Downlink Reference Signals
LTE provides for reference signals to
support Uplink and Downlink transmission. Reference signals occupy pre-defined
Resource Elements on the OFDMA Resource Grid. These do not carry any user data
and only facilitate transmission of user data.
The transmission on the reference signal is
used for channel estimation. Since the sequence that is transmitted on the
downlink reference signal is known by the UE, it can compute the Channel
Response for that Resource Element. The Channel Response for the other Resource
Elements can then be approximated.
The hopping of the reference signals on
Resouce Elements based on a known Pseudo Random Sequence is being considered. This
will better approximate the ever changing radio conditions.
UE also utilises the reference signals for CQI
measurement and reporting.
Three types of downlink reference signals
are defined:
- Cell-specific
reference signals, associated with non-MBSFN transmission
- MBSFN reference
signals, associated with MBSFN transmission
- UE-specific
reference signals
There is one reference signal transmitted
per downlink antenna port.
Cell-specific
reference signals
- Cell-specific reference signals shall be transmitted in all downlink subframes in a cell supporting non-MBSFN transmission. In case the subframe is used for transmission with MBSFN, only the first two OFDM symbols in a subframe can be used for transmission of cell-specific reference symbols.
- Cell-specific reference signals are transmitted on one or several of antenna ports 0 to 3.
- Cell-specific reference signals are defined for sub-carrier separation of 15 KHz only.
- The resource elements which are used for the downlink reference signal on one ports are left unused for transmission on other antenna ports. This reduces the interference and helps the UE to decode the reference signals better.
Mapping of
downlink reference signals (normal cyclic prefix).
MBSFN
reference signals
- MBSFN reference signals shall only be transmitted in subframes allocated for MBSFN transmissions. MBSFN reference signals are transmitted on antenna port 4.
- MBSFN reference signals are defined for extended cyclic prefix only.
Mapping of MBSFN reference signals (extended cyclic
prefix, carrier spacing 15 KHz)
Mapping of MBSFN reference signals (extended
cyclic prefix, carrier spacing 7.5 KHz)
UE-specific
reference signals
- UE-specific reference signals are supported for single-antenna-port transmission of PDSCH and are transmitted on antenna port 5.
- UE specific reference signals are present and are a valid reference for PDSCH demodulation only if the PDSCH transmission is associated with the corresponding antenna port.
- UE-specific reference signals are transmitted only on the resource blocks upon which the corresponding PDSCH is mapped.
Mapping of UE-specific reference signals (normal
cyclic prefix)