5.6 Midamble Allocation for Physical Channels

25.2213GPPPhysical channels and mapping of transport channels onto physical channels (TDD)Release 17TS

Midambles are part of the physical channel configuration which is performed by higher layers. Three different midamble allocation schemes exist:

– UE specific midamble allocation: A UE specific midamble for DL or UL is explicitly assigned by higher layers.

– Default midamble allocation: The midamble for DL or UL is allocated by layer 1 depending on the associated channelisation code.

– Common midamble allocation: The midamble for the DL is allocated by layer 1 depending on the number of channelisation codes currently being present in the DL time slot.

If a midamble is not explicitly assigned and the use of the common midamble allocation scheme is not signalled by higher layers, the midamble shall be allocated by layer 1, based on the default midamble allocation scheme. This default midamble allocation scheme is given by a fixed association between midambles and channelisation codes, see clause A.3, and shall be applied individually to all channelisation codes within one time slot. Different associations apply for different burst types and cell configurations with respect to the maximum number of midambles.

For timeslots employing MBSFN operation burst type 4 is used and hence DL beamforming is not applied, subclause 5.2.4. Furthermore, as this burst type contains only a single midamble, i.e. K_Cell=1, then all physical channels in such timeslots employ the same midamble and thus default and common midamble allocation amount to the same allocation strategies.

5.6.1 Midamble Allocation for DL Physical Channels

Beacon channels shall always use the reserved midambles m⁽¹⁾ and m⁽²⁾, see 5.5. For DL physical channels that are located in the same time slot as the P-CCPCH, midambles shall be allocated based on the default midamble allocation scheme, using the association for burst type 1 and K_Cell =8 midambles. For all other DL physical channels, the midamble is explicitly assigned by higher layers or allocated by layer 1.

5.6.1.1 Midamble Allocation by signalling from higher layers

UE specific midambles may be signalled by higher layers to UE’s as a part of the physical channel configuration, if:

– multiple UEs use the physical channels in one DL time slot; and

– beamforming is applied to all of these DL physical channels; and

– no closed loop TxDiversity is applied to any of these DL physical channels;

or

– PDSCH physical layer signalling based on the midamble is used.

5.6.1.2 Midamble Allocation by layer 1

5.6.1.2.1 Default midamble

If a midamble is not explicitly assigned and the use of the common midamble allocation scheme is not signalled by higher layers, the UE shall derive the midambles from the allocated channelisation codes and shall use an individual midamble for each channelisation code group containing one primary and a set of secondary channelisation codes. The association between midambles and channelisation code groups is given in annex A.3. All the secondary channelisation codes within a set use the same midamble as the primary channelisation code to which they are associated.

Higher layers shall allocate the channelisation codes in a particular order. Secondary codes shall only be allocated if the associated primary code is also allocated. If midambles are reserved for the beacon channels, all primary and secondary channelisation codes that are associated with the reserved midambles shall not be used.

Channelisation codes of one channelisation code group shall not be allocated to different UE’s.

In the case that secondary channelisation codes are used, secondary channelisation codes of one channelisation code group shall be allocated in ascending order, with respect to their numbering, and beginning with the lowest code index in this channelisation code group.

The UE shall assume different channel estimates for each of the individual midambles.

The default midamble allocation shall not apply for those downlink channels that are intended for a UE which will be the only UE assigned to a given time slot or slots for the duration of the assigned channel’s existence (as in the case of high rate services).

5.6.1.2.2 Common Midamble

The use of the common midamble allocation scheme is signalled to the UE by higher layers as a part of the physical channel configuration. A common midamble may be assigned by layer 1 to all physical channels in one DL time slot, if:

– a single UE uses all physical channels in one DL time slot (as in the case of high rate service);

or

– multiple UEs use the physical channels in one DL time slot; and

– no beamforming is applied to any of these DL physical channels; and

– no closed loop TxDiversity is applied to any of these DL physical channels; and

– midambles are not used for PDSCH physical layer signalling.

The number of channelisation codes currently employed in the DL time slot is associated with the use of a particular common midamble. Different associations apply for different burst types and cell configurations with respect to the maximum number of midambles, see annex B.

5.6.2 Midamble Allocation for UL Physical Channels

If the midamble is explicitly assigned by higher layers, an individual midamble shall be assigned to all UE’s in one UL time slot.

If no midamble is explicitly assigned by higher layers, the UE shall derive the midamble from the channelisation code that is used for the data part (except for TFCI/TPC) of the burst. Note that in the event that code hopping is employed the midamble is derived from the channelisation code actually transmitted (i.e. the code used after the hop sequence has been applied – see [9]). The associations between midamble and channelisation code are the same as for DL physical channels.