A novel adaptive cyclic prefix (CP) transmission scheme is proposed for the uplink of orthogonal frequency division multiple access (OFDMA) systems to reduce the power consumption of mobile stations (MSs). In the proposed scheme, an MS adaptively changes its CP length in each frame, while the guard interval is maintained at a fixed duration to avoid frame synchronization problem and the interference problem with frames of other users. Using the proposed scheme, MSs can save power by not transmitting signal during the time difference between the guard interval and the duration of the adaptive CP. We numerically analyze the performance of the proposed scheme in terms of achievable capacity, the amount of power saving, and the feedback overhead of CP values. The result shows that the proposed scheme can reduce MS power consumption by about 20% with a small amount of additional feedback overhead.

This paper proposes a novel system called the cyclic prefix reconstructable time domain synchronous orthogonal frequency division multiplexing ( CPR-TDS-OFDM ) system, which uses a new frame structure and restores the cyclicity of the received OFDM block with low complexity. Simulation results show that the CPR-TDS-OFDM system outperforms the conventional TDS-OFDM system in high-speed fading channels.

In orthogonal frequency division multiplexing (OFDM) system, a guard interval (GI) is used to remove the inter-symbol interference (ISI) due to a multipath channel. It is difficult to set an optimal GI length in the environment whose multipath varies. In this paper, we propose a variable guard interval based on the estimated maximum delay of a multipath channel. The maximum delay is estimated from a channel impulse response (CIR), which is estimated by a preamble symbol. However, since the estimated CIR includes the noise, it is difficult to decide the optimal GI. In order to solve the problem, we introduce the method which selects the path whose signal to noise ratio is high. Additionally, the information of the optimal GI length is required to be transmitted from a receiver to a transmitter. In this paper, we use an acknowledgment (ACK) frame for the feedback of the GI information.

A novel timing synchronizing scheme is proposed for use in inter-vehicle communication (IVC) with an autonomous distributed intelligent transport system (ITS). The scheme determines the timing of packet signal transmission in the IVC network and employs the guard interval (GI) timing in the orthogonal frequency divisional multiplexing (OFDM) signal currently used for terrestrial broadcasts in the Japanese digital television system (ISDB-T). This signal is used because it is expected that the automotive market will demand the capability for cars to receive terrestrial digital TV broadcasts in the near future. The use of broadcasts by automobiles presupposes that the on-board receivers are capable of accurately detecting the GI timing data in an extremely low carrier-to-noise ratio (CNR) condition regardless of a severe multipath environment which will introduce broad scatter in signal arrival times. Therefore, we analyzed actual broadcast signals received in a moving vehicle in a field experiment and showed that the GI timing signal is detected with the desired accuracy even in the case of extremely low-CNR environments. Some considerations were also given about how to use these findings.

In order to evaluate the effect of Nakagami-Rice fading on Orthogonal Frequency Division Multiplex (OFDM) signal transmission when the delay profile exceeds the guard interval, a simple prediction model is developed by extending the Equivalent Transmission-Path (ETP) model for Rayleigh fading. The validity of the model is demonstrated by comparing the calculated values of BER to those obtained by computer simulation. Using the newly developed ETP-OFDM model, digital transmission characteristics of the OFDM signal in a multipath environment when the delay profile exceeds the guard interval are shown as a function of K factor, delay spread, guard interval and OFDM symbol period.

In this letter, we focus on non-pilot-symbol assisted integer frequency offset estimation for multicarrier orthogonal frequency division multiplexing (OFDM) systems. We introduce a frequency offset estimator that is based on the guard interval (GI) present in OFDM signals. We show by simulation that the frequency offset estimator can accurately estimate the frequency misalignment at the sacrifice of limited estimation range.

In order to assess the effect of multipath fading on Orthogonal Frequency Division Multiplex (OFDM) signal transmission when the delay profile exceeds the guard interval, a simple prediction model is developed based on the Equivalent Transmission-Path (ETP) model. This model, which is described in this paper, is referred to as the ETP-OFDM-statistical model. The validity of the model is demonstrated by comparing the calculated digital transmission characteristics to results obtained by computer simulation. Using the newly developed ETP-OFDM-statistical model, digital transmission characteristics of the OFDM signal in a multipath environment when the delay profile exceeds the guard interval are shown as a function of delay spread, guard interval and OFDM symbol period.

When the multipath delay difference exceeds the guard interval (GI), the performance of MIMO-OFDM transmission suffers severely from both the inter-symbol interference (ISI) from the adjacent OFDM symbols and the inter-carrier interference (ICI) within the same symbol. This paper therefore proposes a MIMO-OFDM receiver employing the low-complexity turbo equalization. The proposed receiver initially separates the data streams and suppresses ICI by linear processing. In the iterative processing, it cancels the other data streams as well as ISI and ICI. The MIMO-OFDM turbo equalizer consists of an ISI canceller, an ICI canceller, an optimal detection filter, and a MAP detector. The proposed receiver can improve the transmission performance by exploiting the log-likelihood ratio that the decoding process produces for canceling both ISI and ICI and separating of the spatially multiplexed streams. Computer simulations, which apply the wireless LAN to MIMO, demonstrate that the proposed receiver can provide excellent performance in the severe multipath channels where the delay difference is greater than GI.

This paper proposes an inter-symbol interference (ISI) suppression scheme using only the even-numbered sub-carriers for the fixed-rate OFDM systems with the 2-dimensional modulation. The proposed scheme is based on the principle that the first half of the waveform in the time domain is the same as the second half when an OFDM symbol is composed of only the even-numbered sub-carriers. The feature of the proposed scheme is that, in the case of the maximum multipath delay beyond the duration of the guard interval, the OFDM symbol with only the even-numbered sub-carriers is transmitted in order to generate the extended virtual guard interval and that the high-level modulation with the sub-carrier power enhancement is applied to achieve the constant data rate. In addition, at the receiver, only the second half of the OFDM symbol is used for the FFT processing to avoid the ISI. Moreover, the condition of the maximum multipath delay is notified to the transmitter by using the feedback channel. Numerical results given by computer simulation showed that the proposed scheme provides far better bit error rate (BER) performance than the traditional OFDM transmission using all sub-carriers under the multipath delay beyond the duration of the guard interval.

In an orthogonal frequency division multiplexing (OFDM) system, the bit error performance is degraded in the presence of multiple propagation paths whose excess delays are longer than the Guard Interval (GI), because the orthogonality between subcarriers cannot be maintained. In this paper, we propose a new OFDM demodulation method with a variable-length effective symbol and a multi-stage inter-carrier interference (ICI) canceller, in order to improve the bit error performance in the presence of multipaths whose excess delays are longer than the GI. The influence of the inter-symbol interference (ISI) is eliminated by the variable-length effective symbol, and then the ICI component is reduced by the multi-stage ICI canceller. The principle of the proposed method is explained, and the performance of the proposed method is then evaluated by computer simulation. The results show that the proposed method improves the system availability under more various multipath fading environments without changing the system parameters.

The VLSI implication of the guard interval in orthogonal frequency-division multiplexing (OFDM) systems is described. A new OFDM transmission scheme using cyclic suffix is proposed to reduce the hardware complexity required for implementing the guard interval in the transmitter, and is shown to have the same performance as the conventional approach using cyclic prefix, even with a significantly lower hardware complexity (smaller buffer size and no processing delay).