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Abstract
Satellite and terrestrial components of IMT?Advanced need to be integrated so that the traditional strengths of each component can be fully exploited. LTE/LTE?A is now a recognized foundation of terrestrial 4G networks， and mobile satellite networks should be based on it. Long transmission delay is one of the main disadvantages of satellite communication， especially in a GEO system， and terminal?to?terminal （TtT） design reduces this delay. In this paper， we propose a protocol architecture based on LTE/LTE?A for GEO mobile satellite communication. 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 order to reduce delay and ensure compatibility with a terrestrial LTE/LTE?A system.
Keywords
TtT call； LTE/LTE?Advanced； satellite communication system
I1 Introduction
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 satellite channels， LTE?Advanced has to be modified for satellite radio transmission. Some LTE?Advanced specifications can be used without modification for satellite radio transmission whereas others need to be modified. Similarly， some LTE?Advanced specifications are not relevant and some BMSat specifications 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 geostationary satellite twice. This is called a double?hop connection. Such a connection increases the delay between user terminals and reduces link quality. People in areas where land communication networks are not developed benefit from reduced link delay. In [8]， four single?hop connection methods were proposed 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 double?hop mode but can provide single?hop TtT services. There is a long delay associated with a double?satellite?hopped mobile?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 satellite. However， in a BMSat system based on LTE/LTE?A， differences 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 designed to reduce transmission delay and increase terrestrial compatibility. The paper is organized as follows： section 2 describes the concept of a single?hop TtT call. In section 3， the protocols of four TtT modes are described. In section 4， division 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 SingleHop 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） establishes a single?hop call between two UEs when circumstances permit. 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 data transfer， parameter configuration， and legal interception. The satellite responds by extracting TtT data from the uplink message， 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 packets 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 channel. Then， radio resource control （RRC） information with TTCH requires ciphering and robust header compression （ROHC）. Packet Data Convergence Protocol （PDCP） is configured to compress the header and significantly reduce signaling overhead， as required in LTE [10]. Unacknowledged mode （UM） is used in the radio link control （RLC） layer to provide unidirectional data transfer. This is mainly use by delay?sensitive and error?tolerant real?time applications. Automatic repeat request （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 corresponding 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 control 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 complexity 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 dedicated 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 original protocol. However， in this mode， resource efficiency is reduced 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 dedicated 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 allocated 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 reference signal pattern and cyclic prefix are used in TtT subframes. 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 frequency domain than they are in non?MBSFN transmission. The separation is decreased to every other subcarrier rather than every sixth subcarrier.
In this mode， one vertical protocol path is added without affecting 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 SATGW
and Satellite
For TtT services with a semi?static packet rate， semi?persistent 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 service. 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 resource 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 include 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 using 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 between two MESs， i.e.， TtT users. The EPC starts EPS bearing and sends the initial context setup request to SAT?GW. This request 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） between 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 transmission start signaling to both the UEs to indicate that call establishment 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 single?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 dedicated 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 generates downlink TtT RBs in the TtT processing 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 separate it from the PUSCH， which includes other logical control channel information in TtT mode one
·localize the TCH transport block bearing the TTCH and separate it from the PUSCH， which only includes TTCH logical channel in TtT mode two
·localize the TCH transport block bearing the TTCH and separate it from the PTCH， which only includes the TTCH logical channel in TtT mode three
·localize the TCH transport block bearing the TTCH and separate 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， demodulation， 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 according to the downlink TtT resource allocation. This re?encoding 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 padding the headers of layers two and three. When QPSK modulation is taken into account， a minimum of three PRBs are required 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：
[382+2（CRC）126+2（QPSK）×3=0.4656] （1）
After rate?matching and encoding， TtT RBs are generated. As the time is synchronized， the TtT RBs are buffered for the downlink 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 （according to downlink TtT resource allocation） to generate the downlink 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 information 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 extracted， 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 connection 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 communication. We also designed a detailed call procedure. Four TtT modes for this architecture were then introduced and compared， 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.
References
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[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 connections 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
Satellite and terrestrial components of IMT?Advanced need to be integrated so that the traditional strengths of each component can be fully exploited. LTE/LTE?A is now a recognized foundation of terrestrial 4G networks， and mobile satellite networks should be based on it. Long transmission delay is one of the main disadvantages of satellite communication， especially in a GEO system， and terminal?to?terminal （TtT） design reduces this delay. In this paper， we propose a protocol architecture based on LTE/LTE?A for GEO mobile satellite communication. 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 order to reduce delay and ensure compatibility with a terrestrial LTE/LTE?A system.
Keywords
TtT call； LTE/LTE?Advanced； satellite communication system
I1 Introduction
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 satellite channels， LTE?Advanced has to be modified for satellite radio transmission. Some LTE?Advanced specifications can be used without modification for satellite radio transmission whereas others need to be modified. Similarly， some LTE?Advanced specifications are not relevant and some BMSat specifications 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 geostationary satellite twice. This is called a double?hop connection. Such a connection increases the delay between user terminals and reduces link quality. People in areas where land communication networks are not developed benefit from reduced link delay. In [8]， four single?hop connection methods were proposed 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 double?hop mode but can provide single?hop TtT services. There is a long delay associated with a double?satellite?hopped mobile?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 satellite. However， in a BMSat system based on LTE/LTE?A， differences 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 designed to reduce transmission delay and increase terrestrial compatibility. The paper is organized as follows： section 2 describes the concept of a single?hop TtT call. In section 3， the protocols of four TtT modes are described. In section 4， division 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 SingleHop 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） establishes a single?hop call between two UEs when circumstances permit. 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 data transfer， parameter configuration， and legal interception. The satellite responds by extracting TtT data from the uplink message， 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 packets 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 channel. Then， radio resource control （RRC） information with TTCH requires ciphering and robust header compression （ROHC）. Packet Data Convergence Protocol （PDCP） is configured to compress the header and significantly reduce signaling overhead， as required in LTE [10]. Unacknowledged mode （UM） is used in the radio link control （RLC） layer to provide unidirectional data transfer. This is mainly use by delay?sensitive and error?tolerant real?time applications. Automatic repeat request （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 corresponding 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 control 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 complexity 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 dedicated 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 original protocol. However， in this mode， resource efficiency is reduced 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 dedicated 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 allocated 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 reference signal pattern and cyclic prefix are used in TtT subframes. 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 frequency domain than they are in non?MBSFN transmission. The separation is decreased to every other subcarrier rather than every sixth subcarrier.
In this mode， one vertical protocol path is added without affecting 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 SATGW
and Satellite
For TtT services with a semi?static packet rate， semi?persistent 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 service. 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 resource 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 include 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 using 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 between two MESs， i.e.， TtT users. The EPC starts EPS bearing and sends the initial context setup request to SAT?GW. This request 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） between 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 transmission start signaling to both the UEs to indicate that call establishment 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 single?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 dedicated 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 generates downlink TtT RBs in the TtT processing 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 separate it from the PUSCH， which includes other logical control channel information in TtT mode one
·localize the TCH transport block bearing the TTCH and separate it from the PUSCH， which only includes TTCH logical channel in TtT mode two
·localize the TCH transport block bearing the TTCH and separate it from the PTCH， which only includes the TTCH logical channel in TtT mode three
·localize the TCH transport block bearing the TTCH and separate 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， demodulation， 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 according to the downlink TtT resource allocation. This re?encoding 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 padding the headers of layers two and three. When QPSK modulation is taken into account， a minimum of three PRBs are required 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：
[382+2（CRC）126+2（QPSK）×3=0.4656] （1）
After rate?matching and encoding， TtT RBs are generated. As the time is synchronized， the TtT RBs are buffered for the downlink 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 （according to downlink TtT resource allocation） to generate the downlink 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 information 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 extracted， 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 connection 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 communication. We also designed a detailed call procedure. Four TtT modes for this architecture were then introduced and compared， 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.
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Manuscript received： 2014?11?21