Five types of downlink reference signals are defined:
- Cell-specific reference signals (CRS)
- MBSFN reference signals
- UE-specific reference signals (DM-RS)
- Positioning reference signals (PRS)
- CSI reference signals (CSI-RS)
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 PDSCH transmission. Cell-specific reference signals are transmitted on one or several of antenna ports 0 to 3. Cell-specific reference signals are defined for f 15 kHz only.
The number of Resource Elements (REs) within each Resource Element Group (REG) and the number of REGs within an OFDM symbol is affected by the number of cell-specific reference signals present on all antenna ports. The number and location of cell specific reference signals are dependent on the number of antenna ports and the type of cyclic prefix used. Each antenna port has a unique cell specific reference signal associated with it. As the REG arrangement is affected by cell specific reference signals, the REG arrangement for a one or two antenna port configuration or four antenna port configuration is different. The REG arrangement for each resource block within a subframe and for every antenna port is identical.
To facilitate the estimation of the channel characteristics LTE uses cell specific reference signals (pilot symbols) inserted in both time and frequency. These pilot symbols provide an estimate of the channel at given locations within a subframe. Through interpolation it is possible to estimate the channel across an arbitrary number of subframes.
Cell-specific RS is transmitted in each physical antenna port. It is used for both demodulation and measurement purpose. Its pattern design ensures channel estimation accuracy.
Cell-specific reference signals are used for…
– cell search and initial acquisition,
– downlink channel estimation for coherent demodulation/detection at the UE,
– downlink channel quality measurements.
UE-specific Reference Signals
UE-specific reference signals are supported for transmission of PDSCH and are transmitted on antenna port(s) p 5 , p 7 , p 8 or p 7,8,..., 6 , where is the number of layers used for transmission of the PDSCH. 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 according to Section 7.1 of . UE-specific reference signals are transmitted only on the resource blocks upon which the corresponding PDSCH is mapped.
UE-specific reference signal are transmitted using resource elements with the same index pair k, l regardless of their antenna port p . UE-specific reference signals are denser in frequency but only transmitted when data is transmitted on the corresponding layer.
CSI Reference Signals
CSI reference signals are transmitted on one, two, four or eight antenna ports using p 15 , p 15,16 , p 15,...,18 and p 15,...,22 , respectively. CSI reference signals are defined for f 15 kHz only.
Feedback of channel-state information (CSI) is based on a separate set of reference signals – CSI reference signals. CSI reference signals are relatively sparse in frequency but regularly transmitted from all antennas at the base station.
The CSI reference signal is transmitted in each physical antenna port or virtualized antenna port and is used for measurement purposes only
A cell can be configured with one, two, four or eight CSI-RS. The exact CSI-RS structure, including the exact set of resource elements used for CSI-RS in a rosource block, depends on the number of CSI-RS configured within the cell and may also be different for different cells. More speifically, within a resource-block pair there are 40 possible positions for the reference symbols of CSI-RS and, in a given cell, a subset of corresponding resource elements is used for CSI-RS transmission.
MBSFN reference signals MBSFN reference signals shall be transmitted in the MBSFN region of MBSFN subframes only when the PMCH is transmitted. MBSFN reference signals are transmitted on antenna port 4. MBSFN reference signals are defined for extended cyclic prefix only.
There are 504 unique physical-layer cell identities. The physical-layer cell identities are grouped into 168 unique physical-layer cell-identity groups, each group containing three unique identities. The grouping is such that each physical-layer cell identity is part of one and only one physical-layer cell-identity group. A physical-layer cell identity (2) ID (1) ID cell ID N 3N N is thus uniquely defined by a number (1) ID N in the range of 0 to 167, representing the physical-layer cell-identity group, and a number (2) ID N in the range of 0 to 2, representing the physical-layer identity within the physical-layer cell-identity group.
Primary synchronization signal and Secondary synchronization signal
Both primary and secondary synchronization signals are designed to detect all type of UEs. The synchronization signals always occupy the 62 sub-carrier of the channel, which make the cell search procedure same regardless of channel bandwidth. Although 72 subcarriers (6 RB) are available, only 62 sub-carriers are used so that the UE can perform the cell search procedure. The primary synchronization signal subcarriers are modulated using a frequency domain Zadoff-Chu Sequence. Each subcarrier has the same power level with its phase determined by the root index number in sequence generator as defined in 36.211
The secondary signal is used to identify cell-identity groups. The number and position of subcarrier are same as for the primary synchronization signal: that is the central 62 sub carriers. The sequence generation function utilizes an interleaved concatenation of two length 31 binary sequences as defined in 36.211. The secondary synchronization signal gives a cell-identity group number from 168 possible cell identities N (1, ID).