Conjugate Cancellation Scheme Research Articles

Design Adaptive MIMO Space-Time-Frequency Conjugate Two-Path OFDM Systems

Multiple input multiple output (MIMO) systems can be designed for two operational modes: to improve bit error rate (BER) or to increase capacity without consuming additional power or bandwidth. In this paper, MIMO space-time-frequency (STF) conjugate two-path transmission orthogonal frequency division multiplexing (OFDM) systems with precoders are presented to combat inter-carrier interference and to adaptively switch between these two modes in mobile fading channels. First, the STF architecture is developed to form a 2 × 1 STF-OFDM system. Second, a conjugate cancellation (CC) two-path transmission scheme is integrated with the STF-OFDM system to form a new 2 × 1 STFCC-OFDM system. This system improves BER without expanding power or bandwidth in comparing with the CC scheme. Third, Walsh–Hadamard (WH) transform is employed as the precoder to form the 2 × 1 WHSTFCC-OFDM system. Finally, a 2 × 2 WHSTFCC-OFDM architecture is designed. It can be used to improve BER or switch to two 2 × 1 WHSTFCC-OFDM systems to increase capacity as needed. Simulations indicate that the 2 × 1 WHSTFCC-OFDM system has the best BER, the STFCC-OFDM system, and the STF-OFDM system follow in order. Similarly, the BER of the 2 × 2 WHSTFCC-OFDM system outperforms the 2 × 1 WHSTFCC-OFDM, and the 2 × 2 STFCC-OFDM system follow in order. All these STFCC- and WHSTFCC-OFDM schemes are simple and backward compatible with the existing OFDM system. They may serve as baseband building blocks in OFDM transceivers and can be adaptively switched between 2 × 1 and 2 × 2 architectures as required for the fourth generation and the future planned fifth generation MIMO systems. This adaptive MIMO system can also be applied to the regular or bad weather environment in the disaster conditions for increasing capacity or enhancing the reliability of the communication system.

A novel MIMO‐GFDM receiver for next generation communication

AbstractOrthogonal frequency‐division multiplexing is not suitable for fifth‐generation applications such as machine‐type communication, tactile internet, and internet of things. Generalized frequency‐division multiplexing (GFDM) is presented to address these requirements on the basis of the pulse‐shaping filter architecture, which makes GFDM have lower out‐of‐band emission and relatively less cyclic prefix and a become candidate for fifth‐generation systems. Because of the filter architecture, the subcarrier is nonorthogonal, which leads to serious intercarrier interference and is sensitive to carrier frequency offset (CFO). For reducing the intercarrier interference, we propose a 2‐path conjugate transmission scheme in the transmitter, and an adaptive receiver based on the modified phase rotated conjugate cancellation scheme is derived. By maximizing the carrier‐to‐interference ratio, we derive the optimal phase rotation and present an adaptive normalized block least mean squared algorithm to search the ideal result. With the proposed adaptive receiver, the CFO variation due to mismatch oscillator frequency and channel effect can be tracked effectively without feeding back the estimated information to the transmitter. In this paper, we propose a novel receiver with channel equalization and CFO compensation for multiple‐input–multiple‐output–GFDM systems in time‐varying channels. So far, no related research works exist. In the simulations, we compare the proposed receiver with several existing schemes in different time‐varying channels and modulations. The proposed scheme outperforms the existing schemes.

Intercarrier interference reduction in MC-CDMA system through second order duobinary coded phase rotated conjugate cancellation scheme.

Multi-carrier code division multiple access (MC-CDMA) technique is one of the strong candidates for next generation wireless mobile communication systems. Multi-carrier systems are very much sensitive to carrier frequency offset (CFO) results in intercarrier interference (ICI). To mitigate ICI without any spectral loss, a second order duobinary coded phase rotated conjugate cancellation algorithm is proposed in this paper. In the conventional phase rotated conjugate cancellation (PRCC) technique, one path carries the MC-CDMA signal with a phase spin of ϕ and the other path carries the conjugate of the first path signal with -ϕ phase spin. This artificial phase rotation allows the transmitter to tune the transmitted signals so that the ICI effects could be mutually cancelled at the receiver. Although the PRCC technique reduces the spectral efficiency, the limitation can be overcome by the joint second order duobinary coding scheme with PRCC technique. In the proposed method, the correlative coding between the binary symbols modulated on adjacent subcarriers is used to reduce the ICI without any spectral loss. Simulation results show that the proposed PRCC method provides better carrier to interference ratio (CIR) and bit error rate (BER) performances compared to the conventional conjugate cancellation (CC) technique.

Intercarrier interference cancellation using general phase rotated conjugate transmission for OFDM systems

In this paper, we propose a general phase rotated conjugate cancellation (PRCC) scheme for intercarrier interference (ICI) cancellation in orthogonal frequency division multiplexing (OFDM) systems. We also derive the optimal phase rotation for this general scheme such that the carrier-to-interference ratio would be maximized. It is shown that the previous conjugate cancellation (CC) scheme is equivalent to a special case of our proposed scheme. The general PRCC scheme not only inherits advantages of the conventional CC scheme, such as backward compatibility with the existing OFDM systems, low receiver complexity, and two-path diversity, but also provides better performance, especially at high frequency offset situations.

A Novel Intercarrier Interference Cancellation for MIMO-OFDM Systems

The main impairment of an orthogonal frequency division multiplexing (OFDM) systems is an intercarrier interference (ICI) effect caused by a frequency offset. A zero-padded technique designed for a conjugate cancellation scheme in MIMO-OFDM systems for ICI cancellation is proposed by padding zero between two consecutive symbols over a space-frequency domain. At the receiver, the ICI cancellation and space-frequency diversity combining techniques are proposed. In this paper, a performance analysis is investigated and analyzed. In comparison with the conventional complex conjugate scheme, the outage probability expressions of the proposed system and the repetition-coded conjugate cancellation scheme are derived for quasi-static Rayleigh fading channels. Simulation results show the close agreement with those obtained by theoretical analysis, which could be used to estimate the proposed system performance. Finally, simulation results in time-varying frequency-selective fading channels, a bit error rate (BER) performance of the proposed system is significantly improved over both the ordinary zero-padded MIMO-OFDM systems and the repetition-coded conjugate cancellation scheme when the frequency offset is not greater than 10% of subcarrier spacing. Furthermore, the proposed system is shown to be able to attain significant diversity gain.