclusions

The proposed scheme has shown a remarkable performance. Further re"nement of parameters ispossible once the operational channel characteristic is known. The scrambled link makes the



improvement o!ered by the coding jeopardized, hence it would be better if this requirement wasremoved by improving the physical layer implementation of the protocol. Finally, we expect thatthis scheme is not only bene"cial for our LEO microsatellite systems but also for other systemsthat are similar in nature.

Acknowledgements

Author Y. R. would like to thank Mr Mark Allery, Mr John Pa!ett, and Dr Je!Ward of SSC forproviding information. This work was carried out at and supported by SSC.

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Authors: biographies:

Yusep Rosmansyah received the BSc degree in Electrical Engineering from the Institut TeknologiBandung (ITB), Indonesia, in 1993, and the MSc degree in Satellite Communications Engineer-ing from the University of Surrey, U.K. in 1996. He is currently conducting research inmodulation and coding for packetized communication systems towards a PhD degree at theCCSR, University of Surrey, U.K.

Peter Sweeney obtained his BA Honours in Applied Physics from the Oxford University in 1971and PhD from the Cambridge University Engineering Department in 1977 for research in laserinterferometric methods of measuring electron densities in crossed "eld devices. After workingfor seven years in industry, he joined the University of Surrey in 1983 where he is now a SeniorLecturer in the CCSR. He specializes in error control coding, particularly in low complexitytrellis decoding of block codes. He also lectures for several external organizations as well as forMSc and short courses at the University of Surrey.

Martin N. Sweeting received the BSc Honours degree in Electronic and Electrical Engineeringfrom the University of Surrey in 1974 and the PhD in 1979 for research in electrically short H.F.communications antennas. After working at Marconi Space and Defence Systems on theMARECS satellite, he returned to the University of Surrey, where he is now a Professor, topioneer the development of UoSAT series microsatellites. He holds a personal Chair in SatelliteEngineering at University of Surrey and is the Director of the SSC. He is a member of the BNSCSpace Technology Advisory Board and the UK CCIR(8) committees; and is also Chairman ofAMSAT-UK. He was awarded the OBE, the Royal Academy of Engineering Silver Medal, theQueen's Anniversary Prize for Higher Education, and is a Fellow of the Royal Academy ofEngineering and the University of Surrey. In 1998, his company SSTL won the Queen's Awardfor Technological Achievement.



Documents

A turbo-coded hybrid ARQ for low earth orbit microsatellite communications