Conventional base stations and terminals that perform wireless communication periodically transmit and receive control signals. The base station and the terminal convert information on reception conditions such as received signal field strength by using the control signals to determine the mutual channel state. The base station that has determined the channel state of the terminal specifies, for the terminal, an operation in a multi-antenna transmission and reception scheme, such as a modulation and coding scheme (MCS) or spatially multiplexed signal separation, and the radio resources to be allocated to the terminal, so as to implement adaptive radio transmission in accordance with the state of the channel between the base station and the terminal. When establishing adaptive radio transmission, the base station and the terminal need to receive control signals accurately. Control signal transmission requires high reception performance in order to resist noise interference and radio propagation path fluctuations due to fading.
To obtain high reception performance when transmitting control signals, a transmitting-side apparatus typically performs processing to increase the redundancy on the control signals by applying a low code rate in an error-correction code or by applying repetition processing or spread processing using a spreading sequence or the like. Further, the transmitting-side apparatus typically uses a modulation scheme having noise interference resistance in which the distance between signal points is large, with a modulation level such as binary phase-shift keying (BPSK) or quadrature phase-shift keying (QPSK). Hereinafter, transmission of control signals is referred to as control signal transmission, and radio transmission of data performed after exchange of control signals is referred to as data transmission.
A terminal moving at high speed tends to be degraded in the performance of detecting signals to be demodulated because deviation occurs between the channel value estimated using pilot signals that have a known pattern between the terminal and a base station and the channel value of the signals to be demodulated. Patent Literature 1 described below discloses a technique that is in response to this problem and that involves assigning control signals to symbol positions adjacent to pilot signals during control signal transmission in a Long Term Evolution (LTE) system standardized by 3GPP. By arranging control signals and pilot signals adjacently, an error in the estimation of a control signal channel is reduced, so that degradation in detection performance experienced by control signal transmission due to channel fluctuations can be reduced.
In the uplink in an LTE system, single-carrier block transmission (hereinafter, referred to as SCBT) is used, which has lower peak performance and places a smaller load on power amplifiers than the orthogonal frequency-division multiplex (OFDM) used in the downlink. It is called multi-access scheme single-carrier frequency-division multiple access (SC-FDMA) (Non Patent Literature 1 described below). In the LTE uplink, the physical uplink shared channel (PUSCH) is sometimes used to transmit control signals. When the PUSCH is used, a transmitting-side apparatus separately encodes data signals and control signals, then multiplexes them, and applies interleaving to a multiplexed sequence to generate an SC-FDMA symbol. A pilot signal is assigned to the third and thirteenth subframes, and an SC-FDMA symbol containing control signals is assigned the positions near the pilot signals to prevent degradation in the performance of receiving the control signals.
When SCBT is adapted for use in a high-speed mobile environment, a transmitting-side apparatus disposes an SC-FDMA control symbol, which is an SC-FDMA symbol containing control signals, near an SC-FDMA pilot symbol, which is an SC-FDMA symbol made up of pilot signals. However, even when a receiving-side apparatus can accurately detect the SC-FDMA control signal symbol, its reception performance for an SC-FDMA data symbol, which is an SC-FDMA symbol made up of data signals and is away from the SC-FDMA pilot symbol, may be degraded under the influence of a channel estimation error due to channel fluctuations. In particular, when a high modulation level is used in SC-FDMA data symbols, degradation in reception performance becomes conspicuous. By increasing the rate of insertion of SC-FDMA pilot symbols and disposing SC-FDMA data symbols near SC-FDMA pilot symbols, degradation in the performance of receiving the SC-FDMA data symbols due to channel fluctuations can be avoided. On the other hand, the increased rate of insertion of SC-FDMA pilot symbols leads to a reduction in transmission efficiency.
Patent Literature 2 described below discloses a technique that is in response to this problem and that involves achieving efficient insertion of pilot signals while maintaining low peak performance in SCBT. The dispersed arrangement of pilot signals implemented in OFDM can be implemented also in SCBT.
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