1. Field of the Invention
The present invention relates to a mobile telecommunications technology, and more particularly, to a technique for receiving and transmitting downlink reference signals. Herein, downlink reference signals for data demodulation are efficiently received and transmitted in a single user mode or a multi-user mode.
2. Discussion of the Related Art
In a mobile telecommunications system, a user equipment (UE) may receive information from a base station through a downlink, and the UE may also transmit information through an uplink. Information transmitted or received by the UE may include data and diverse control information. And, depending upon the type and usage of the information transmitted or received by the UE, a variety of physical channels may exist.
FIG. 1 illustrates a general view showing the physical channels used in a mobile telecommunications system, such as a 3rd generation partnership project (3GPP) long term evolution (LTE) system and a general method for transmitting signals. When power of a UE is turned off and then turned back on, or when using a UE newly introduced to a cell, in step 101, the UE performs an initial cell search process in order to be in synchronization with the base station. In order to do so, the UE receives a primary synchronization channel (P-SCH) and a secondary synchronization channel (S-SCH) from the base station, so as to be in synchronization with the base station, thereby being able to acquire information such as cell ID. Thereafter, the UE receives a physical broadcast channel from the base station, thereby being capable of acquiring broadcast information within the cell. Meanwhile, during an initial cell searching step, the UE receives a downlink reference signal (DL RS), thereby being able to verify the downlink (DL) channel status. After completing the initial cell search, in step 102, the UE may receive a physical downlink control channel (PDCCH) and a physical downlink shared channel (PDSCH) based upon the physical downlink control channel information, so as to acquire more detailed system information.
Meanwhile, in case the UE has not completed its access to the base station, the UE may perform a random access procedure, as shown in step 103 to step 106, in a later process in order to complete its access to the base station. For this, the UE transmits a characteristic sequence as a preamble through a physical random access channel (PRACH) (S103). Then, the UE may receive a response message respective to the random access through the physical downlink control channel (PDCCH) and its corresponding physical downlink shared channel (PDSCH) (S104). Subsequently, with the exception of a handover, in case of a contention-based random access, the UE may perform a contention resolution procedure, such as transmitting additional physical random access channels (PRACHs) (S105) and receiving the respective physical downlink shared channels (PDSCHs) (S106). After performing the above-described procedure, the UE may receive physical downlink control channel (PDCCH)/physical downlink shared channel (PDSCH) (S107) and may transmit physical uplink shared channel (PUSCH)/physical uplink control channel (PUCCH) (S108), as a general uplink/downlink (UL/DL) signal transmission procedure.
FIG. 2 illustrates a block view showing a signal processing procedure for transmitting a downlink signal from a base station. In the 3GPP LTE system, the base station may transmit at least one or more code words via downlink. Each of the at least one or more code words may be processed through a scrambling module 301 and a modulation mapper 302 as a complex symbol. Thereafter, the complex symbol is mapped to multiple layers by a layer mapper 303. Herein, a precoding module 304 multiplies each layer by a selected precoding matrix depending upon the channel status, thereby allocating (or assigning) the processed layers to each transmission antenna. Each transmission signal processed as described above for the respective antenna is mapped to a time-frequency resource element, which is to be used by a resource element mapper 305 for transmission. Subsequently, each of the transmission signals passes through an OFDM signal generator 306 so as to be transmitted through the respective antenna.
Hereinafter, a downlink reference signal that is used in the 3GPP LTE system will be described in detail. The 3GPP LTE system uses antenna number 0 to antenna number 5 as its logical antenna ports. Herein, each antenna port is not divided (or classified) by a physical division (or classification). Therefore, the question of mapping each logical antenna index to which actual physical antenna index would relate to the implementation by each manufacturer.
In the 3GPP LTE system, three different types of reference signals are used as downlink reference signals. The three types include cell-specific reference signals (non-associated with MBSFN transmission), MBSFN reference signals associated with MBSFN transmission, and UE-specific reference signals. A cell-specific reference signal corresponds to a reference signal generated by using a cell ID for each cell as an initial value. Herein, antenna port 0 to antenna port 3 may be used for transmitting the cell-specific reference signals. An MBSFN reference signal is used for acquiring downlink channel information respective to the MBSFN transmission. Herein, antenna port 4 may be used for transmitting the MBSFN reference signal. Meanwhile, in the 3GPP LTE system, a UE-specific reference signal is supported for a single antenna port transmission of the PDSCH. Herein, antenna port 5 may be used for transmitting the UE-specific reference signal. The UE may receive from an upper layer (or higher layer) (e.g., a MAC layer or higher) information on whether such user-specific reference signals exist so as to be used for PDSCH demodulation.
FIG. 3 illustrates an example of a specific reference signal being mapped in a time-frequency resource region and transmitted, when the 3GPP LTE system uses a general cyclic prefix. Referring to FIG. 3, the horizontal axis represents a time region, and the vertical axis represents a frequency region. In the time-frequency region shown in FIG. 3, the smallest squared region corresponds to 1 OFDM symbol in the time region and to 1 subcarrier in the frequency region. In the 3GPP LTE system, when a normal cyclic prefix (CP) is used, one slot includes 7 OFDM symbols, and one sub-frame includes 2 slots. FIG. 3 illustrates a pattern where a UE-specific reference signal being transmitted through antenna port 5 is mapped to the time-frequency region throughout even-numbered slots and odd-numbered slots, thereby being transmitted.