Hello everyone!
Today, I want to share information about an uplink reference signal and a downlink reference signal.
Uplink reference signal
The uplink RS is used for synchronization and uplink channel estimation between the E-UTRAN and the UE.
There are two types of uplink RS:
DMRS: accompanying pilot signal during PUSCH and PUCCH transmission.
SRS: pilot signal when the PUSCH and PUCCH are not transmitted.
Features of uplink reference signals
DMRSs are transmitted only in the corresponding PUSCH and PUCCH allocated bandwidths.
The position of the DMRS in the timeslot varies according to the formats of the PUSCH and PUCCH.
The SRS bandwidth is greater than the bandwidth allocated to a single UE. The purpose is to provide a reference for the eNodeB to estimate the full-bandwidth uplink channel.
SRSs are transmitted in the last symbol of each subframe. The period and bandwidth are configurable.
A resource for transmitting an SRS is code division multiplexing. Multiple users can use different sounding sequences and use the same resources at the same time.
Figure 1 DMRS location map
Downlink reference signal
Functions like CDMA/UMTS pilot signals. It is used to demodulate downlink physical channels and measure channel quality.
There are three types of downlink RS:
Cell-specific reference signal: CQI measurement is based on CRSs（Mandatory parameters）
MBSFN Specific RS（Optional parameters）
UE-specific RS（Optional parameters）
Features of downlink reference signals
The RS is essentially a pseudo-random sequence known to the terminal.
For each antenna port, the frequency domain spacing of the RS is six subcarriers.
REs occupied by reference signals must be left blank on the same REs of other antenna ports.
When the number of antenna ports increases, the total pilot overhead of the system also increases, and the available data RE decreases.
LTE reference signals are discretely distributed, while CDMA/UMTS pilot signals are continuous.
The denser the RS distribution, the more accurate the channel estimation, but the higher the overhead, which affects the system capacity.
Figure 2 Cell-specific reference signals
UE-specific reference signal
UE-specific RS is used for downlink beamforming. Different beams carry UE-specific reference signals. The UE-specific reference signal is sent on antenna port 5.
Figure 3 UE-specific reference signal
RS and physical channels are closely related and can be learned together.
You're welcome to read and comment, and I'll share Cell Selection in my next post.
