Introduction:
Compressed mode, also known as the Slotted Mode, is needed when making measurements on another frequency (inter-frequency) or on a different radio technology (inter-RAT). In the Compressed Mode the transmission and reception are stopped for a short time and the measurements are performed on other frequency or RAT in that time. After the time is over the transmission and reception resumes. To make sure that the data is not lost, the data is compressed in the frame making empty space where measurements can be performed.
Compressed mode is not necessary. If the UE has a second receiver it can make measurements on that receiver while continuing with the transmission/reception on the first receiver. This does not happen in practise as the cost would go up. The UE capabilities define whether a UE requires compressed mode in order to monitor cells on other FDD frequencies and on other modes and radio access technologies. UE capabilities indicate the need for compressed mode separately for the uplink and downlink and for each mode, radio access technology and frequency band.
A UE shall support compressed mode for all cases for which the UE indicates that compressed mode is required. A UE does not need to support compressed mode for cases for which the UE indicates that compressed mode is not required. For these cases, the UE shall support an alternative means of making the measurements. The UE shall support one single measurement purpose for one transmission gap pattern sequence. The measurement purpose of the transmission gap pattern sequence is signalled by higher layers.
The figure above gives an idea of how the frame is compressed for performing measurements. In compressed frames, TGL slots from Nfirst to Nlast are not used for transmission of data. As illustrated in figure, the instantaneous transmit power is increased in the compressed frame in order to keep the quality (BER, FER, etc.) unaffected by the reduced processing gain. The amount of power increase depends on the transmission time reduction method. What frames are compressed, are decided by the network. When in compressed mode, compressed frames can occur periodically, as illustrated in figure, or requested on demand. The rate and type of compressed frames is variable and depends on the environment and the measurement requirements.
Parameterisation of the compressed mode [3]
In response to a request from higher layers, the UTRAN shall signal to the UE the compressed mode parameters.
A transmission gap pattern sequence consists of consecutive occurrences of transmission gap pattern 1, where transmission gap pattern 1 consists of one or two transmission gaps. See figure below.
The following parameters characterise a transmission gap pattern:
- TGSN (Transmission Gap Starting Slot Number): A transmission gap pattern begins in a radio frame, henceforward called first radio frame of the transmission gap pattern, containing at least one transmission gap slot. TGSN is the slot number of the first transmission gap slot within the first radio frame of the transmission gap pattern;
- TGL1 (Transmission Gap Length 1): This is the duration of the first transmission gap within the transmission gap pattern, expressed in number of slots;
- TGL2 (Transmission Gap Length 2): This is the duration of the second transmission gap within the transmission gap pattern, expressed in number of slots. If this parameter is not explicitly set by higher layers, then TGL2 = TGL1;
- TGD (Transmission Gap start Distance): This is the duration between the starting slots of two consecutive transmission gaps within a transmission gap pattern, expressed in number of slots. The resulting position of the second transmission gap within its radio frame(s) shall comply with the limitations of [1]. If this parameter is not set by higher layers, then there is only one transmission gap in the transmission gap pattern;
- TGPL1 (Transmission Gap Pattern Length): This is the duration of transmission gap pattern 1, expressed in number of frames;
The following parameters control the transmission gap pattern sequence start and repetition:
- TGPRC (Transmission Gap Pattern Repetition Count): This is the number of transmission gap patterns within the transmission gap pattern sequence;
- TGCFN (Transmission Gap Connection Frame Number): This is the CFN of the first radio frame of the first pattern 1 within the transmission gap pattern sequence.
In addition to the parameters defining the positions of transmission gaps, each transmission gap pattern sequence is characterised by:
- UL/DL compressed mode selection: This parameter specifies whether compressed mode is used in UL only, DL only or both UL and DL;
- UL compressed mode method: The methods for generating the uplink compressed mode gap are spreading factor division by two or higher layer scheduling and are described in [1];
- DL compressed mode method: The methods for generating the downlink compressed mode gap are spreading factor division by two or higher layer scheduling and are described in [1];
- downlink frame type: This parameter defines if frame structure type 'A' or 'B' shall be used in downlink compressed mode. The frame structures are defined in [1];
- scrambling code change: This parameter indicates whether the alternative scrambling code is used for compressed mode method 'SF/2'. Alternative scrambling codes are described in [4];
- RPP: Recovery Period Power control mode specifies the uplink power control algorithm applied during recovery period after each transmission gap in compressed mode. RPP can take 2 values (0 or 1). The different power control modes are described in [5];
- ITP: Initial Transmit Power mode selects the uplink power control method to calculate the initial transmit power after the gap. ITP can take two values (0 or 1) and is described in [5].
The UE shall support simultaneous compressed mode pattern sequences which can be used for different measurements. The following measurement purposes can be signalled from higher layers:
- FDD
- TDD
- GSM carrier RSSI measurement
- Initial BSIC identification
- BSIC re-confirmation.
The UE shall support one compressed mode pattern sequence for each measurement purpose while operating in FDD mode, assuming the UE needs compressed mode to perform the respective measurement. In case the UE supports several of the measurement purposes, it shall support in parallel one compressed mode pattern sequence for each supported measurement purpose where the UE needs compressed mode to perform the measurement. The capability of the UE to operate in compressed mode in uplink and downlink is given from the UE capabilities.
The GSM measurements Initial BSIC identification and BSIC re-confirmation are defined in [6].
Higher layers will ensure that the compressed mode gaps do not overlap and are not scheduled to overlap the same frame. The behaviour when an overlap occurs is described in [7]. UE is not required to support two compressed mode gaps in a frame.
In all cases, higher layers have control of individual UE parameters. Any pattern sequence can be stopped on higher layers' command.
The parameters TGSN, TGL1, TGL2, TGD, TGPL1, TGPRC and TGCFN shall all be integers.
Different Methods of Frame Compression
There are three different methods through which the frame compression can be achieved
- Puncturing: The symbol rate can be reduced by puncturing the data bits. This method is practical only for short TGLs. This is because there is an upper limit to the amount of data that can be punctured and then receovered at the other end. The advantage is that it is a simple method and there is no need to change the speading code as in the other method. This method has been removed from the latest version of the specifications.
- Changing the Spreading Factor (SF): In this method the frame in which compression is to be carried out, SF is reduced by 2 so the same amount of data can be transmitted in half the time and the remaining time measurements can be done. This is the most popular method used in practice as its very straightforward.
- Higher Layer scheduling: The higher layers are aware of the compressed mode schedule, so they could lower the data rate in the frame measurements need to be done thus avoiding the need for new spreading factor and new channelisation codes. Higher layers sets restrictions so that only a subset of the allowed TFCs are used in a compressed frame. The maximum number of bits that will be delivered to the physical layer during the compressed radio frame is then known and a transmission gap can be generated. Note that in the downlink, the TFCI field is expanded on the expense of the data fields and this shall be taken into account by higher layers when setting the restrictions on the TFCs. Compressed mode by higher layer scheduling shall not be used with fixed starting positions of the TrCHs in the radio frame.
Frame structure in the uplink [1]
The frame structure for uplink compressed frames is illustrated in figure below:
Frame structure types in the downlink [1]
There are two different types of frame structures defined for downlink compressed frames. Type A maximises the transmission gap length and type B is optimised for power control. The frame structure type A or B is set by higher layers independent from the downlink slot format type A or B.
- With frame structure of type A, the pilot field of the last slot in the transmission gap is transmitted. Transmission is turned off during the rest of the transmission gap (figure a). In case the length of the pilot field is 2 bits and STTD is used on the radio link, the pilot bits in the last slot of the transmission gap shall be STTD encoded assuming DTX indicators as the two last bits in the Data2 field.
- With frame structure of type B, the TPC field of the first slot in the transmission gap and the pilot field of the last slot in the transmission gap is transmitted. Transmission is turned off during the rest of the transmission gap (figure b). In case the length of the pilot field is 2 bits and STTD is used on the radio link, the pilot bits in the last slot of the transmission gap shall be STTD encoded assuming DTX indicators as the two last bits of the Data2 field. Similarly, the TPC bits in the first slot of the transmission gap shall be STTD encoded assuming DTX indicators as the two last bits in the Data1 field.
(a) Frame structure type A
(b) Frame structure type B
Transmission gap position [1]
(a) Transmission gap position
(b) Transmission gap positions with different Nfirst
Transmission gaps can be placed at different positions as shown in figures a and b (above) for each purpose such as interfrequency power measurement, acquisition of control channel of other system/carrier, and actual handover operation.
When using single frame method, the transmission gap is located within the compressed frame depending on the transmission gap length (TGL) as shown in figure a(1). When using double frame method, the transmission gap is located on the center of two connected frames as shown in figure a(2).
Parameters of the transmission gap positions are calculated as follows.
TGL is the number of consecutive idle slots during the compressed mode transmission gap:
TGL = 3, 4, 5, 7, 10,14
Nfirst specifies the starting slot of the consecutive idle slots,
Nfirst = 0,1,2,3,…,14.
Nlast shows the number of the final idle slot and is calculated as follows;
If Nfirst + TGL £ 15, then Nlast = Nfirst + TGL –1 ( in the same frame ),
If Nfirst + TGL > 15, then Nlast
= (Nfirst + TGL – 1) mod 15 ( in the next frame ).
When the transmission gap spans two consecutive radio frames, Nfirst and TGL must be chosen so that at least 8 slots in each radio frame are transmitted.
References:
[1] 3GPP TS 25.212: Multiplexing and channel coding
[2] 3GPP TS 25.331: Radio Resource Control (RRC) Protocol Specifications
[3] 3GPP TS 25.215: Physical layer - Measurements (FDD)
[4] 3GPP TS 25.213: Spreading and modulation (FDD)
[5] 3GPP TS 25.214: Physical layer procedures (FDD)
[6] 3GPP TS 25.133: Requirements for support of radio resource management