A brief on Transmit diversity- OLTD,STTD,TSTD and CLTD and its uses.

Broadly speaking there are two types of transmit diversity

Open loop transmit diversity(OLTD) and Closed loop transmit diversity(CLTD)
and we have defined it under FEATURE_TRANSMIT_DIVERSITY_SUPPORT.

1.Open loop transmit diversity

a.Space time block coding based transmit antenna diversity(STTD)
STTD encoding is used for DPCH,F-DPCH,AICH,E-RGCH,E-HICH,PCCPCH,SCCPCH (Refer table 10 in 25.211 for full list)
The STTD encoding is optional in UTRAN but STTD support is mandatory at the UE.
Alamouti coding is used for STTD.The presence or absence of STTD encoding in PCCPCH
is determined by higher layer message or by demodulating the SCH channel (value of variable "a" in SCH)
or by a combination of the above two schemes. If P-CCPCH is STTD encoded, then a = +1 in SCH.
If P-CCPCH is not STTD encoded,then a = -1 in SCH.

In case the diversity antenna is present in UTRAN and the P-CCPCH is to be transmitted using OLTD,
the data bits of the P-CCPCH are STTD encoded. In the 10ms frame,15 slot sequence,the last two
data bits in even numbered slots are STTD encoded together with the first two data bits in the following slot,
except for slot #14 where the two last data bits are not STTD encoded and instead transmitted
with equal power from both the antennas


b.Time switched transmit diversity for SCH(TSTD).
TSTD encoding is applicable only for SCH.TSTD for the SCH is optional in UTRAN, while TSTD support is mandatory in the UE.
In the 10ms frame,15 slot sequence,for TSTD,on antenna 1,both Primary Synchronisation Code(PSC)
and Secondary Synchronisation Code(SSC) are transmitted in even numbered slots, and on antenna 2,
both PSC and SSC are transmitted in odd numbered slots.


2.Closed loop transmit diversity
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 S sub-field or D sub-field of the
Feed Back Information(FBI) bits of uplink DPCCH. Atmost two bits are available for FBI signalling.
The S sub-field is used for SSDT signalling and D sub-field is used for closed loop mode transmit diversity.

For the calculation of phase and power settings and signalling these settings to the UTRAN we use CLTD.
The UE uses the CPICH to separately estimate the channels seen from each antenna. Once every slot,
the UE computes the phase adjustment,φ in mode 1, and for mode 2 the amplitude 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 in mode 1 and for mode 2 the amplitude adjustment
to maximise the total UE received power from the cells in the active set.


a.Closed loop mode 1 transmit diversity
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.This information
is signalled via FBI bits of uplink DPCCH. We use Antenna Verification algorithm to achieve CLTD
and it is applied on A-DPCH and HS-DSCH channels(for R99 it is DPCH).In closed loop mode 1,
if channel estimates are taken from the Primary CPICH, the performance will also suffer if the UE
cannot detect errors since the channel estimates will be taken for the incorrect phase settings.
To mitigate this problem, antenna verification can be done, which can make use of antenna specific
pilot patterns of the dedicated physical channel.


b.Closed loop mode 2 transmit diversity
In this mode both phase and amplitude adjustments are done
and signalled to UTRAN access point through FBI bits of uplink DPCCH.
CLTD-mode 2 is removed from the 3GPP 25.214 after release 5.11.0.
We do not support CLTD mode 2 and there is no CLTD on pilot.
CPICH only supports STTD or no-diversity.