10 Logical Channels

03.523GPPLower layers of the GSM Cordless Telephony System (CTS) radio interfaceStage 2TS

For the GSM-CTS system, eight logical channels have been specified. There is one traffic channel TCH/F, and there are six signalling channels CTSBCH, CTSARCH, CTSAGCH, CTSPCH, SACCH, and FACCH.

10.1 CTS Beacon Channel (CTSBCH)

The CTSBCH logical channel is used to provide frequency and synchronisation information in the downlink direction. From this information the CTS-MS is able to synchronise and to recognise the identity of the CTS-FP. It is made up of a pair of CTSBCH-SB and CTSBCH-FB transmitted in every 52-multiframe.

Signalling mechanisms have been defined in order to reduce emissions from both CTS-FP and CTS-MS.

In cases where the CTS-FP has no resources to handle accesses from a CTS-MS, the CTSBCH shall indicate that no CTS-MS shall attempt to access the CTS-FP.

In order to avoid continuous broadcasting of the CTSPCH, signalling is provided on the CTSBCH that indicates the presence of the CTSPCH.

10.1.1 CTSBCH format

– burst:

– Frequency Correction Burst (FB) is used for the CTSBCH-FB;

– Synchronisation Burst (SB) is used for the CTSBCH-SB.

– channel coding scheme: CTSBCH-SB uses the same channel coding scheme as the SCH, specified in GSM 05.03 subclause 4.7. No channel coding is required for CTSBCH-FB.

10.1.2 CTSBCH timing

The timing of the CTSBCH is as follows (see figure 1):

– frame position:

– FN mod 52 =25 the CTSBCH-FB is transmitted;

– FN mod 52 =51 the CTSBCH-SB is transmitted.

– timeslot position: a pair of CTSBCH-SB and CTSBCH-FB shall have the same timeslot position within one 52-multiframe but the position can change from one 52-multiframe to another according to the Beacon timeslot management procedure (see subclause 11.2).

10.1.3 CTSBCH radio frequency channel

The CTSBCH is transmitted on the CTSBCH frequency channel, according to the CTSBCH frequency selection, see subclause 12.2.2.

10.1.4 CTSBCH-SB information

The CTSBCH-SB carries 25 information bits. These 25 bits shall be divided into five fields as follows:

– a status field indicating whether the CTS-FP has any radio resource available;

– a flag indicating the presence of the CTSPCH in the next 52-multiframe;

– a flag indicating whether the CTS-FP is currently performing timeslot shifting (see subclause 11.2) on the CTSBCH;

– a field indicating the timeslot number of the CTSARCH, CTSAGCH and CTSPCH;

– the FPBI field indicating the identity of the CTS-FP, in such a way that invalid attachment attempts by CTS-MS which are not enrolled (see GSM 03.56) with this CTS-FP are minimised.

10.2 CTS Access Request Channel (CTSARCH)

The CTSARCH is transmitted in the uplink by the CTS-MS to request dedicated radio resources from the CTS-FP; it is also used during the Alive check procedure (see clause 12).

10.2.1 CTSARCH format

– Burst: Synchronisation Bursts (SB) are used for the CTSARCH.

NOTE: due to the short distance between the CTS-FP and the CTS-MS, there is no need to use a burst of reduced length like the Access Burst (as specified in GSM 05.02 subclause 5.2).

– Channel coding scheme: same as for the SCH, specified in GSM 05.03 subclause 4.7.

10.2.2 CTSARCH timing

The timing of the CTSARCH is as follows (see figure 1):

– frame position:

– FN mod 52 = 2 to 9 for CTSARCH used for the non-hopping access procedure, see subclause 12.3.3.2;

– FN mod 52 = 10 to 15 for CTSARCH used for the hopping access procedure (see subclause 12.3.3.2) and the alive check procedure (see subclause 12.3.2 and subclause 12.3.5).

– timeslot position: the CTSARCH is transmitted on a timeslot number which shall be indicated in the CTSBCH-SB information bits (see subclause 10.1.4).

10.2.3 CTSARCH radio frequency channel

The CTSARCH radio frequency channel is:

– the CTS beacon frequency for the CTSARCH used for the non-hopping access procedure, see subclause 12.3.3.2;

– mapped on a predefined set of frequencies by the Total Frequency Hopping for the CTSARCH used for the hopping access procedure (see subclause 12.3.3.2) and the alive check procedure (see subclause 12.3.2).

Refer to subclause 10.8.2 for background on the choice of the radio frequency channel.

10.2.4 CTSARCH information

The CTSARCH carries 25 information bits forming an access request message. These 25 bits shall be divided into two fields as follows:

– a field indicating the cause of the type of the access request;

– a field carrying the CTS Mobile Subscriber Identity (CTSMSI) allocated to the CTS-MS.

10.3 CTS Access Grant Channel (CTSAGCH)

The CTSAGCH is used in the downlink by the CTS-FP to grant a dedicated RR connection to a CTS-MS that has requested radio resources by the use of the CTSARCH.

10.3.1 CTSAGCH format

– Burst: Normal Bursts (NB) are used for the CTSAGCH.

– Training sequence: it is determined by the three LSBs of the FPBI. These three bits form the 3-bit training sequence code (TSC) which selects one of the eight training sequences specified in GSM 05.02 subclause 5.2.3.

– Channel coding and interleaving schemes: same as for the SACCH, as specified in GSM 05.03 subclause 4.1, over 4 consecutive bursts.

10.3.2 CTSAGCH timing

The timing of the CTSAGCH is as follows (see figure 1):

– frame position:

– FN mod 52 = 16 to 19 for the CTSAGCH used for the non-hopping access procedure, see subclause 12.3.3.2;

– FN mod 52 = 20 to 23 for the CTSAGCH used for the hopping access procedure, see subclause 12.3.3.2.

– timeslot position: the CTSAGCH is transmitted on a timeslot number which shall be indicated in the CTSBCH‑SB information bits (see subclause 10.1.4).

10.3.3 CTSAGCH radio frequency channel

The CTSAGCH radio frequency channel is:

– the CTS beacon frequency for the CTSAGCH used for the non-hopping access procedure, see subclause 12.3.3.2;

– mapped on a predefined set of frequencies by the Total Frequency Hopping for the CTSAGCH used for the hopping access procedure, see subclause 12.3.3.2.

Refer to subclause 10.8.2 for background on the choice of the radio frequency channel.

10.3.4 CTSAGCH information

The CTSAGCH carries 23 octets of information, forming a message which directs the CTS-MS to a channel on which the RR connection can be continued. This message provides the CTS-MS with channel and timing information.

10.4 CTS Paging Channel (CTSPCH)

The CTSPCH is used in the downlink by the CTS-FP to broadcast information for paging (see subclause 12.3.3.3) and alive check (see subclause 12.3.2) procedures. The presence of the CTSPCH on the physical channel is indicated by a signalling information in the CTSBCH (see subclause 10.1.4).

10.4.1 CTSPCH format

– Burst: Normal Bursts (NB) are used for the CTSPCH.

– Training sequence: it is determined by the three LSBs of the FPBI. These three bits form the 3-bit training sequence code (TSC) which selects one of the eight training sequences specified in GSM 05.02 subclause 5.2.3.

– Channel coding and interleaving schemes: same as for the SACCH, as specified in GSM 05.03 subclause 4.1, over 4 consecutive bursts.

10.4.2 CTSPCH timing

The timing of the CTSPCH is as follows (see figure 1):

– frame position: FN mod 52 = 2 to 5;

– timeslot position: the CTSPCH is transmitted on a timeslot number which shall be indicated in the CTSBCH-SB information bits (see subclause 10.1.4).

10.4.3 CTSPCH radio frequency channel

The CTSPCH radio frequency channel is mapped on a predefined set of frequencies by the Total Frequency Hopping algorithm.

Refer to subclause 10.8.2 for background on the choice of the radio frequency channel.

10.4.4 CTSPCH information

The CTSPCH contains 23 octets of information, forming the following possible messages:

– a message used for the paging procedure, see subclause 12.3.3.3;

– a message used for alive check procedure, see subclause 12.3.2.

10.5 SACCH

In GSM-CTS, the TDMA frames where the CTSBCH is transmitted (FN mod 26 = 25) are not available for any SACCH transmission, in contrary to GSM. This results in the requirement that the SACCH multiframe shall span 104 TDMA frames (480 ms) as for GSM, however the interleaving scheme of the 4 SACCH bursts shall be so that no SACCH burst is sent in the TDMA frames: FN mod 104 = 25, 51, 77 or 103.

Therefore, the mapping in time of the 4 SACCH/CTS frames onto the physical channel shall be as follows:

– TN = 0 and 1 FN mod 104 = 12, 38, 64, 90;

– TN = 2 and 3 FN mod 104 = 38, 64, 90, 12;

– TN = 4 and 5 FN mod 104 = 64, 90, 12, 38;

– TN = 6 and 7 FN mod 104 = 90, 12, 38, 64.

The timing of the SACCH on the 26-multiframe is shown on figure 2.

The SACCH radio frequency channel is mapped on a predefined set of frequencies by the Total Frequency Hopping algorithm, as defined in subclause 10.8.2, except in the case of the non-hopping access procedure (for the attachment or enrolment of a CTS-MS), where the SACCH is mapped on the CTS beacon frequency.

SACCH is a point-to-point dedicated control channel used to transmit signalling messages for the layered GSM-CTS protocol.

10.6 FACCH

The standard GSM FACCH as defined in GSM 05.02 is used in GSM-CTS. Full rate FACCH/F is supported.

The timing of the FACCH on the 26-multiframe is shown on figure 2.

The FACCH radio frequency channel is mapped on a predefined set of frequencies by the Total Frequency Hopping algorithm, as defined in subclause 10.8.2, except in the case of the non-hopping access procedure (for the attachment or enrolment of a CTS-MS), where the FACCH is mapped on the CTS beacon frequency.

FACCH is a point-to-point dedicated control channel used to transmit signalling messages for the GSM-CTS layered protocol.

10.7 TCH

The traffic channel TCH used in GSM-CTS is the standard GSM traffic channel TCH as defined in GSM 05.02.

The supported channel types shall be:

– TCH/F.

The supported channel modes shall be in the Phase 1 of the GSM-CTS:

– speech v1: full rate speech coder;

– speech v2: enhanced full rate speech coder;

– signalling only.

Discontinuous transmission (DTX) shall be supported on the speech TCH.

The timing of the TCH on the 26-multiframe is shown on figure 2.

The TCH radio frequency channel is mapped on a predefined set of frequencies by the Total Frequency Hopping algorithm, as defined in subclause 10.8.2, except in the case of the non-hopping access procedure (for the attachment or enrolment of a CTS-MS), where the TCH is mapped on the CTS beacon frequency.

10.8 Mapping of the Logical Channels onto Physical Channels

10.8.1 Mapping in time of the logical channels onto the physical channels

The following figures give the mapping in time of the logical channels onto the physical channels.

Figure 1: 52-multiframe structure

Figure 2: 26-multiframe structure

10.8.2 Mapping in frequency of the logical channels onto the physical channels

Prior to its attachment with a CTS-FP, the CTS-MS can not perform hopping as it is not aware of:

– the list of frequencies on which it shall hop: the TFH list (see subclause 12.2.2);

– the hopping sequence to use with the CTS-FP: this is computed from a set of parameters of the TFH algorithm (see below).

Therefore, the CTSBCH and the channels used in the attachment of a CTS-MS to access the CTS-FP (see non-hopping access procedure: subclause 12.3.3.2.1) are transmitted on the CTS beacon frequency.

After attachment, the CTS-MS has obtained the required information i.e. the TFH list and the parameters for the TFH algorithm, therefore hopping can be performed. All logical channels (except the CTSBCH) are mapped by the TFH algorithm on the TFH list.

The TFH algorithm shall be the Lempel-Greenberger algorithm concatenated with a non-repeating code (LG/NR). The hopping sequence is computed from a codeword which is continuously changing according to the LG/NR algorithm.

10.8.3 Permitted Channel Combinations

Due to the change of CTSBCH timeslot position from one 52-multiframe to another (see subclause 11.2), the following channel combinations are allowed on a physical channel:

(i) CTSBCH + CTSPCH + CTSARCH + CTSAGCH;

(ii) CTSPCH + CTSARCH + CTSAGCH;

(iii) CTSBCH;

(iv) CTSBCH + TCH/F + FACCH/F + SACCH/CTS;

(v) TCH/F + FACCH/F + SACCH/CTS.

Channel combinations (i), (iii) and (iv) shall be mutually exclusive in one 52-multiframe, because the CTSBCH is transmitted only once per 52-multiframe.

Channel combinations (i) and (ii) shall be also mutually exclusive.