Terminal⁃to⁃Terminal Calling for GEO Broadband Mobile Satellite Communication

Qing Wang，Minjiong Zhu，Jie Zhou，Zhen Gao，and Jinsheng Yang

（School of Electrical Information Engineering，Tianjin University，Tianjin 300072，China）

Terminal⁃to⁃Terminal Calling for GEO Broadband Mobile Satellite Communication

Qing Wang，Minjiong Zhu，Jie Zhou，Zhen Gao，and Jinsheng Yang

（School of Electrical Information Engineering，Tianjin University，Tianjin 300072，China）

Satellite and terrestrial components of IMT⁃Advanced need to be integrated so that the traditional strengths of each compo⁃nent can be fully exploited.LTE/LTE⁃A is now a recognized foundation of terrestrial 4G networks，and mobile satellite net⁃works should be based on it.Long transmission delay is one of the main disadvantages of satellite communication，espe⁃cially in a GEO system，and terminal⁃to⁃terminal（TtT）design reduces this delay.In this paper，we propose a protocol archi⁃tecture based on LTE/LTE⁃A for GEO mobile satellite commu⁃nication.We propose a detailed call procedure and four TtT modes for this architecture.We describe the division of tasks between the satellite gateway（SAT⁃GW）and satellite as well as TtT processing in the physical layer of the satellite in or⁃der to reduce delay and ensure compatibility with a terrestrial LTE/LTE⁃A system.

TtT call；LTE/LTE⁃Advanced；satellite communication system

1 Introduction

I ntegrated terrestrial and satellite communication has been addressed for many years and is at the forefront of R&D within the satellite community［1］.The ITU has made recommendations for the development of the satellite radio interface of IMT⁃Advanced［2］.

The broadband mobile satellite（BMSat）radio interface is mainly used for broadband mobile satellite services that use geostationary（GEO）satellites［3］.BMSat is derived from the terrestrial LTE⁃Advanced specifications［4］-［6］and enables access to LTE⁃Advanced core networks.

Because there are differences between terrestrial and satel⁃ lite channels，LTE⁃Advanced has to be modified for satellite ra⁃dio transmission.Some LTE⁃Advanced specifications can be used without modification for satellite radio transmission whereas others need to be modified.Similarly，some LTE⁃Ad⁃vanced specifications are not relevant and some BMSat specifi⁃cations have no corresponding LTE⁃Advanced specification.In［7］，a kind of narrowband transmission scheme was proposed for allocating limited bandwidth to user equipment（UE）.Such a scheme is particularly necessary for a satellite channel，which is power⁃constrained［7］.

In a conventional mobile satellite communication systems using GEO satellites，communication passes through the geo⁃stationary satellite twice.This is called a double⁃hop connec⁃tion.Such a connection increases the delay between user termi⁃nals and reduces link quality.People in areas where land com⁃munication networks are not developed benefit from reduced link delay.In［8］，four single⁃hop connection methods were pro⁃posed and compared.With these methods，communication frames are transferred between user terminals via satellite only once［8］.

A terminal⁃to⁃terminal（TtT）call can be established in dou⁃ble⁃hop mode but can provide single⁃hop TtT services.There is a long delay associated with a double⁃satellite⁃hopped mo⁃bile⁃to⁃mobile service.Therefore，a single⁃hopped service can be routed directly through the satellite—from any terminal in any spot beam to any other terminal in any other spot beam.In single⁃hop mode，two mobile Earth stations（MESs）engaged in a call communicate directly via the satellite.In［9］，the authors define a TtT call at a circuit⁃switched L⁃L channel on the satel⁃lite.However，in a BMSat system based on LTE/LTE⁃A，differ⁃ences between the uplink SC⁃FDMA and downlink OFDM are challenging to TtT call specification design.

In this paper，we propose a GEO BMSat TtT architecture and related call procedure based on LTE/LTE⁃A.These are de⁃signed to reduce transmission delay and increase terrestrial compatibility.The paper is organized as follows：section 2 de⁃scribes the concept of a single⁃hop TtT call.In section 3，the protocols of four TtT modes are described.In section 4，divi⁃sion of tasks between SAT⁃GW and satellite are defined.In section 5，TtT call singling processing is defined.Section 6 concludes the paper.

2 Concept of Single-Hop TtT

The BMSat network should support single⁃hop voice calls，i. e.，TtT calls，between two UEs on the same satellite network. During TtT call processing，the gateway station（GS）establish⁃es a single⁃hop call between two UEs when circumstances per⁃mit.When the TtT call has been established，the voice data is transferred via satellite directly，which avoids the long delay. The satellite gateway（SAT⁃GW）is responsible for non⁃TtT da⁃ta transfer，parameter configuration，and legal interception. The satellite responds by extracting TtT data from the uplinkmessage，TtT processing，and downlink TtT MAP combining. The TtT concept is shown in Fig.1.

TtT call has following characteristics：

·It uses single⁃hop mode only for voice services.

·It supports mobility management（handover）.

·It supports end⁃to⁃end ciphering.

·It supports legal interception.

·It does not use hybrid automatic repeat request（HARQ）.

·It uses semi⁃persistent scheduling.

·It responds to any release requirement immediately during communication.

3 TtT Mode

To distinguish TtT data from non⁃TtT data，a TtT radio bearer（TRB）is used for a TtT call.When establishing a TRB，an Evolved Universal Terrestrial Radio Access Network（E⁃UTRAN）is used to decide how to transfer the pack⁃ets of an Evolved Packet System（EPS）bearer across the radio interface.An EPS bearer is mapped one⁃to⁃one to a TRB，a TRB is mapped one⁃to⁃one to a TtT traffic channel（TTCH）logical chan⁃nel.Then，radio resource control（RRC）information with TTCH requires cipher⁃ing and robust header compression（RO⁃HC）.Packet Data Convergence Protocol（PDCP）is configured to compress the header and significantly reduce signal⁃ing overhead，as required in LTE［10］.

Unacknowledged mode（UM）is used in the radio link control（RLC）layer to provide unidirectionaldata transfer. This is mainly use by delay⁃sensitive and error⁃tolerant real⁃time applications.Automatic repeat re⁃quest（ARQ）is not included in RLC layer for TtT service. Then，in the medium access control（MAC）layer，the TTCH is mapped to

·A downlink shared channel（DL⁃SCH）or uplink shared channel（UL⁃SCH）multiplexes with other logical channels and is identified by a TtT logical channel ID（LCID）.The DL⁃SCH or UL⁃SCH is mapped one⁃to⁃one to the correspond⁃ing physical downlink shared channel（PDSCH）or physical uplink shared channel（PUSCH）.This is TtT mode one，i.e.，TtT TTCH in MAC PDU.

·A separate TtT channel（TCH）.A MAC protocol data unit（PDU）only bears the TTCH logical channel，and the TCH is mapped to the physical layer in three ways：

1）The TCH is mapped to the PDSCH，which is identified by a TtT radio network temporary identifier（T⁃RNTI）or by one bit in the physical downlink control channel（PDCCH）.This is TtT mode two，i.e.，TtT TCH in PDSCH.

2）The TCH is mapped to a separate physical TtT channel（PTCH），which is periodically scheduled in dedicated physical resource blocks（PRBs）.This is TtT mode three，i.e.，TtT PTCH in dedicated PRBs.

3）TCH is mapped to a separate PTCH，which is periodically scheduled in a dedicated subframe.This is TtT mode four，i.e.，TtT PTCH in a dedicated TtT subframe.

3.1 Mode One：TtT TTCH in MAC PDU

The TTCH LCID is defined in the MAC layer.The TTCH and all other logical channels are mapped to the DL⁃SCH or UL⁃SCH in one MAC PDU（Fig.2）.

In this mode，the physical layer is not modified.However，it is inefficient to multiplex logical channels because useless con⁃trol information has to be processed in the satellite during TtT transmission.

3.2 Mode Two：TtT TCH in PDSCH

In TCH transmission，the MAC PDU only bears the TTCH. When the TCH is mapped to the PDSCH（Fig.3），the TCH is identified along with other transmission channels by using the T⁃RNTI，which is similar to the PCH，or by using one bit in the PDCCH when the same RNTI is used with the PCH or DL⁃SCH.Mode two is highly efficient because only the TTCH needs to be processed.Also，a separate TCH enables greater flexibility to support network coding.However，search com⁃plexity is doubled when the T⁃RNTI is added，and the physical layer has to be modified.

3.3 Mode Three：TtT PTCH in Dedicated PRBs

The TCH is periodically mapped to the PTCH using dedicat⁃ed PRBs，which are similar to PBCHs.With the PRCH，three PRBs are scheduled every 20 ms（Fig.4）.In this mode，one vertical protocol path is added without interfering with the orig⁃inal protocol.However，in this mode，resource efficiency is re⁃duced for dedicated PRBs if no TtT services are required，and the physical layer also has to be modified.

3.4 Mode Four：TtT PTCH in Dedicated TtT Subframe

The TCH is periodically mapped to the PTCH using a dedi⁃cated subframe，which is similar to the PMCH.The TtT control channel（TCCH）and TTCH are mapped two⁃to⁃one to the TCH and is then mapped one⁃to⁃one to the PTCH.TtT data are scheduled by higher level signaling.The PDCCH is only allo⁃cated uplink resources but not for PTCH transmission（Fig.5）.

A UE that is measuring a neighboring cell does not need to know in advance the allocation of TtT and non⁃TtT subframes. The UE can take advantage of the fact that a different refer⁃ence signal pattern and cyclic prefix are used in TtT sub⁃frames.An extended cyclic prefix（CP）is used，i.e.，the prefix is approximately 17 μs instead of approximately 5 μs.In this mode，the reference symbols are spaced closer in the frequen⁃cy domain than they are in non⁃MBSFN transmission.The sep⁃aration is decreased to every other subcarrier rather than every sixth subcarrier.

In this mode，one vertical protocol path is added without af⁃fecting the original protocol.However，resource efficiency is lower for the dedicated subframe if there are not enough TtT users.In this case，the physical layer has to be modified.

4 Cooperation Between SAT-GW and Satellite

For TtT services with a semi⁃static packet rate，semi⁃persis⁃tent scheduling may be needed to reduce the control signaling overhead.

The SAT⁃GW is responsible for non⁃TtT service processing，legal interception，and semi⁃persistent configuration of TtT ser⁃vice.Semi⁃persistent scheduling involves allocating resources to both TtT and non⁃TtT services and leaving TtT PRBs in the DL⁃MAP blank.The satellite inserts TtT resource blocks into the DL⁃MAP for multiplexing with non⁃TtT services.TtT re⁃source blocks are produced by an additional TtT processing module at the satellite.

The tasks performed between the SAT⁃GW and satellite are shown in Fig.6.

5 TtT Call Processing

The signaling procedures for TtT call processing，which in⁃clude random access，paging and handover，are completed in double⁃hop mode，as with LTE.

A TtT call can only be made in RRC⁃connected state and us⁃ ing TRB bearing（Fig.7）.

5.1 TtT Call Establishment

A TtT call is established in double⁃hop mode.The signaling process includes paging，random access，and establishment of an RRC connection and is coordinated with LTE.

After receiving the initial UE message，the Evolved Packet Core（EPC）recognizes that a TtT call is being established be⁃tween two MESs，i.e.，TtT users.The EPC starts EPS bearing and sends the initial context setup request to SAT⁃GW.This re⁃quest includes the TtT setup.Security is established on the RRC connection，and the radio bearer，including TRB，is also established on the connection.A virtual circuit for the call is established between the TtT UEs，both of which are informed of the TtT states.A dedicated control channel（DCCH）be⁃tween the SAT⁃GW and satellite is established.The SAT⁃GW configures the satellite parameters，which include downlink and uplink TtT resource allocation results and TtT mode，through the control channel.The SAT⁃GW also informs UEs of the downlink and uplink TtT resource allocation results.The SAT⁃GW generates the downlink MAP（DL⁃MAP）for non⁃TtT services and transmits it to the satellite through DCCH.TtT RBs are left blank in the DL⁃MAP.

In TtT mode one，the PRBs are semi⁃scheduled for DL⁃SCH with TTCH in PD⁃SCH.In TtT mode two，the PRBs are semi⁃scheduled for TCH in PDSCH.In TtT mode three，the PRBs for PTCH are periodically scheduled.And in TtT mode four，the subframes for PTCH are periodically scheduled.

After receiving the DL⁃MAP，the satellite sends a TtT trans⁃mission start signaling to both the UEs to indicate that call es⁃tablishment is finished.At this time，the TtT call changes to signaling double⁃hop/voice single⁃hop mode.

5.2 TtT Voice Communication

In voice communication，TtT voice data is transmitted in sin⁃gle⁃hop mode.

5.2.1 TtT UE Sending TtT Traffic

TtT uplink traffic sent by a TtT UE is carried by the TRB and mapped to the TTCH.In mode one，the TTCH is mapped to the UL⁃SCH，multiplexed with other logical channels，and transmitted via the PUSCH.In mode two，the TTCH is mapped to the TCH and transmitted in the PUSCH.In mode three，the TTCH is mapped to the TCH and transmitted via the PTCH in dedicat⁃ed PRBs.In mode four，the TTCH is mapped to the TCH and transmitted via the PTCH in dedicated subframes.

5.2.2 TtT Processing at the Satellite

The satellite extracts the TtT user data from the uplink traffic and gener⁃ates downlink TtT RBs in the TtT pro⁃cessing module.Fig.8 shows the TtT processing module.

The satellite separates TtT data from received uplink traffic according to the parameters provided by the SAT⁃GW.The uplink resource allocation is used to：

·localize the UL⁃SCH transport block bearing the TTCH and sepa⁃ rate it from the PUSCH，which includes other logical control channel information in TtT mode one

·localize the TCH transport block bearing the TTCH and sep⁃arate it from the PUSCH，which only includes TTCH logical channel in TtT mode two

·localize the TCH transport block bearing the TTCH and sep⁃arate it from the PTCH，which only includes the TTCH logi⁃cal channel in TtT mode three

·localize the TCH transport block bearing the TTCH and sep⁃arate it from the PTCH subframe，which only includes the TTCH logical channel in TtT mode four.

The satellite then finishes TtT transport block decoding，which involves channel estimation，equalization，IDFT，demod⁃ulation，descrambling，deinterleaving，deconcatenation，and channel decoding.Finally，the satellite recovers the bitstream of the UL⁃SCH or TCH transport block.

In TtT mode one，the recovered UL⁃SCH transport block bit stream includes other logical channel data.However，in TtT modes two to four，the recovered TCH transport block bit stream includes only TTCH logical channel data.

Next，the satellite encodes the TtT transport block again ac⁃cording to the downlink TtT resource allocation.This re⁃encod⁃ing requires rate⁃matching based on the number of REs in the allocated RB.

For VoIP services，244 bits of user data are generated in 20 ms，and the whole packet is approximately 300 bits after pad⁃ding the headers of layers two and three.When QPSK modula⁃tion is taken into account，a minimum of three PRBs are re⁃quired when ITBS=7，NPRB=3，and the transport block size is 328 bits［11，Sec.7.1.7.2］.The number of usable REs in an allocated RB depends on whether one，two，or three OFDM symbols are used for the control signal.Thus，the number of REs in each RB is 126，138 or 150.If there are three OFDM control symbols，the downlink channel coding rate is：

After rate⁃matching and encoding，TtT RBs are generated. As the time is synchronized，the TtT RBs are buffered for thedownlink TtT MAP.

The TtT RBs are read by the buffer and mapped to the DL⁃SCH or TCH and are then combined with the DL⁃MAP（accord⁃ing to downlink TtT resource allocation）to generate the down⁃link TtT MAP.In this case，the TtT RBs are mapped to the DL⁃SCH in TtT mode one and to the TCH in the other three modes.The TtT RBs are scheduled in TtT modes one and two. PTCH PRBs and PTCH subframe are scheduled in TtT modes three and four，respectively.

5.2.3 TtT UE Receiving Downlink TtT MAP

In TtT modes one and two，TtT UE in the downlink decodes PDSCH and the corresponding DL⁃SCH according to CRC scrambling codes，such as SI⁃RNTI，P⁃RNTI，RA⁃RNTI，SPS⁃C⁃RNTI，C⁃RNTI，and T⁃RNTI.In order to recover the DL⁃SCH bitstream in TtT mode one，and to retrieve the TCH bitstream in TtT mode two，PDCCH uses the DCI information.This infor⁃mation is used to decide the size of the transport block and modulation order.Then，the PDCCH separates TTCH from the DL⁃SCH/TCH bitstream and recovers the user data in a higher layer.

In TtT modes three and four，the TtT PTCH RBs are extract⁃ed，and the bitstream of the TCH transport block，the TTCH，and the user data are recovered step by step.

5.2.4 TtT Call Release

The SAT⁃GW releases the TtT virtual circuit and RRC con⁃nection when the UE is inactive.Call release is signaled in double⁃hop mode，as in an LTE system.

6 Conclusion

In this paper，we have proposed a protocol architecture based on LTE/LTE⁃A for GEO mobile satellite TtT communica⁃tion.We also designed a detailed call procedure.Four TtT modes for this architecture were then introduced and com⁃pared，and the scheme for cooperation between SAT⁃GW and the satellite was analyzed.Our next step is to come up with a more comprehensive design within this protocol architecture in the areas of frequency synchronization，time control，ciphering， and interception schemes.

［1］F.Bastia，C.Bersani，E.A.Candreva，et al.，“LTE adapta⁃tion for mobile broadband satellite networks，”EURASIP Journal on Wireless Communications and Networking，pp. 1-13，2009.doi：10.1155/2009/989062.

［2］Vision and Requirements for the Satellite Radio Interface（s）of IMT⁃Advanced，ITU Std.Rep.ITU⁃R M.2176，2010.

［3］BMSat Radio Interface Specifications；Introduction to the BMSat Family，CCSA Std.BMSat⁃36.001.2，2013.

［4］LTE；E⁃UTRA；Physical Channels and Modulation，3GPP Std.3GPP TS36.211，2013.

［5］LTE；E⁃UTRA；Multiplexing and Channel Coding，3GPP Std.3GPP TS36.212，2013.

［6］LTE；E⁃UTRA；Physical Layer Procedures，3GPP Std. 3GPP TS 36.213，2013.

［7］H.W.Kim，K.Kang，and B.⁃J.Ku，“Narrowband uplink transmission in LTE⁃based satellite radio interface，”in Fourth International Conference on Advances in Satellite and Space Communications，Chamonix，France，2012，pp.104-107.

［8］K.Ebina，N.Kataoka，M.Ueba，and H.Mizuno，“Investigation of single⁃hop con⁃nections between user terminals in geostationary mobile satellite communication systems，”in Global Telecommunications Conference，2001，vol.4，pp.2764-2768.doi：10.1109/GLOCOM.2001.966277.

［9］GEO⁃Mobile Radio Interface Specifications；Part 3：Network Specifications；Sub⁃Part 18：Terminal⁃to⁃Terminal Call（TtT），ETSI Std.GMR⁃103.296（ETSI TS 101 376⁃3⁃18），2001.

［10］S.Sesia，I.Toufik，and M.Baker，LTE⁃The UMTS Long Term Evolution：From Theory to Practice.Torquay，UK：Wiley，2009.

［11］BMSat Radio Interface Specifications；Evolved Universal Satellite Radio Access（E⁃USRA）and Evolved Universal Satellite Radio Access Network（E⁃USRAN）；Physical layer procedures（Release 1），CCSA Std.BMSat⁃36.213，2013.

Manuscript received：2014⁃11⁃21

Biographiesphies

Qing Wang Wang（wqelaine@tju.edu.cn）received her BEng，MEng.，and PhD degrees in electronic engineering from Tianjin University，China，in 2004，2007，and 2010. Since 2010，she has worked in the School of Electronic and Information Engineer⁃ing，Tianjin University，and is currently an associate professor there.From October 2007 to April 2009，she was a visiting scholar in the School of Electrical and Elec⁃tronic Engineering，Nanyang Technological Univeristy，Singapore.Her research in⁃terests include wireless communication，passive radar，and radio propagation.

Minjiong Zhug Zhu（zhuminjiong@sina.cn）received his BS degree in electronic informa⁃tion engineering from Tianjin University，China，in 2014.He is currently pursuing the MS degree there.His research include software⁃defined networking and 5G mo⁃bile communication.

Jie Zhou Zhou（zhoujie_tju@163.com）received her BS and MS degrees in communication engineering from Tianjin University，China，in 2011 and 2015.Her research inter⁃ests include architecture design for wireless mobile communication，especially for satellite mobile communication systems，as well as physical⁃layer algorithms in wire⁃less mobile communication.

Zhen Gaon Gao（zgao@tju.edu.cn）received his BS，MS and PhD degrees in electrical and information engineering from Tianjin University，China，in 2005，2007 and 2011. From October 2008 to November 2010，he was a visiting scholar at Georgia Tech，USA，working on the design and implementation for OFDM⁃based cooperative com⁃munication.From July 2011 to November 2014，he was an assistant researcher with the Wireless and Mobile Communication Research Center，Tsinghua University，China.He has been an associate professor at Tianjin University since December 2014.His research interests include mobile satellite communication，fault⁃tolerant signal processing，and wireless communication systems.

Jinsheng Yang Yang（jsyang@tju.edu.cn）is an associate professor in the School of Elec⁃tronic Information Engineering，Tianjin University，China.His research interests in⁃clude the channel characterizations and modeling for wireless communications and networking⁃based information solutions.