In some wireless communication systems, multiple mobile transceivers, also known as ‘user equipment’ (UE), e.g., cellular telephones, communicate simultaneously with stationary transceivers, e.g., base stations. In such communications systems, reference signals are necessary for coherent demodulation of RF signals and detection of data at the receiver. The reference signals are also used to estimate a quality of uplink channels from the mobile transmitters to the receiver for channel-dependent frequency-scheduling and/or time-domain scheduling.
Allocating more resources, such as power, time and frequency for the reference signals improves channel state information (CSI) estimation, and thus, leads to better demodulation of the data with a reduced bit error rate (BER). However, increasing the resources for the reference signals reduces the spectral efficiency of the system.
In such systems, a sub-frame is a basic unit of transmission. The sub-frame includes short blocks and long blocks. The short blocks can include the reference signals. The long blocks do not include reference signals. In 3GPP, the sub-frame corresponds to a transmission time interval (TTI). That is, the entire sub-frame is transmitted during one continuous, uninterrupted time interval. In the art and herein, the terms ‘TTI’ and ‘sub-frame’ are used interchangeably.
In the prior art, each TTI includes an equal number of reference signals so that the receiver can estimate the CSI before the data in the TTI are demodulated and detected.
In 3GPP LTE, two reference signal patterns are currently being considered. With frequency division multiplexing (FDM), each UE sends its uplink reference signal across a distinct set of sub-carriers, R1-060246, “EUTRA SC-FDMA Uplink Resource Block, Resource Allocation and Pilot/Reference Signal Design & TP,” Motorola; incorporated herein by reference. The reference signals of any two UEs are orthogonal to each other and do not overlap in the frequency-domain.
With code division multiplexing (CDM), each UE sends its uplink reference signal across a common set of sub-carriers, R1-060784, “Orthogonal Pilot Channel Structure for E-UTRA Uplink,” NTT DoCoMo, Fujitsu, Mitsubishi Electric, NEC, Panasonic, Sharp; incorporated herein by reference. Orthogonality between the reference signals of two UEs is achieved in the ‘code-domain’ by ensuring that the UEs transmit sequences across time and frequency that are orthogonal to each other. For example, the reference signals are generated by taking cyclic shifts of a single constant amplitude zero auto-correlation (CAZAC) sequence. Examples of CAZAC sequences are given in B. M. Popovic, “Generalized Chirp-like Polyphase Sequences with Optimal Correlation Properties,” IEEE Trans. Info. Theory, vol. 38, pp. 1406-1409, July 1992; and D. C. Chu, “Polyphase codes with good periodic correlation properties,” IEEE Trans. Inform. Theory, vol. IT18, pp. 531-532, 1972; both incorporated herein by reference.
In general, the overhead associated with the uplink reference signals is considerable as the reference signals are required to enable the receiver to estimate the channel quality of all the UEs that intend to communicate with it, and to aid the receiver station to coherently demodulate the data from the signals received from the respective transmitting UEs. Generating the reference signals consumes power and takes time. Transmitting the reference signals consumes power. Estimating the CSI from the received reference signals consumes power and takes time. The reference signals consume precious bandwidth and valuable space in the sub-frames that could otherwise be used by data and control signals.
Therefore, it is desired to reduce the overhead associated with the reference signals used for coherent signal demodulation and data detection on uplinks from transmitters to a receiver so that the number of UEs that transmit signals simultaneously can be increased.