HARQ–ARQ Interaction Method for LTE-based Mobile Satellite Communication System 2014

INTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS AND NETWORKINGInt. J. Satell. Commun. Network. 2014; 32:377–392Published online 10 February 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/sat.1052

HARQ–ARQ interaction method for LTE-based mobile satellitecommunication system

Tae Chul Hong,�,� , Kun-Seok Kang, Bon-Jun Ku and Do-Soeb Ahn

Electronics and Telecommunications Research Institute, 218 Gajeongro, Yuseong-gu, Daejeon, 305-700, Korea

SUMMARY

Hybrid automatic repeat request (HARQ) and automatic repeat request (ARQ) of the terrestrial long-term evolu-tion (LTE) system are designed considering the very short propagation delay of terrestrial environment. Therefore,when HARQ and ARQ are applied to the LTE-based mobile satellite communication system, the inefficiencyis caused by the long propagation delay of satellite environment. This paper proposes the interaction methodbetween HARQ and ARQ for the decrease of inefficiency in the LTE-based mobile satellite communication sys-tem. The existing concept of layered architecture about HARQ and ARQ is also maintained in our interactionmethod. Simulation results reveal that our proposed scheme can provide the larger spectrum efficiency than thatof the non-interaction scheme in all environments. The performance improvement can be up to 2.04 times throughthe interaction method. Copyright © 2014 John Wiley & Sons, Ltd.

Received 7 October 2012; Revised 9 July 2013; Accepted 1 August 2013

KEY WORDS: HARQ; ARQ; cross layer; LTE; GEO; satellite communication

1. INTRODUCTION

Third generation partnership project (3GPP) long-term evolution (LTE) system adopts various errorcorrection methods in the physical layer and upper layers [1]. Channel coding schemes for error protec-tion are used in physical layer, retransmission techniques for error correction are used in upper layers.There are two retransmission techniques in 3GPP LTE system. One is hybrid automatic repeat request(HARQ) between physical layer and medium access control (MAC) layer, the other is automatic repeatrequest (ARQ) in radio link control (RLC) layer [2, 3]. HARQ is performed in a lower layer of a layerwhere conventional ARQ is performed, and thus, errors are corrected by ARQ when the errors are notcorrected by HARQ. In general, the retransmission of ARQ is carried out after reaching the maximumnumber of retransmission attempts of HARQ. In terrestrial communication environment, the propaga-tion delay is very short, so the retransmission of HARQ is carried out rapidly. By the way, the satellitecommunication system has a long round trip delay (RTD) in comparison with the terrestrial communi-cation system. Because of this, in satellite communication system, the retransmission of HARQ is notcarried out rapidly. Therefore, HARQ and ARQ operations without considering a long RTD can causethe inefficiency. Firstly, the transmission gap exists. When the ARQ transmitter requests a feedbackpacket to the ARQ receiver after the transmission attempt of packet, which is located in an upper boundof the transmission window, the ARQ transmitter cannot transmit a packet until receiving a feedbackpacket [3]. In this case, a long RTD of satellite communication system causes a long transmission gap.The long transmission gap causes a throughput loss. Secondly, two or more HARQ processes can takein charge of the same packet transmission. When ARQ receive a feedback packet in RLC layer, thepacket, which is retransmitted by a HARQ process, can be regarded as a transmission failed packet

�Correspondence to: Tae Chul Hong, Electronics and Telecommunications Research Institute, 218 Gajeongro, Yuseong-gu,Daejeon, 305-700, Korea.�E-mail: [email protected]

Copyright © 2014 John Wiley & Sons, Ltd

378 T. C. HONG ET AL.

Figure 1. Network architecture of long-term evolution based satellite communication.

by ARQ. In other words, when the packet transmission is not ended by a HARQ process, the ARQcan order the same packet transmission to the other HARQ process. This is because, the ARQ doesnot know the transmitting packets state information in HARQ processes. A throughput loss is alsoincurred from the same packet handling of multiple HARQ processes. In this paper, we try to decreasethe throughput loss using a HARQ/ARQ interaction method. In a case of 2G/3G mobile communica-tion systems, a network controller, such as a base station controller, a radio network controller, and thelike, separately exists from a base station. By the way, RLC layer is located in a network controller,and MAC layer is located in a base station. Therefore, the signaling cost between HARQ and ARQcannot be ignored, and thus, an interaction between HARQ and ARQ may not be effective. However,in a case of LTE mobile communication system, a network controller locates in the same place. There-fore, an interaction method between HARQ and ARQ can be applied to the systems with very lowsignaling cost.

In recent times, the multibeam geostationary Earth orbit (GEO) based mobile satellite service isprovided using large antenna technique, so LTE standard can be applied to the satellite communicationsystem [4, 5]. In case of LTE-based satellite communication system, all components of LTE networklocate in the same place such as Figure 1. This architecture causes very low signaling cost betweenMAC and RLC layers. Therefore, it is not hard to apply an interaction method between HARQ of MAClayer and ARQ of RLC layer in LTE-based satellite communication system. In this environment, ourproposed interaction method between HARQ and ARQ can improve the system throughput, and thisis validated by simulations.

The remainder of this paper is organized as follows. In Section 2, we first describe HARQ and ARQin GEO satellite communication system. In Sections 3 and 4, we propose the HARQ/ARQ interac-tion method and demonstrate simulation results of the performance, respectively. Finally, concludingremarks are given in Section 5.

2. HARQ AND ARQ IN SATELLITE COMMUNICATIONS

2.1. Hybrid automatic repeat request

The 3GPP LTE system adopts N-channel stop and wait (SAW) HARQ for the decrease of physicallayer complexity [6, 7]. In case of the terrestrial LTE system, the RTD between a base station and a

Copyright © 2014 John Wiley & Sons, Ltd Int. J. Satell. Commun. Network. 2014; 32:377–392DOI: 10.1002/sat

HARQ–ARQ INTERACTION METHOD 379

mobile terminal is short, so the LTE system uses eight-channel SAW HARQ. However, in case of theLTE based satellite communication system, the RTD between a base station and a mobile terminal isvery large, so the number of channel of N-channel SAW HARQ should be increased. When a GEOsatellite is used, N should be 500 or more for the continuous packet transmission. If the channels ofN-channel SAW HARQ are insufficient for the continuous transmission, the system throughput perfor-mance could be reduced [8]. Figure 2 shows the transmitted packets during satellite RTD, accordingto N. In Figure 2, we confirm that N should be determined considering the RTD for the continuouspacket transmission. The max throughput of N-channel SAW HARQ can be derived as follows:

MaxT hroughput DN �HARQPacketSi´e

RTD(1)

Regardless of N, the continuous packet transmission may not be possible according to the selectionof a packet scheduler in the overloaded state. On the other hand, the transmitter cannot transmit apacket in spite of the decision of a packet scheduler because of inappropriate N. Therefore, the numberof channel of HARQ should be determined for enabling the continuous packet transmission in the lessloaded state.

Figure 2. N-channel stop and wait (SAW) hybrid automatic repeat request (HARQ) in satellite communicationsystem.

Table I. Definition of variables for the transmission window and the receiving window.

Variable Description

VT(A) A sequence number (SN) value of a subsequent data packet for whichan acknowledgement is to be received and is provided to thetransmission window, as a lower value

VT(S) An SN value to be assigned to a newly generated and transmitted data packet.An initial value of the VT(S) is set to zero, and the VT(S) is updated wheneveran acknowledgement mode data protocol data unit havingSND VT(S) is transferred

VT(MS) VT(A)C a size of the transmission window, and is provided to the transmissionwindow, as an upper value

VR(R) An SN value of a sequentially and completely received last data packet and isused as a lower value of the receiving window

VR(H) A greatest SN value from among SN values of received data packetsVR(MR) VR(R)C the size of the transmission window and has an SN value of a first

data packet of which exceeds a size of the receiving window



Figure 3. Transmission window of automatic repeat request. PDU, protocol data unit.

Figure 4. Receiving window of automatic repeat request. PDU, protocol data unit.

2.2. Automatic repeat request

Selective repeat ARQ is used in RLC of a 3GPP LTE system[3, 7]. It performs retransmission based onthe transmission window and the receiving window. The transmission window controls the transmis-sion operation based on a VT(A), VT(S), and VT(MS) values, and the receiving window controls thereceiving operation based on a VR(R), VR(H), and VR(MR) values. The variables for the transmissionwindow and the receiving window are defined in Table I.

Figures 3 and 4 show the transmission window and the receiving window of ARQ according tothe definition of Table I. The transmission window is operated by the feedback of a receiver and thetransmitting actions of a transmitter. The feedback packet of a receiver is a status protocol data unit(STATUS PDU) [3]. When the polling of a transmitter or the expired event of a timer is occurred, anARQ receiver makes STATUS PDU using the receiving window. A STATUS PDU contains the infor-mation for acknowledged packets and negative acknowledged packets. Therefore, the transmitter cancontrol the transmission window through the information of a STATUS PDU. In the satellite commu-nication environment, the state of a receiver can be changed during the transferring time of a STATUSPDU. For the long propagation delay, the receiver can receive the packets, which are regarded as thenegative acknowledged packets in a STATUS PDU. In this case, the transmitter carries out the unnec-essary retransmission, so the timing of a STATUS PDU transmission can affect the system throughputperformance.

2.3. Inefficiency of LTE ARQ schemes in a satellite environment

In the satellite communication system, a long RTD causes the inefficiency of HARQ and ARQ oper-ations. Figure 5 shows an example of HARQ and ARQ operations in the satellite environment. In



Figure 5. Example of hybrid automatic repeat request (HARQ) and automatic repeat request (ARQ) operation.PDU, protocol data unit; NACK, negative acknowledgement; ACK, acknowledgement; STATUS PDU, status

protocol data unit.

Figure 5, we assume four-channel SAW HARQ, so the transmitter transmits protocol data unit (PDU)4with the polling message about a STATUS PDU. At (A) of Figure 5, the HARQ process, which takescharge in PDU1, receives a negative acknowledgement (NACK), so the HARQ process retransmitsPDU1. By the way, at (B) of Figure 5, an ARQ transmitter receives a STATUS PDU with the informa-tion of a NACK about PDU1, so the ARQ transmitter orders the retransmission of PDU1 to a HARQprocess. It is not required to retransmit PDU1 at (B) of Figure 5, so this retransmission wastes thecommunication resources. At (C) of Figure 5, the ARQ receiver successfully received from PDU1 toPDU4, but the ARQ transmitter will know it at (D) of Figure 5. Therefore, the transmission of a newPDU will be continued at (D) of Figure 5.

If a round trip time is short, the retransmission of a HARQ process would be carried out quickly,and the transmission discontinuation interval of RLC would be shortened. However, in the satellitecommunication environment, the retransmission of a HARQ process is not carried out quickly, so thetransmission discontinuation interval of RLC is elongated.

3. HARQ/ARQ INTERACTION METHOD

As described in the previous sections, each of HARQ and ARQ independently performs an assignedtask in a corresponding layer, and an interaction between HARQ and ARQ has possibility for improv-ing the transmission efficiency. Therefore, we propose an interaction method for enhancing thetransmission performance. For the interaction between HARQ and ARQ, we define messages, whichare required to exchange between MAC and RLC layers. Figure 6 shows the required messages forthe interaction method. In the proposed method, HARQ uses three types of message and ARQ usesone type of message. Firstly, when a HARQ process receives a positive acknowledgement (ACK), theprocess sends a message, which contains the ARQ sequence number (SN) of a acknowledged packet,to ARQ. Secondly, when a HARQ process receives the NACK after the maximum retransmission, theprocess requests a retransmission to ARQ about the transmission failed packet. Finally, when ARQ

Figure 6. Information exchange between hybrid automatic repeat request (HARQ) and automatic repeat request(ARQ) for interaction. RLC, radio link control; MAC, medium access control; ACK, acknowledgement.



Figure 7. Medium access control (MAC) and radio link control (RLC) operation for hybrid automaticrepeat request (HARQ)/automatic repeat request (ARQ) interaction. ACK, acknowledgement; NACK, negative

acknowledgement.

Figure 8. Transmission window modification. STATUS PDU, status protocol data unit; HARQ, hybrid automaticrepeat request.

requests the information about transmitting packets, the MAC sends a message, which contains theSNs of transmitting packets at HARQ processes. In the case of the message of ARQ, when an ARQtransmitter receives a STATUS PDU, it requests the information about transmitting packets of HARQprocesses. Figure 7 shows the operation of MAC and RLC about the processing new defined mes-sages. Based on received messages from HARQ, the ARQ transmitter checks that the packets, whichare transmitting at HARQ processes, exist among negative acknowledged packets of a STATUS PDU,and excludes those packets from the retransmission operation. For these operations, we modify thetransmission window architecture.

Figure 8 shows the modified transmission window architecture. We divide the transmission windowinto two windows. The one is the transmission window, and the other is the waiting window. Thetransmission window is basically operated according to the defined HARQ messages, and the waitingwindow is operated according to the STATUS PDU. The transmission window is operated based on theVT(A), VT(S), and VT(MS), and the waiting window is operated based on the VT(CA). The VT(CA)is the SN value of a subsequent data packet for which a positive ACK is to be received via the STATUSPDU and is provided to the waiting window as a lower value. The transmission window operates withthe same manner of 3GPP LTE except the VT(A) processing. In 3GPP LTE, the VT(A) is updated onthe basis of a received positive ACK of a STATUS PDU. However, in our proposed method, the VT(A)is updated on the basis of an interaction message with respect to the reception of a positive ACK atHARQ. On the other hand, the VT(CA) of the waiting window is updated on the basis of a positiveACK of a STATUS PDU. In this case, the transmission window operates according to interactionmessages from the HARQ, so the transmitter can promptly transmits a subsequent PDU. In addition,the ARQ transmitter can check the packets, which are negative acknowledged by a STATUS PDU,are retransmitted at the HARQ process through interaction messages. The differentiation between theoperations of MAC layer and the operations of RLC layer is maintained through the waiting window.



Figure 9. Example of interaction between hybrid automatic repeat request (HARQ) and automatic repeat request(ARQ). STATUS PDU, status protocol data unit; PDU, protocol data unit; NACK, negative acknowledgement;

ACK, acknowledgement; RLC, radio link control.

In addition, the transmitter can cope with the NACK to ACK error through the waiting window. TheNACK to ACK error is that the transmitter misunderstands a NACK feedback packet as a positiveACK feedback packet. In this case, if the waiting window does not exist, the RLC packet, which iscontained in acknowledged packet information message form HARQ, is discarded instantly, so thetransmitter cannot manage the NACK to ACK error. However, in our proposed window architecture, anRLC packet is not discarded directly by the message from HARQ, and it is discarded by the STATUSPDU. Therefore, the packet, which belongs to the NACK to ACK error, can be retransmitted.Our proposed method does not affected by the types of HARQ, so all types of HARQ can be appliedto the our proposed method. In addition, the proposed method does not need to exchange signalingbetween network nodes, so the additional delay according to the application of HARQ/ARQ interactionis dependent on hardware platform. In these days, the hardware processing speed is very fast, so theHARQ/ARQ interaction method can be achieved within the constraint of LTE protocol delay.

Figure 9 shows an example of the operation of the proposed interaction method. In Figure 9, weassume four-channel SAW HARQ as Figure 5, so the transmitter transmits PDU4 with the pollingmessage about a STATUS PDU. At (A) of Figure 9, the operation is the same as that of Figure 5, butthe transmitter transmits PDU5, PDU6, and PDU7 according to the interaction messages from HARQprocesses. At (B) of Figure 9, the ARQ transmitter receives a STATUS PDU, and the transmitterknows PDU1 should be retransmitted. In that time, the ARQ transmitter requests the information abouttransmitting packets and receives an interaction message from HARQ. Therefore, the ARQ transmittercan recognizes PDU1 is being retransmitted at a HARQ process. From this reason, the ARQ transmitterdoes not order PDU1 retransmission to a HARQ process, unlink Figure 5. At (C) of Figure 9, the ARQtransmitter knows the successful transmission of PDU1 from the interaction message of HARQ.

In comparison with the normal operation of Figure 5, the throughput of LTE-based satellite com-munication system can be increased 2.4 times through the proposed interaction method. In addition, itcan be less than one-half the transmission discontinuation interval of the normal operation of Figure 5.The situation of Figures 5 and 9 is only an example, so we do the system level simulations to validatethe performance of the proposed method in realistic environments.

4. PERFORMANCE EVALUATION

We do system level simulations for the performance validation of the our proposed method. We expectthat the throughput performance is improved by the proposed method, so we check the spectral effi-ciencies in simulations. Simulation parameters are defined in Table II. In simulations, we assumeHARQ type II, and the additional coding gains of incremental redundancies are based on Reference[9]. Considering a GEO satellite communication system, we assume the RTD is 500 ms, and the satel-lite channel model by Fontan et al. [10] is reflected to system level simulations. The channel modelby Fontan et al. [10] is composed of three-state(line of sight (LOS), moderate shadowing, and deepshadowing) Markov chain and five environments(urban, suburban, open, intermediate tree shadow,and deep tree shadow). Our simulations use urban, suburban, open environment models of elevationangle 40ı. Link level simulation results are applied to systems level simulations through link-to-systemmapping method [11]. The LTE standard defines 29 combinations of modulation and coding scheme



Table II. Simulation parameters.

General parameter

Number of beams 19 (2 tier)Distance between beam centers 180 kmNumber of terminals in a cell 25/50/100/200Terminals distribution UniformDownlink carrier frequency 2.1 GHzDownlink carrier frequency 2.0 GHzBandwidth per beam 5 MHzFrequency reuse factor 6Modulation and coding scheme (MCS) modes MCS 0 – MCS 16Target BLER of MCS mode 10�2

Back off for MCS mode decision 2 dBNumber of stop and wait HARQ processes 500Maximum number of HARQ retransmission 3RLC mode Acknowledged modeARQ transmission window size 1500 PDUsMaximum number of ARQ retransmission 2Channel environment Open, suburban, and urbanScheduling method Proportional fairnessPower control Full power allocationTraffic model Full bufferLink-to-system mapping Effective exponential SINR mapping

Satellite parameterAntenna heights 36,000 kmRound trip delay 500 msNumber of transmit antenna 1Number of receive antenna 1Antenna radiation pattern ITU�R S.672�4Antenna gain 50 dBiPower for communication payload (200 W per beam) 3.8 kWLoss of nonlinearity 3 dBOther loss 1.5 dBSystem noise temperature 450 K

User terminal parameterAntenna height 1.5 mNumber of transmit antenna 1Number of receive antenna 1Antenna radiation pattern OmnidirectionalAntenna gain 0 dBiUser terminal power 2 WOther loss 3 dBSystem noise temperature 290 K

BLER, block error rate; HARQ, hybrid automatic repeat request; RLC, radio link control; ARQ,automatic repeat request; PDU, protocol data unit; SINR, signal-to-interference plus noise ratio.

(MCS) [12]. We consider Quadrature Phase Shift Keying (QPSK) and 16 Quadrature Amplitude Mod-ulation (QAM) are used in LTE-based satellite communications system, so MCS modes about QPSKand 16QAM are only used in our simulations. Table III shows the MCS mode definitions about QPSKand 16QAM, and link level simulation results about MCS 0 – MCS 16 are shown in Figure 10. TheLTE system uses the resource block(RB) of 180 kHz as a smallest resource unit. In our simulations,we assume that one user uses only one RB at a time. The satellite antenna beam pattern of ITU-RS.672-4 is applied to simulations [13]. The proposed method is about operations within a transmitteror a receiver entity, so it does not affect the fairness among users. The scheduler manages the fairnessamong users. For excluding the scheduling effect, we use the proportional fairness scheduler, which isused in popular [14].



Table III. Modulation and coding scheme (MCS)mode definition in the long-term evolution system.

MCS mode Modulation/coderate

0 QPSK/0.1061 QPSK/0.1442 QPSK/0.1783 QPSK/0.2284 QPSK/0.2835 QPSK/0.3506 QPSK/0.4177 QPSK/0.4948 QPSK/0.5619 QPSK/0.65010 16QAM/0.35811 16QAM/0.41412 16QAM/0.46913 16QAM/0.53614 16QAM/0.60315 16QAM/0.62516 16QAM/0.669

QPSK, Quadrature Phase Shift Keying; QAM, Quadra-ture Amplitude Modulation.

Figure 10. Block error rate (BLER) according to the MCS and symbol energy to noise ratio (Es=N0).

4.1. Status protocol data unit transmission period

As mentioned in the previous section, a STATUS PDU of ARQ is transmitted with the polling of atransmitter or the expired event of a timer, and the transmission timing of that can affect the systemthroughput performance. Therefore, we do simulations for examining the effect of the STATUS PDUtransmission period. Figures 11 and 12 show the simulation results of uplink and downlink, respec-tively. We assume 50 terminals per beam, and three channel environments are used. In legend, N meansa case of the no-interaction method between HARQ and ARQ, and I means a case of the interactionmethod between HARQ and ARQ. In Figures 11 and 12, the performance of the interaction method isbetter than that of no-interaction method. The uplink and down spectral efficiencies of the interaction



Figure 11. Spectral efficiency according to the STATUS protocol data unit (PDU) transmission period (Uplink).

Figure 12. Spectral efficiency according to the STATUS protocol data unit (PDU) transmission period(Downlink).

method are from 1.14 to 2.04 times and from 1.02 to 1.98 times larger than those of no-interactionmethod, according to the STATUS PDU transmission period. The spectral efficiencies of the interac-tion method are almost similar values regardless of the STATUS PDU transmission period. On theother hand, the spectral efficiencies of no-interaction method are affected by the STATUS PDU trans-mission period. When the STATUS PDU period is large, the probability of the occurrence of inefficientoperations between HARQ and ARQ can be decreased. Therefore, the performance can be improvedbasically. When the STATUS PDU transmission period is larger than 3 s, the proper performance ofthe no-interaction method can be expected. In case of the downlink, the spectral efficiencies of subur-ban and urban environments are also increased at the STATUS PDU period of 4 s. This is because thescheduler can know whether the transmitter can transmit a packet or not in real time. The transmitter offull buffer traffic model always has packets for transmission, but the transmitter may not send a packet



Figure 13. Spectral efficiency according to number of terminals per beam (Uplink).

according to the operations of HARQ and ARQ in real situation. In erroneous environments as sub-urban and urban, the operations of most of transmitters are not synchronized, so some of transmitterscan transmit packets when some of transmitter cannot transmit packets due to the operations of HARQand ARQ. Therefore, if this information can be reflected to the scheduler in real time, the throughputcan be improved. By the way, this information can be only reflected on the downlink scheduling in realtime, because the information for uplink scheduling from user terminals is useless due to the long prop-agation delay. In Figure 12, the performance of the no-interaction method for open environment is notimproved at the STATUS PDU period of 4 s. In almost error-free condition such as an open environ-ment, the operations of most of transmitters are synchronized, so when a transmitter cannot transmit apacket, the others are almost the same situation. Therefore, the scheduling gain cannot be achieved.

4.2. Number of terminals

We also do simulations for examining the effect of the number of terminals per beam. Figures 13 and14 show the simulation results of uplink and downlink, respectively. In these simulations, we assumethe STATUS PDU period of 3 s, and three channel environments are used. From Figures 13 and 14,we confirm that the spectral efficiencies of the interaction method are almost similar values regardlessof the number of terminals per beam, and the uplink and down spectral efficiencies of the interactionmethod are from 1.16 to 1.35 times and from 1.16 to 1.32 times larger than those of the no-interactionmethod, according to the number of terminals per beam. In Figures 13 and 14, we also confirm thatthe spectral efficiencies without the interaction method are almost similar values except the case of 25terminals per beam. When 25 terminals exist in a beam, the spectral efficiencies of the no-interactionmethod are lower than those of the other no-interaction cases. The spectral efficiencies of the cases of50 or more terminals are about 15% larger than that of the case of 25 terminals. This result shows whenthe sufficient amount of terminals does not exist in a beam, the throughput loss due to the inefficientoperations of HARQ and ARQ is increased. The scheduling gain can be made by the large number ofterminals, so it can decrease the negative effect of inefficient operations of HARQ and ARQ. However,in the real situation, we cannot control the number of active terminals for reducing the inefficiencybetween the HARQ and ARQ operations.

4.3. Transmission window size of ARQ

We do simulations for examining the effect of the transmission window size of ARQ. Figures 15and 16 show the simulation results of uplink and downlink, respectively. We assume 50 terminals per



Figure 14. Spectral efficiency according to number of terminals per beam (Downlink).

Figure 15. Spectral efficiency according to the combination of the transmission window size of automatic repeatrequest (ARQ) and the period of the STATUS protocol data unit (PDU) transmission (Uplink).

beam, and three channel environments are used. In these simulations, when the transmission windowsize is changed from 500 PDUs to 2500 PDUs, the period of the STATUS PDU transmission is alsochanged from 1 to 5 s. We keep the ratio for the transmission window size of 1500 PDUs and theSTATUS PDU transmission period of 3 s. Figures 15 and 16 show that the interaction method providesthe better spectral efficiency regardless of the transmission window size of ARQ. The uplink anddown spectral efficiencies of the interaction method are from 1.12 to 1.59 times and from 1.12 to 1.77times larger than those of the no-interaction method. When the interaction method is not used, thelarger transmission window size of ARQ provides the larger spectral efficiency. The memory is usedfor implementing the transmission window of ARQ, so the no-interaction method requires the morememory for the throughput performance improvement. In case of the interaction method, the shortperiod of the STATUS PDU transmission and the small size of transmission window of ARQ can beapplied, because the throughput performance of the interaction method is not affected by the period of



Figure 16. Spectral efficiency according to the combination of the transmission window size of automatic repeatrequest (ARQ) and the period of the STATUS protocol data unit (PDU) transmission (Downlink).

Figure 17. Spectral efficiency according to the round trip delay (Uplink).

the STATUS PDU transmission and the size of the transmission window of ARQ. Therefore, to achievean optimal throughput performance, the interaction method does not need much memory similar to thecase of the no-interaction method. Although the waiting window of the interaction method requires theadditional memory except an initial memory, the amount of required memory is smaller than that ofthe no-interaction method to achieve an optimal throughput performance. When the period of theSTATUS PDU transmission is short, the amount of required memory for the waiting window is moredecreased. Therefore, the interaction method, comparable with the no-interaction method betweenHARQ and ARQ, can provide the larger spectral efficiency with the small size of memory.

4.4. Round trip delay

Finally, we do simulations for examining the effect of RTD. Figures 17 and 18 show the simula-tion results of uplink and downlink, respectively. In these simulations, we assume 25 terminals for



Figure 18. Spectral efficiency according to the round trip delay (Downlink).

investigating the effect of RTD without the scheduling gain. In the LTE system, 25 RBs exists in5 MHz bandwidth, so one RB can be assigned to every terminal every time. The combination of thetransmission window size of 2500 PDUs and the STATUS PDU transmission period of 5 s is applied tothe no-interaction method. In previous simulations, the no-interaction method has the best throughputperformance through this combination. In case of the interaction method, the combination of the trans-mission window size of 1500 PDUs and the STATUS PDU transmission period of 2 s is applied. Thethroughput performance of the interaction method is not affected by the combinations of the transmis-sion window size and the STATUS PDU transmission period, so we select a combination at random.In Figures 17 and 18, when RTD is 10 ms, the spectral efficiency difference between the no-interactionmethod and the interaction method is very small. However, when RTD is larger than 100 ms, the spec-tral efficiency difference between the no-interaction method and the interaction method is increased.The spectral efficiencies of the no-interaction method and the interaction method are reduced aboutaverage 18.4% and 3.6%, respectively, comparing the case of 500 ms RTD with the case of 10 msRTD. This result shows that our proposed method improves the throughput performance in a long RTDenvironment. The RTD of the satellite communication system is varied from about 10 ms to 500 msaccording to the orbit of a satellite, so if the interaction method is not applied to the satellite commu-nication system, the large throughput loss can be caused by a long RTD [15]. From these results, weconfirm that the proposed interaction method is used successfully in the satellite communications.

5. CONCLUSION

In satellite communications, various inefficiencies are caused by a long propagation delay. One of themis the inefficiency from HARQ and ARQ operations. In this paper, we propose the interaction methodbetween HARQ and ARQ for reducing the inefficiency. The proposed method has better performancethan that of the no-interaction method between HARQ and ARQ in all environments. The performanceof the interaction method is not affected by the period of STATUS PDU transmission, the number ofterminals, and the transmission window size of ARQ, so the performance is maintained at a constantlevel, which is better than the best performance of the no-interaction method between HARQ and ARQ.The proposed scheme can provide a maximum 104% increase of the spectrum efficiency comparedwith the no-interaction method between HARQ and ARQ at a particular environment. In addition, thethroughput loss of the interaction method due to RTD is 14.8 percentage points lower than that of theno-interaction method. Many schemes that can compensate disadvantages of satellite environment are



required for LTE-based mobile satellite communication system. We think that the interaction methodbetween HARQ and ARQ would be the important one of the compensation schemes.

ACKNOWLEDGEMENT

This research was funded by the MSIP (Ministry of Science, ICT & Future Planning), Korea in the ICT R&DProgram 2013.

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AUTHORS’ BIOGRAPHIES

Tae Chul Hong received his BS and MS degrees in electrical and electronics engineer-ing from Yonsei University in 2000 and 2003, respectively. He has worked for ETRI from2003. He is currently a Senior Member of research staff at Satellite Wireless ConvergenceResearch Department of ETRI in Daejoen, Korea. His research interests include LTE-basedmobile satellite system, stratospheric communications system, satellite MBMS, scheduling,and network QoS.

Kun-Seok Kang received his BS and MS degrees in School of Electronics and ElectricalEngineering from Kyungpook National University, Korea, in 1997 and 1999, respectively.He is currently a Senior Member of research staff at Satellite Wireless ConvergenceResearch Department of ETRI, Korea and has worked for the development of efficienttransmission algorithms for satellite communications. His research interests include satellitecommunications, coding technique, and multicarrier transmission.



Bon-Jun Ku received his BS and MS degree in School of Electronic and Electrical Engi-neering from Kyungpook National University in 1995 and 1999, respectively. He receivedPhD degree in Electrical and Electronics Engineering from Chungbuk National Universityin 2010. He is currently a Team Leader of Satellite Smart Communication Research Teamin ETRI. His research interests include satellite communications system, stratosphericcommunications system, and antenna engineering.

Do-Soeb Ahn received his BS and MS degrees in Electronics Engineering from Kyung-pook National University in 1988 and 1990, respectively and his PhD degree in ElectronicsEngineering from Chungnam National University in 2010. He has worked for ETRI from1990. He is currently a Director of the Satellite Wireless Convergence Research Division ofETRI in Daejeon, Korea and working in the area of satellite wireless communication.


HARQ–ARQ Interaction Method for LTE-based Mobile Satellite Communication System 2014

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