4.4 Compressed mode

25.2123GPPMultiplexing and channel coding (FDD)Release 17TS

In compressed frames, TGL slots from N_first to N_last are not used for transmission of data. As illustrated in figure 11, 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 (see subclause 4.4.3). What frames are compressed, are decided by the network. When in compressed mode, compressed frames can occur periodically, as illustrated in figure 11, or requested on demand. The rate and type of compressed frames is variable and depends on the environment and the measurement requirements.

Figure 11: Compressed mode transmission

4.4.1 Frame structure in the uplink

The frame structure for uplink compressed frames is illustrated in figure 12.

Figure 12: Frame structure in uplink compressed transmission

4.4.2 Frame structure types in the downlink

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 13(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 13(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

Figure 13: Frame structure types in downlink compressed transmission

4.4.2A Frame structure in the downlink for F-DPCH

There is only one type of frame structure defined for downlink F-DPCH compressed frames: transmission is turned off during the whole transmission gap i.e. in slots N_first to N_last.

4.4.2B Frame structure in the downlink for F-TPICH

There is only one type of frame structure defined for downlink F-TPICH compressed frames: transmission is turned off during the whole transmission gap i.e. in slots N_first to N_last. TPI will not be transmitted in either of the both slots in the case where only one of the two slots carrying the TPI information overlaps with the transmission gap.

4.4.3 Transmission time reduction method

When in compressed mode, the information normally transmitted during a 10 ms frame is compressed in time. The mechanisms provided for achieving this are reduction of the spreading factor by a factor of two , and higher layer scheduling. In the downlink and the uplink, all methods are supported. The maximum idle length is defined to be 7 slots per one 10 ms frame. The slot formats that are used in compressed frames are listed in [2].

In downlink when DL_DCH_FET_Config is configured by higher layers, and in uplink when UL_DPCH_10ms_Mode is configured by higher layers, the same slot format is used in compressed frames and normal frames.

In case F-DPCH is configured in the downlink, no transmission time reduction method is needed during compressed frames. The same slot format is used in compressed frames and normal frames.

4.4.3.1 Void

4.4.3.2 Compressed mode by reducing the spreading factor by 2

The spreading factor (SF) can be reduced by 2 during one compressed radio frame to enable the transmission of the information bits in the remaining time slots of the compressed frame. This method is not supported for SF=4.

On the downlink, UTRAN can also order the UE to use a different scrambling code in a compressed frame than in a non-compressed frame. If the UE is ordered to use a different scrambling code in a compressed frame, then there is a one-to-one mapping between the scrambling code used in the non-compressed frame and the one used in the compressed frame, as described in [3] subclause 5.2.1.

4.4.3.3 Compressed mode by higher layer scheduling

Compressed frames can be obtained by higher layer scheduling. Higher layers then set 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. Compressed mode by higher layer scheduling shall not be used when DL_DCH_FET_Config is configured by higher layers.

4.4.4 Transmission gap position

Transmission gaps can be placed at different positions as shown in figures 14 and 15 for each purpose such as interfrequency power measurement, acquisition of control channel of other system/carrier, and actual handover operation.

The restrictions listed below apply to DPCCH/DPCCH2/S-DPCCH/DPDCH in the uplink and DPCH or F-DPCH in the downlink.

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 14 (1). When using double frame method, the transmission gap is located on the center of two connected frames as shown in figure 14 (2).

Figure 14: Transmission gap position

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

N_first specifies the starting slot of the consecutive idle slots,

N_first = 0,1,2,3,…,14.

N_last shows the number of the final idle slot and is calculated as follows;

If N_first + TGL  15, then N_last = N_first + TGL –1 ( in the same frame ),

If N_first + TGL > 15, then N_last = (N_first + TGL – 1) mod 15 ( in the next frame ).

When the transmission gap spans two consecutive radio frames, N_first and TGL must be chosen so that at least 8 slots in each radio frame are transmitted.

Figure 15: Transmission gap positions with different Nfirst

4.4.5 Transmission gap position for E‑DCH

In the following, the transmission gap position for E‑DCH during compressed frames is specified for the case when E‑DCH TTI length is 10 ms. Slots that are not idle due to uplink compressed mode are termed "available".

The parameter n_first and n_last are used to determine the transmission gap position due to uplink compressed mode in the current radio frame. If the start of the transmission gap is allocated in the current frame n_first=N_first else n_first=0. If the end of a transmission gap is allocated in the current frame n_last=N_last else n_last=14.

4.4.5.1 E‑DPDCH Transmission Gap Position during Initial Transmissions

If an initial transmission overlaps with a compressed frame the starting slot of the consecutive E‑DPDCH idle slots within the E‑DCH TTI is n_first, and n_last is the final idle slot within the 10 ms E‑DCH TTI. The number of transmitted slots n_tx1 is given by n_tx1=14+n_first–n_last.

If the initial transmission occurs in a non‑compressed uplink frame, n_tx1=15.

4.4.5.2 E‑DPDCH Transmission Gap Position during Retransmissions

If the current retransmission occurs in a compressed frame the maximum number of slots available for the retransmission is given by n_max=14+n_first–n_last. Else the maximum number of slots available for the retransmission n_max is 15.

If the initial transmission was compressed and in the retransmission more than n_tx1 slots are available for transmission (n_max>n_tx1), the last n_dtx=n_max–n_tx1 available slots of the E‑DPDCH frame are E‑DPDCH idle slots. The parameter n_tx1 refers to the number of transmitted slots calculated as defined in 4.4.5.1 for the corresponding initial transmission.

The E‑DPDCH transmission gap in case a retransmission occurs in a compressed frame or a retransmission occurs in a non‑compressed frame for which initial transmission was compressed is defined as follows:

If n_max  n_tx1

E‑DPDCH idle slots are slots n_first, n_first+1, .., n_last

Else

If a retransmission occurs in a compressed frame and n_first < n_tx1

E‑DPDCH idle slots are the slots n_first, n_first+1, .., n_last and 15-n_dtx, 15-n_dtx+1, ..,14

Else

E‑DPDCH idle slots are the slots n_tx1, n_tx1+1, ..,14

4.4.5.3 E‑DPCCH Transmission Gap Position

If a transmission overlaps with an uplink compressed frame the starting slot of the compressed mode gap within the E‑DCH TTI is n_first, and n_last is the final E‑DPCCH idle slot within the 10 ms E‑DCH TTI.