4 Functional division between BSC and BTS

08.523GPPBase Station Controller - Base Tranceiver Station (BSC-BTS) Interface - Interface PrinciplesTS

4.1 General

In Technical Specification 3GPP TS 08.02 the functional division between MSC and BSS is described. This section describes the further subdivision of functions between BSC and BTS/TRX required for the A-bis interface. A summary can be found in table 4.1. Some general requirements on the functionality of the A-bis interface are also specified.

4.2 Terrestrial channel management

There is a unique mapping from traffic channels on the radio path to the terrestrial traffic channels. BSC makes the choice of radio channel and thereby also of the terrestrial channel for a call.

4.3 Radio channel management

4.3.1 Channel configuration management

The channel configuration is controlled between the BSC and OMC. Current configuration is downloaded from OMC to BSC which then controls the use of the radio channels (TDMA time slots for BCCH/CCCH, TCHs, SDCCHs etc).

4.3.2 SDCCH (Stand alone DCCH) and TCH management

4.3.2.1 Frequency hopping management

The hopping sequences for each BTS (cell) is downloaded from OMC to BSC. It is then the responsibility of BSC to download this information to each BTS and also to send the corresponding BCCH information to be transmitted in the BCCH time slots.

4.3.2.2 Channel selection, link supervision and channel release

These functions are controlled by BSC. For channel selection BSC has to have information on blocked radio channels and also on interference level on idle channels.

In the assignment messages to MS (Immediate Assign, Assign Command and Handover Command), a Starting Time parameter is included. This starting time is based on the frame number on the (new) BTS. Before sending the assign message to MS, BSC has to be informed on the current frame number in BTS.

When assigning a channel, BSC shall inform BTS on relevant parameters, e.g. channel type, channel coding, rate adaption, starting time.

4.3.2.3 Power control

The ordered MS power level is sent in the 16 bit L1-header of SACCH- blocks on the downlink and the actual power level used by MS is reported in the corresponding L1-header on the uplink. This header is inserted (downlink) and extracted (uplink) by BTS/TRX.

The determination of required power level in MS is based on uplink radio measurements made by BTS/TRX and reported to BSC. The basic control of this power is performed by BSC and the dynamic regulation is performed by BSC or optionally by BTS. If BTS supports dynamic MS power regulation, BSC can indicate whether BTS is to regulate the MS power and if so, also the parameters required by BTS.

The required TRX transmission power level on a channel is based on reported measurements performed by MS. The dynamic control of this power is optional. If supported, the basic control is performed by BSC and the dynamic regulation is performed by BSC or optionally by BTS. If BTS supports dynamic TRX transmission power regulation, BSC can indicate whether BTS is to regulate the transmission power and if so, also the parameters required by BTS.

4.3.2.4 Idle channel observation

Idle channels are monitored by BTS.

4.3.3 BCCH/CCCH management

TRX knows the timing of BCCH/CCCH slots (not known by BSC). The actual timing of BCCH/CCCH blocks therefore has to be made by BTS/TRX, including the scheduling of Paging Request messages on paging sub- channels.

BCCH information is downloaded to BTS.

4.3.4 Random access

Detection of a random access attempt has to be made by TRX which then sends a message to BSC containing the required timing advance, the frame number of the access attempt and the 8 bit Channel Request message sent by MS in the access burst. This information is then included by BSC in the following Immediate Assign message sent to MS.

4.3.5 Channel coding/decoding

The error protection coding and decoding is made by BTS/TRX.

Different coding and interleaving schemes are used for speech and data calls. This information has to be signalled from BSC to BTS on a per call basis.

4.3.6 Transcoding/rate adaption

The A-bis interface has to allow for the transcoder/rate adaptor being positioned either inside or outside BTS.

4.3.7 Timing advance

Timing advance has to be determined by TRX.

When MS is on a dedicated channel (SDCCH, TCH), the required timing advance (TA) is sent to MS and the actual timing advance is reported by MS in the 16 bit L1-header of the SACCH blocks.

At handover access, TA is determined by TRX and reported to MS in the PHYsical INFOrmation message sent by BTS/TRX.

At random access, TA is determined by TRX but in those cases, TA has to be reported to BSC for inclusion in the IMMediate ASSIGN message sent to MS by BSC.

4.3.8 Radio resource indication

BTS reports on status (interference level, blocking etc.) of idle channels to BSC on a regular basis.

4.3.9 Measurements

MS measures the downlink received level and quality from the serving cell and the received level from surrounding cells. The results from these measurements are reported by MS in Measurement Report messages on SACCH.

Uplink received level and quality are measured by BTS/TRX. The parameters for the uplink measurements are equivalent to the parameters used by MS for the corresponding downlink measurements (averaging period and dynamic range).

The support of forwarding this basic raw measurement data over the A-bis interface is mandatory. Additionally, BTS and BSC may optionally support some pre-processing in BTS of this data.

4.3.10 LAPDm functions (Layer 2)

Layer 2 on the radio interface (LAPDm) is terminated at BTS/TRX. Between BTS and BSC, LAPD is used.

4.3.11 Paging

Paging is initiated by MSC via BSC.

BSC determines the paging group to be used based on IMSI of the MS to be paged. The paging group value is sent to BTS together with the TMSI or IMSI.

Based on the paging group information, BTS/TRX will build the relevant PAGING REQUEST message and execute the transmission of the message in the correct paging sub-channel.

4.3.12 Handover

No handover recognition or decision is made by BTS.

However, BTS/TRX has to detect the handover access made by a handed over MS. BTS/TRX also checks the Handover Reference value sent by MS in the handover access burst against the Handover Reference value received from BSC in the channel activation command. In case of an asynchronous handover, BTS/TRX then builds the PHYsical INFOrmation message (including the Timing Advance value) and sends it to MS. The handover access is also reported to BSC.

4.3.13 Encryption

Encryption and decryption is made of the complete outgoing bit stream (except for preambles/synch words). This therefore has to be made by TRX. The encryption key has to be downloaded to BTS/TRX before encryption starts. Special control messages are therefore required between BSC and BTS/TRX.

4.3.14 Mobility management and call control

All mobility management and call control functions recide in MSC.

TABLE 4.1: Summary of functional division between BTS and BSC/MSC

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TABLE 4.1. (continued): Summary of functional division between BTS and BSC/MSC

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NOTE 1: Although the transcoder is always controlled by the BTS, it can optionally be located outside the BTS (e.g. at the BSC or at the MSC site). In that case, remote control is performed by BTS using inchannel signalling.

NOTE 2: Including averaging comparable to what is done in the MS.

NOTE 3: The support of power level determination in BTS is optional.

NOTE 4: The support of forwarding all raw measurement data from MS/TRX over the A-bis interface and the processing of it in BSC is mandatory. The BTS/BSC may additionally support also some pre- processing in BTS of this raw data.