UE Transmission Antenna Selection

This is a feature supported by Rel 8. It is a feature that enable UE to select between Antenna port 0 and 1 when it support multiple Tx Antenna.

Why would a UE with two antennas send from only one of them? A typical UE has two receive antennas but only one transmit chain. Antenna selection lets that single transmit chain use the antenna with the better uplink channel at each moment. So the UE gets some diversity gain without a second power amplifier. This page covers how the feature is switched on, what it rules out, and how the eNB signals the antenna to use.

How do the UE and the network turn it on ?

Antenna selection needs two things. The UE must say that it can do it, and the network must tell the UE which mode to use. Both parts are in RRC, and both have been there since the first LTE release.

UE notifies Network via ue-TxAntennaSelectionSupported in UE Capability Information message on whether it support this function or not.

Network notifies UE (when UE support this feature) via ue-TransmitAntennaSelection on how UE is supposed to apply this fuction.

The two fields are shown below. The capability field ue-TxAntennaSelectionSupported sits in PhyLayerParameters of UE-EUTRA-Capability. The configuration field ue-TransmitAntennaSelection sits in AntennaInfoDedicated, and AntennaInfoDedicated-r10 carries the same choice for Release 10 and later.

Following is based on 36.331 v19.3.0 (Release 19)

PhyLayerParameters ::=              SEQUENCE {
    ue-TxAntennaSelectionSupported      BOOLEAN,
    ue-SpecificRefSigsSupported     BOOLEAN
}

AntennaInfoDedicated ::=            SEQUENCE {
    transmissionMode                    ENUMERATED {
                                            tm1, tm2, tm3, tm4, tm5, tm6,
                                            tm7, tm8-v920},
    codebookSubsetRestriction           CHOICE {
        n2TxAntenna-tm3                     BIT STRING (SIZE (2)),
        n4TxAntenna-tm3                     BIT STRING (SIZE (4)),
        n2TxAntenna-tm4                     BIT STRING (SIZE (6)),
        n4TxAntenna-tm4                     BIT STRING (SIZE (64)),
        n2TxAntenna-tm5                     BIT STRING (SIZE (4)),
        n4TxAntenna-tm5                     BIT STRING (SIZE (16)),
        n2TxAntenna-tm6                     BIT STRING (SIZE (4)),
        n4TxAntenna-tm6                     BIT STRING (SIZE (16))
    }       OPTIONAL,                                                           -- Cond TM
    ue-TransmitAntennaSelection         CHOICE{
        release                         NULL,
        setup                           ENUMERATED {closedLoop, openLoop}
    }
}

The setup value picks one of two modes, and 36.213 clause 8.7 defines what each one means. With closedLoop, the eNB chooses the antenna. The UE transmits PUSCH in subframe n from the antenna that the most recent DCI format 0 in subframe n-4 or earlier indicated. With openLoop, the UE chooses by itself, and the specification does not say how. If the feature is released, or the UE does not support it, the UE transmits from UE port 0.

Carrier aggregation adds one detail. The meaning of ue-TxAntennaSelectionSupported is that all bands of a supported band combination switch antennas together. From Release 13, a UE can report per band combination which uplink bands switch together, with txAntennaSwitchUL-r13 in bandParameterList-v1380. For bands that switch together, the UE may assume the same antenna port in each DCI format 0 of a subframe.

The capability is a single BOOLEAN : ue-TxAntennaSelectionSupported in PhyLayerParameters, present since Release 8.
closedLoop puts the eNB in charge : the UE follows the antenna indicated by DCI format 0, four subframes later at the earliest.
openLoop leaves the choice to the UE : 36.213 does not specify how the UE selects the antenna.
Without the feature, the UE uses UE port 0 : this is the default when antenna selection is released or not supported.

What else must the UE give up ?

Antenna selection assumes one transmit chain. So any feature that needs two uplink antenna ports at once, or that would make the antenna choice unclear, cannot run with it. 36.213 lists these exclusions as things the UE is not expected to be configured with.

According to 36.213 8.7 UE transmit antenna selection,

A UE configured with transmit antenna selection for a serving cell is not expected to

• be configured with more than one antenna port for any uplink physical channel or signal for any configured

serving cell, or

• be configured with trigger type 1 SRS transmission on any configured serving cell, or

• be configured with simultaneous PUCCH and PUSCH transmission, or

• be configured with demodulation reference signal for PUSCH with OCC for any configured serving cell

• receive DCI Format 0 indicating uplink resource allocation type 1 for any serving cell.

The list has changed since this quote was taken. In 36.213 v19.4.0, the OCC item is gone, and a new last item says the UE is not expected to be configured with a SCG. The clause also says that the UE is not expected to have both SRS-Antenna-Switching-2T4R and ue-TransmitAntennaSelection. So antenna selection does not combine with dual connectivity either.

The first item is the core one. More than one antenna port for PUSCH, PUCCH or SRS would need more than one transmit chain, and that is exactly what antenna selection avoids. The specification does not give reasons for the other items. Note, though, that the last item concerns DCI format 0, which is the DCI that carries the antenna command in the next section.

One antenna port for every uplink channel : the UE is not configured with multi-port PUSCH, PUCCH or SRS.
No simultaneous PUCCH and PUSCH : the network does not configure simultaneousPUCCH-PUSCH together with antenna selection.
The current list adds SCG and removes OCC : check 36.213 v19.4.0 clause 8.7 rather than the older quote.

How does the eNB tell the UE which antenna to use ?

With closedLoop, the eNB must send an antenna command with every uplink grant. DCI format 0 has no spare bit for it. Instead, the eNB encodes the command in the CRC of the DCI, so the payload size does not change.

When this is configured by high layer, network notifies UE on which antenna it should use for each transmission by masking CRC field of PDCCH (DCI) as shown below (See 36.212 5.3.3.2 CRC attachment for further details)

DCI CRC scrambled with the RNTI and the UE transmit antenna selection mask

Read the diagram above from the top. The A payload bits a₀ to a_A-1 pass through unchanged. The 16 CRC parity bits become b_A to b_A+15. Each of them is added modulo 2 to one RNTI bit x_rnti,k and to one mask bit x_AS,k. The result is c_A to c_A+15. The last parity bit is labelled a₁₅ in the drawing, but it is the parity bit p₁₅.

The mask has only two values : 36.212 Table 5.3.3.2-1 gives all zeros for UE port 0, and zeros with a final 1 for UE port 1.
Port 0 looks like an ordinary DCI : the all-zero mask leaves the RNTI scrambling unchanged.
The UE finds the port by the CRC check : in practice the UE tests the CRC with each mask, and the mask that passes names the antenna.
The mask applies to DCI format 0 and 6-0A only : other DCI formats keep the plain RNTI scrambling.

How does the eNB know which antenna is better? It measures the SRS. When antenna selection is enabled, the UE alternates the antenna from one SRS transmission to the next. Without frequency hopping, the antenna index for SRS is a(n_SRS) = n_SRS mod 2 (36.213 clause 8.2). So the eNB receives SRS from both antennas in turn and compares them. Release 15 extended SRS antenna switching to four antennas, with ue-TxAntennaSelection-SRS-1T4R-Config and ue-TxAntennaSelection-SRS-2T4R-NrOfPairs. When 1T4R is configured together with ue-TransmitAntennaSelection, the UE switches SRS over all four antennas, but it still selects PUSCH from the first two.

Reference

3GPP TS 36.212 v19.3.0 - clause 5.3.3.2 CRC attachment, Table 5.3.3.2-1 UE transmit antenna selection mask
3GPP TS 36.213 v19.4.0 - clause 8.2 SRS antenna index, clause 8.7 UE transmit antenna selection
3GPP TS 36.331 v19.3.0 - PhyLayerParameters, AntennaInfoDedicated, txAntennaSwitchUL-r13, ue-TxAntennaSelection-SRS-1T4R-Config; v8.10.0 for the Release 8 fields