7 Closed loop mode 1 transmit diversity
25.2143GPPPhysical layer procedures (FDD)Release 17TS
The general transmitter structure to support closed loop mode transmit diversity for DPCH transmission is shown in figure 3. Channel coding, interleaving and spreading are done as in non-diversity mode. The spread complex valued signal is fed to both TX antenna branches, and weighted with antenna specific weight factors w1 and w2. The weight factor w1 is a constant scalar and the weight factor w2 is complex valued signal.
The weight factor w2 (actually the corresponding phase adjustment) is determined by the UE, and signalled to the UTRAN access point (i.e. cell transceiver) using the FBI field of uplink DPCCH.
For the closed loop mode 1 different orthogonal dedicated pilot symbols in the DPCCH are sent on the 2 different antennas. Closed-loop transmit diversity for DPCH is not supported with downlink slot formats 17 and 18 in [1].
Figure 3: The generic downlink transmitter structure to support closed loop mode transmit diversity for DPCH transmission.
Closed loop mode 1 characteristics are summarised in the Table 8. The use of the closed loop mode 1 is controlled via higher layer signalling.
Table 8: Summary of number of feedback information bits per slot, NFBD, feedback command length in slots, NW, feedback command rate, feedback bit rate, number of phase bits, Nph, per signalling word, number of amplitude bits, Npo, per signalling word and amount of constellation rotation at UE for the closed loop mode 1.
|
NFBD |
NW |
Update rate |
Feedback bit rate |
Npo |
Nph |
Constellation rotation |
|
1 |
1 |
1500 Hz |
1500 bps |
0 |
1 |
/2 |
7.1 General procedure
The UE uses the CPICH to separately estimate the channels seen from each antenna.
Once every slot, the UE computes the phase adjustment, , that should be applied at the UTRAN access point to maximise the UE received power. During soft handover, the UE computes the phase adjustment to maximise the total UE received power from the cells in the active set. In the case that a HS-PDSCH is associated with a DPCH for which closed loop mode 1 is applied, the antenna weights applied to the HS-PDSCH, are the same as the antenna weights applied to the associated DPCH. In case a HS-PDSCH is associated with a DPCH during soft handover, the UE may emphasize the radio link transmitted from HS-DSCH serving cell, respectively, when calculating the antenna weights. An example of how the computations can be accomplished is given in Annex A.2.
The UE feeds back to the UTRAN access point the information on which phase/power settings to use. Feedback Signalling Message (FSM) bits are transmitted in the FBI field of uplink DPCCH slot(s) (see [1]). Each message is of length NW = Npo+Nph bits and its format is shown in the figure 4. The transmission order of bits is from MSB to LSB, i.e. MSB is transmitted first. FSMpo and FSMph subfields are used to transmit the power and phase settings, respectively.
Figure 4: Format of feedback signalling message. FSMpo transmits the power setting and FSMph the phase setting.
The adjustments are made by the UTRAN Access Point at the beginning of the downlink DPCCH pilot field. The downlink slot in which the adjustment is done is signalled to L1 of UE by higher layers. Two possibilities exist:
1) When feedback command is transmitted in uplink slot i, which is transmitted approximately 1024 chips in offset from the received downlink slot j, the adjustment is done at the beginning of the pilot field of the downlink slot (j+1) mod 15.
2) When feedback command is transmitted in uplink slot i, which is transmitted approximately 1024 chips in offset from the received downlink slot j, the adjustment is done at the beginning of the pilot field of the downlink slot (j+2) mod 15.
Thus, adjustment timing at UTRAN Access Point is either according to 1) or 2) as controlled by the higher layers.
In case of soft handover, Layer 1 shall support different adjustment timing values for different radio links in the same active set.
The timing of the weight adjustment of the HS-PDSCH is such that the HS-PDSCH weight adjustment is done at the HS-PDSCH slot border, respectively, M chips after the adjustment of the associated DPCH, where 0 M < 2560.
7.2 Determination of feedback information
The UE uses the CPICH transmitted both from antenna 1 and antenna 2 to calculate the phase adjustment to be applied at UTRAN access point to maximise the UE received power. In each slot, UE calculates the optimum phase adjustment, , for antenna 2, which is then quantized into having two possible values as follows:
(1)
where:
(2)
If = 0, a command ‘0’ is send to UTRAN using the FSMph field. Correspondingly, if = , command ‘1’ is send to UTRAN using the FSMph field.
Due to rotation of the constellation at UE the UTRAN interprets the received commands according to table 9 which shows the mapping between phase adjustment, , and received feedback command for each uplink slot.
Table 9: Phase adjustments, i, corresponding to feedback commands for the slots i of the UL radio frame
|
Slot # |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
|
|
FSM |
0 |
0 |
/2 |
0 |
/2 |
0 |
/2 |
0 |
/2 |
0 |
/2 |
0 |
/2 |
0 |
/2 |
0 |
|
1 |
|
-/2 |
|
-/2 |
|
-/2 |
|
-/2 |
|
-/2 |
|
-/2 |
|
-/2 |
|
|
The weight w2 is then calculated by averaging the received phases over 2 consecutive slots. Algorithmically, w2 is calculated as follows:
(3)
where:
(4)
For antenna 1, w1 is constant:
(5)
7.2.1 End of frame adjustment
In closed loop mode 1 at frame borders the averaging operation is slightly modified. Upon reception of the FB command for slot 0 of a frame, the average is calculated based on the command for slot 13 of the previous frame and the command for slot 0 of the current frame, i.e. i from slot 14 is not used:
(6)
where:
– = phase adjustment from frame j-1, slot 13.
– = phase adjustment from frame j, slot 0.
7.2.2 Normal initialisation
For the first frame of transmission UE determines the feedback commands in a normal way and sends them to UTRAN.
Before the first FB command is received, the UTRAN shall use the initial weight .
Having received the first FB command the UTRAN calculates w2 as follows:
(7)
where:
= phase adjustment from slot 0 of the first frame.
7.2.3 Operation during compressed mode
7.2.3.1 Downlink in compressed mode and uplink in normal mode
When downlink is in compressed mode but uplink is operating normally (i.e. not compressed) the UTRAN continues it’s Tx diversity related functions in the same way as in non-compressed downlink mode.
In downlink transmission gaps there are uplink slots for which no new estimate of the phase adjustment is calculated. During these slots the following rules are applied in UE when determining the feedback command:
1) If no new estimate of phase adjustment i exists corresponding to the feedback command to be sent in uplink slot i:
– If 1 < i < 15:
– the feedback command sent in uplink slot i-2 is used;
– else if i = 0:
– the feedback command sent in uplink slot 14 of previous frame is used;
– else if i = 1:
– the feedback command sent in uplink slot 13 of previous frame is used;
– end if.
2) When transmission in downlink is started again in downlink slot Nlast+1 (if Nlast+1 = 15, then slot 0 in the next frame) the UE must resume calculating new estimates of the phase adjustment. The feedback command corresponding to the first new estimate of i must be sent in the uplink slot which is transmitted approximately 1024 chips in offset from the downlink slot Nlast+1.
7.2.3.2 Both downlink and uplink in compressed mode
During the uplink transmission gaps no FB commands are sent from UE to UTRAN. When transmission in downlink is started again in downlink slot Nlast+1 (if Nlast+1 = 15, then slot 0 in the next frame) the UE must resume calculating new estimates of the phase adjustment. The feedback command corresponding to the first new estimate of i must be sent in the uplink slot which is transmitted approximately 1024 chips in offset from the downlink slot Nlast+1.
The UTRAN continues to update the weight w2 until the uplink transmission gap starts and no more FB commands are received. When the transmission in downlink resumes in slot Nlast+1, the value of w2, calculated after receiving the last FB command before the start of the uplink transmission gap, is applied to antenna 2 signal.
After the UE resumes transmission in uplink and sends the first FB command, the new value of w2 is calculated as follows:
– S1 = {0, 2, 4, 6, 8, 10, 12 14}.
– S2 = {1, 3, 5, 7, 9, 11, 13}.
– i = number of uplink slot at which the transmission resumes.
– j = number of uplink slot at which the last FB command was sent before the start of the uplink transmission gap.
– Do while () or ():
– j = j-1;
– if j < 0;
– j = 14;
– end if;
– end do;
– calculate w2 based on FB commands received in uplink slots i and j.
Note that for Nlast = 13 the end of frame adjustment procedure shall be based on the FB commands for the last odd slot prior to the uplink transmission gap and slot 0.
7.2.3.3 Uplink in compressed mode and downlink in normal mode
The UTRAN continues to update the value of w2 until the uplink transmission gap starts and no more FB commands are received. Then, the value of w2 calculated after receiving the last FB command before the uplink transmission gap is applied to the antenna 2 signal. When the UE resumes transmission in uplink, it shall send FB commands according to subclause 7.2 equations 2 and 3 (normal operation) and the UTRAN Access Point shall interpret the FB commands according to Table 9.
The calculation of w2 by the UTRAN following the uplink transmission gap, and before the first two FB commands following the gap are received is not specified.
7.2.4 Initialisation during compressed mode
7.2.4.1 Downlink in compressed mode
When closed loop mode 1 is initialised during the downlink transmission gap of compressed mode there are slots for which no estimate of the phase adjustment is calculated and no previous feedback command is available.
In this case, if the UE is required to send feedback in the uplink, the FB command to the UTRAN shall be ‘0’.
When transmission in downlink is started again in slot Nlast+1 (if Nlast+1 = 15, then slot 0 in the next frame), the UTRAN shall use the initial weight . The UE must start calculating estimates of the phase adjustment. The feedback command corresponding to the first estimate of i must be sent in the uplink slot which is transmitted approximately 1024 chips in offset from the downlink slot Nlast+1. Having received this feedback command the UTRAN calculates w2 as follows:
(8)
where:
= phase adjustment in uplink slot i , which is transmitted approximately 1024 chips in offset from the downlink slot Nlast+1.
, if slot i is even ( ) and
, if slot i is odd ( )
7.2.4.2 Uplink in compressed mode
Initialisation of closed loop mode 1 operation during uplink compressed mode only is not specified.