3GPP LTE (3rd Generation Partnership Project Long-term Evolution, hereinafter referred to as “LTE”) uplink uses cyclic shift sequences, which are orthogonal sequences, as pilot signals to reduce interference among sequences. A cyclic shift sequence can be generated by cyclically shifting a pilot sequence by a cyclic shift amount on the time axis. For example, FIG. 1 shows a cyclic shift sequence (m=0) and a cyclic shift sequence (m=1) with pilot sequence length N=12 and cyclic shift amount A=6.
In FIG. 1, while the cyclic shift sequence (m=0) is configured in order of a(0) to a(11), the cyclic shift sequence (m=1) is configured, by cyclically shifting the cyclic shift sequence (m=0) by Δ(=6) samples, in order of a(6) to a(11), a(0) to a(5).
The cyclic shift amount is determined by a base station apparatus (hereinafter abbreviated to “base station”) and reported from the base station to a terminal station apparatus (hereinafter abbreviated to “terminal”) per scheduling (per subframe). Eight types “0, 2, 3, 4, 6, 8, 9, 10” (3 bits) are defined for reporting the cyclic shift amount. These correspond to a cyclic shift amount of “0, 2, 3, 4, 6, 8, 9, 10”×symbol length/12 (ms).
Since sequences can be separated with low inter-sequence interference by assigning cyclic shift sequences of different cyclic shift amounts to different terminals, cyclic shift sequences are used for pilot signal transmission in MU-MIMO (Multiple User-Multiple Input Multiple Output). In MU-MIMO, a plurality of terminals transmit data signals at the same time and the same frequency, spatially multiplex the data signals and thereby improve system throughput. At this time, it is also preferable that a plurality of terminals transmit pilot signals at the same time and the same frequency from the standpoint of frequency utilization efficiency. Therefore, cyclic shift sequences, which are orthogonal sequences, for pilot signals and the cyclic shift sequences are transmitted at the same time and the same frequency. The reception side can separate pilot signals using the nature of orthogonal sequences, and can thereby accurately estimate a channel state of each terminal.
On the other hand, in LTE-Advanced (hereinafter referred to as “LTE-A”) uplink, studies are being carried out on supporting SU-MIMO (Single User-Multiple Input Multiple Output) to improve throughput, whereby one terminal transmits data signals from a plurality of antenna ports at the same time and the same frequency and spatially multiplexes the data signals using virtual communication channels (hereinafter referred to as “streams”) in the space.
Here, the “antenna port” refers to a logical antenna (antenna group) made up of one or a plurality of physical antennas. That is, the antenna port does not always refer to one physical antenna, but may also refer to an array antenna made up of a plurality of antennas. For example, the antenna port may be made up of a plurality of physical antennas and defined as a minimum unit whereby a base station or terminal can transmit different pilot signals. Furthermore, the antenna port may also be defined as a minimum unit for multiplying a weight of a precoding vector. Hereinafter, a case will be described as an example where an “antenna port” and a physical antenna have a one-to-one correspondence for simplicity of explanation.
SU-MIMO requires pilot signals for each stream and studies are being carried out on code-multiplexing pilot signals of each stream using a cyclic shift sequence, which is an orthogonal sequence, for the purpose of reducing inter-sequence interference.
Here, in an ideal environment in which there is no channel variation, a cyclic shift sequence is an orthogonal sequence and no inter-sequence interference occurs. On the other hand, in a real environment with a channel variation, complete orthogonality is not established and a certain degree of inter-sequence interference occurs. Especially when the number of streams increases and the cyclic shift sequence multiplexing number increases, inter-sequence interference also increases. Therefore, in LTE-A, studies are being carried out on reducing inter-sequence interference using a Walsh sequence as well as cyclic shift sequences adopted in LTE.
In multiplexing using Walsh sequences, pilot signals of a first slot (slot #1) and a second slot (slot #2) making up a subframe are multiplied by Walsh sequence w1=[1 1] or Walsh sequence w2=[1 −1] (see FIG. 2). That is, Walsh sequence w1 uses pilot signals similar to those conventional ones in first and second slots and Walsh sequence w2 uses pilot signals similar to those conventional ones in the first slot and uses pilot signals with an inverted phase (180 degree rotation) in the second slot.
As a method of reporting a cyclic shift amount, in LTE, the base station reports in three bits using a control information channel (Physical Downlink Control Channel, PDCCH) to be reported to each terminal per scheduling. Furthermore, in LTE-A, studies are being carried out on adding one bit indicating whether a Walsh sequence of each terminal is w1 or w2 using a control information channel (PDCCH), the base station reporting the Walsh sequence to each terminal and each terminal switching between the Walsh sequences.
Furthermore, in order to reduce inter-sequence interference of cyclic shift sequences between streams in SU-MIMO, Walsh sequence w1 is used for pilot signals of odd-numbered streams and Walsh sequence w2 is used for pilot signals of even-numbered streams (see FIG. 3).
Here, the “stream number” is a number indicating order in which data is assigned. For example, when data is transmitted with only one stream, suppose a stream transmitted from one antenna port is stream #0 and when data is transmitted with two streams, the stream transmitted from an antenna port different from the above-described port is stream #1. By setting different Walsh sequences depending on whether a stream number is an odd number or even number, it is possible to reduce inter-sequence interference between pilot signals of neighboring streams (see Non-Patent Literature 1). Furthermore, since there is no need for reporting a bit indicating a Walsh sequence, which will be used in the second (stream #1) and subsequent streams, the amount of reporting the cyclic shift amount can be reduced.