Differential Modulation Scheme Research Articles

Differential Modulations for Multinode Cooperative Communications

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper proposes and analyzes differential modulation schemes for two cooperation protocols in multinode cooperative wireless networks; namely, multinode differential amplify-and-forward scheme (DiffAF) and multinode differential decode-and-forward scheme (DiffDF). In the DiffAF scheme, with knowledge of long-term average of received signals from all communication links, the destination efficiently combines signals from direct and all multiple-relay links to improve communication reliability. In the DiffDF scheme, by utilizing a decision threshold at each relay-destination link, the destination efficiently combines signals from the direct link and each relay link whose signal amplitude is larger than the threshold. For the DiffAF scheme, an exact bit error rate (BER) formulation based on optimum combining is provided for differential <formula formulatype="inline"> <tex>${M}$</tex></formula>-ary phase shift keying (DMPSK) modulation, and it serves as a performance benchmark of the proposed DiffAF scheme. In addition, BER upper bounds, BER lower bounds, and simple BER approximations are derived. Then, optimum power allocation is provided to further improve performance of the DiffAF scheme. Based on the tight BER approximation, the optimum power allocation can be simply obtained through a single dimensional search. In case of the DiffDF scheme, the performance of DMPSK modulation is analyzed. First, a BER formulation for DMPSK modulation is derived. Next, an approximate BER formulation of the DiffDF scheme is obtained, and a tractable BER lower bound is derived to provide further insights. Then, the performance of the DiffDF scheme is enhanced by jointly optimizing power allocation and decision thresholds with an aim to minimize the BER. Finally, simulation results under the two proposed cooperation protocols are given to validate their merit and support the theoretical analysis. </para>

All-optical packet header and payload separation for optical packet switched networks

A novel all-optical header and payload separation technique that can be utilized in unslotted optical packet switched networks is presented. The technique uses a modified terahertz optical asymmetric demultiplexer (TOAD) for packet header extraction with differential modulation scheme, and two semiconductor optical amplifiers (SOAs) that perform a simple XOR operation between the packet and its self-derived header to get the separated payload. The main virtue of this system is its simple structure and that it does not need any additional continuous pulses. Through numerical simulations, the operating characteristics of the scheme are illustrated. In addition, the parameters of the system are discussed and designed to optimize the operation performance.

Differential UWB Communications With Digital Multicarrier Modulation

As a high-rate alternative of impulse radio (IR) ultrawideband (UWB) communications, analog and digital multicarrier UWB radios with coherent detection have been introduced. In this paper, we investigate digital multicarrier differential (MCD) modulation and demodulation schemes for UWB communications. In our approach, the differential encoding and decoding are carried out across multiple digital carriers in the frequency domain. Hence, channel estimation is bypassed by using neighboring carriers as reference carriers. Compared with the transmitted reference TR-UWB which relies on delaying and correlating the received signal segments in time domain, our MCD-UWB avoids the need for analog delay elements. In addition, our digital-carrier based approach does not incur any spectrum expansion. We also show that our approach enables variable data rates, and remains operational even in the presence of severe interframe interference (IFI). In addition, the small spacing between differentially encoded carriers ensures that the data rate is not limited by the channel coherence bandwidth. Furthermore, our schemes are flexible in providing variable-rate transmissions. Simulations are also carried out to corroborate our theoretical analysis.

Angle differential‐QAM scheme for resolving phase ambiguity in continuous transmission system

AbstractAn angle differential quadrature amplitude modulation (ADQAM) scheme is proposed to solve phase ambiguity problem in non‐data‐aided continuous transmission system with square QAM constellation. Starting from the 16‐ADQAM case, we derive differential encoding and decoding schemes in terms of two differential angles and use a solar system analogy for explanation. The 16‐ADQAM system incurs only about 0.5‐dB performance degradation compared with the coherent 16‐QAM system under AWGN channel. Generalization of flat fading channel and higher‐level ADQAM is straightforward. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Punctured super-orthogonal space-time trellis codes

In this paper, we propose punctured super-orthogonal space-time trellis codes. First, considering a system with two transmit antennas, we present the basic design criteria for the punctured super-orthogonal codes. Then, staged decoding, a sub-optimal decoding technique used in single-input single-output systems, is extended to the proposed punctured codes. To reduce the complexity of staged decoding, we propose two complementary design rules of the punctured super-orthogonal codes. After that, we discuss the constraints under which the punctured codes can be designed without metric distortion. Compared with the conventional codes, the punctured super-orthogonal codes can provide lower decoding complexity. Also, they allow an easy implementation of various spectral efficiencies with a single encoder/decoder. Furthermore, the puncturing scheme can be generalized to the case of more than two transmit antennas. In addition, we implement differential modulation schemes based on the punctured codes. Such implementations do not require channel estimation at the transmitter and the receiver. Simulation results demonstrate that the performance of the punctured super-orthogonal space-time trellis codes is close to that of the corresponding conventional codes in the literature.

A Differential Modulation Scheme for MIMO Links with Distributed Transmit Antennas

This letter proposes a differential en/decoding scheme for multiple-input multiple-output (MIMO) communication links with distributed transmit antennas, in which neither the transmitter nor the receiver knows channel knowledge. The proposed scheme is based on the layered differential space-time codes we present. Simulation results show that the bit error rate (BER) performance is nonidentical in different channel propagation delay cases. The optimum delay time for two transmit antennas system, for example, is about 0.6Ts (where Ts denotes symbol period) when the links achieve better performance.

A Simple Differential Modulation Scheme for Quasi-Orthogonal Space-Time Block Codes with Partial Transmit Diversity

We report a simple differential modulation scheme for quasi-orthogonal space-time block codes. A new class of quasi-orthogonal coding structures that can provide partial transmit diversity is presented for various numbers of transmit antennas. Differential encoding and decoding can be simplified for differential Alamouti-like codes by grouping the signals in the transmitted matrix and decoupling the detection of data symbols, respectively. The new scheme can achieve constant amplitude of transmitted signals, and avoid signal constellation expansion; in addition it has a linear signal detector with very low complexity. Simulation results show that these partial-diversity codes can provide very useful results at low SNR for current communication systems. Extension to more than four transmit antennas is also considered.

All-Optical-Packet Header and Payload Separation for Unslotted Optical-Packet-Switched Networks

A novel all-optical header and payload separation technique that can be utilized in unslotted optical-packet-switched networks is proposed. The technique uses two modified terahertz optical asynchronous demultiplexers: One is for packet header extraction with differential modulation scheme, and the other performs a simple xor operation between the packet and its self-derived header to get the separated payload. The main virtue of this system is simple structure and low-power consumption. Through numerical simulations, the operating characteristics of the scheme are illustrated. In addition, the parameters of the system are discussed and designed to optimize the performance of the scheme

General differential modulation scheme for quasi-orthogonal space–time block codes with partial or full transmit diversity

A general and simple differential modulation scheme that can be applied to both partial-diversity quasi-orthogonal space–time block codes and full-diversity quasi-orthogonal space–time block codes is reported. A new class of quasi-orthogonal coding structures is presented for various number of transmit antennas. Differential encoding and decoding can be simplified to differential Alamouti codes by grouping the signals in the transmitted matrix and decoupling the detection of data symbols, respectively. For the codes with partial transmit diversity, the new scheme can achieve constant amplitude of transmitted signals, and avoid signal constellation expansion; in addition, it has a linear signal detector with very low complexity. Simulation results show that these partial-diversity codes can provide very useful results at low signal-to-nose ratio for current communication systems. For codes with full transmit diversity achieved by constellation rotation, the proposed scheme has performance equal to the best full-rate quasi-orthogonal schemes previously described in the literature with the benefit of a simpler detector. Moreover, a simple linear detector is also presented for the case when two orthogonal ASK constellations are used. Extension to more than four transmit antennas is also considered.

A Differential Space–Time Modulation Scheme for Correlated Rayleigh Fading Channels: Performance Analysis and Design

Differential space-time modulation (DSTM) is an encoding technique for multiantenna systems that allows the receiver to detect the transmitted signals without the knowledge of the fading channels. It can be viewed as an extension of the differential phase-shift keying scheme in single antenna systems. In this paper, we investigate the performance of multiantenna systems employing DSTM under spatially correlated Rayleigh fading channels. We present three expressions for calculating the pairwise error probability (PEP) of the DSTM scheme. The first expression allows us to numerically calculate the exact PEP. The remaining two are closed-form expressions of the exact PEP at asymptotically high signal-to-noise ratios and the PEP upper bound. It is found that when the distance matrix is proportional to the identity matrix, the error probability is determined by the channel correlation matrix and the initial transmitted code matrix. Based on the new PEP upper bound, we derive a criterion for optimizing the initial code matrix for this special case. Simulation results show that the performance of the differential space-time coded systems can be significantly improved by designing the initial transmitted code matrix according to the proposed design criterion

Differential Super-Orthogonal Space-Time Trellis Codes

In this paper, we present two differential trellis coded space-time modulation schemes when the channel state information is not available at the transmitter and at the receiver. First, we extend the design of super-orthogonal space-time trellis codes to improve their performance. Then, based on these extended super-orthogonal codes, we propose the first differential scheme. Compared with the trellis coded differential unitary space-time modulation scheme, not only does the proposed differential scheme achieve full diversity, but also it provides identical or higher coding gain and much lower decoding complexity. In order to achieve further improved coding gain in addition to full diversity for more than two transmit antennas, we propose a second differential scheme which is based on super-quasi-orthogonal codes. Such a differential scheme outperforms the differential modulation scheme based on super-orthogonal codes with simpler decoding. Simulation results demonstrate the good performance of our schemes

High rate trellis coded differential unitary space-time modulation via super unitarity

We introduce a new trellis coded differential unitary space-time modulation scheme for multiple-antenna wireless systems. In the new scheme, the constellation expansion is executed using a super unitarity technique so that the number of available unitary matrices is increased but the transmitted symbol alphabet is kept the same. A novel set partitioning strategy is then applied to the expanded matrix set. This scheme avoids the rate-loss problem suffered by traditional code design. We provide some examples using the proposed code construction method. Compared with existing ones, the new codes can not only achieve lower transmitter and receiver complexity, but also provide higher coding gains

`Print and Scan' Resilient Data Hiding in Images

Print-scan resilient data hiding finds important applications in document security and image copyright protection. This paper proposes methods to hide information into images that achieve robustness against printing and scanning with blind decoding. The selective embedding in low frequencies scheme hides information in the magnitude of selected low-frequency discrete Fourier transform coefficients. The differential quantization index modulation scheme embeds information in the phase spectrum of images by quantizing the difference in phase of adjacent frequency locations. A significant contribution of this paper is analytical and experimental modeling of the print-scan process, which forms the basis of the proposed embedding schemes. A novel approach for estimating the rotation undergone by the image during the scanning process is also proposed, which specifically exploits the knowledge of the digital halftoning scheme employed by the printer. Using the proposed methods, several hundred information bits can be embedded into images with perfect recovery against the print-scan operation. Moreover, the hidden images also survive several other attacks, such as Gaussian or median filtering, scaling or aspect ratio change, heavy JPEG compression, and rows and/or columns removal

Novel header extraction for optical packet-switched networks using a modified terahertz optical asymmetric demultiplexer with differential modulation scheme

A novel scheme of header extraction for optical packet switched networks is proposed for the first time. The system is based on a modified terahertz optical asymmetric demultiplexer (TOAD) with a differential modulation scheme. Numerical analysis and simulation show that more than 15-dB contrast ratio of the separated header at 2.5 Gbit/s to the suppressed payload at 40 Gbit/s can be achieved. In addition, the semiconductor optical amplifier (SOA) TOAD parameters are discussed and designed to optimize the performance of the proposed scheme.

A new differential modulation for coded OFDM with multiple transmit antennas

A new differential modulation scheme is presented for coded orthogonal frequency-division multiplexing (OFDM) systems with multiple transmit antennas in frequency- and time-selective channels. In contrast to an earlier scheme that involves differential modulation in space and time (ST) across two code matrices, the new scheme performs it in space and frequency (SF) within only one code matrix. As such, the shortest coherence time of the channel that can be handled is reduced, which makes the differential SF scheme more resistant to fast fading. Along with a suitable spectral encoder and interleaver, both the differential ST and SF schemes offer joint spatio-spectral diversity and coding gain. Numerical simulation confirms that SF is indeed more resistant to time-selective fading than ST; however, the former also suffers some performance loss caused by frequency-selective fading and is preferred only when fast fading is prevalent.

Single-symbol-decodable differential space-time modulation based on QO-STBC

We present a novel differential space-time modulation (DSTM) scheme that is single-symbol decodable and can provide full transmit diversity. It is the first known singlesymbol- decodable DSTM scheme not based on Orthogonal STBC (O-STBC), and it is constructed based on the recently proposed Minimum-Decoding-Complexity Quasi-Orthogonal Space-Time Block Code (MDC-QOSTBC). We derive the code design criteria and present systematic methodology to find the solution sets. The proposed DSTM scheme can provide higher code rate than DSTM schemes based on O-STBC. Its decoding complexity is also considerably lower than DSTM schemes based on Sp(2) and double-symbol-decodable QOSTBC, with negligible or slight trade-off in decoding error probability performance.

Differential space-frequency modulation via smooth logical channel for broadband wireless communications

In this letter, a differential space-frequency modulation (DSFM) scheme is proposed for multiple input multiple-output (MIMO)-orthogonal frequency-division multiplexing (OFDM) systems in broadband wireless communications. We assume that the fading channels keep constant only within each OFDM block, and may change independently from one OFDM block to another. The differential schemes proposed for MIMO-OFDM systems in the literature cannot successfully decode with such a rapidly fading channel, since the successful decoding of the previously existing schemes relies on the assumption that the fading channel keeps constant within a period of several OFDM blocks, and it changes slowly from a period of several OFDM blocks to another. In our proposed DSFM scheme, the transmitted signals are differentially encoded in the frequency domain within each OFDM block. Thus, the differential decoding can be performed over subcarriers within each single OFDM block. Furthermore, if a statistical channel power-delay profile (PDP) is known at the transmitter, we propose to create a smooth logical channel to improve the performance of the DSFM scheme. We obtain the smooth logical channel by sorting the channel frequency responses over subcarriers from a statistical point of view. If the logical channel is not smooth enough, we further consider a pruning process in which we use only the good part of the channel and get rid of the bad part of the channel. Simulation results show that the proposed DSFM scheme over a smooth logical channel (with pruning, if necessary) performs well for various channel PDPs.

Closed-form blind MIMO channel estimation for orthogonal space-time block codes

In this paper, a new computationally simple approach to blind decoding of orthogonal space-time block codes (OSTBCs) is proposed. Using specific properties of OSTBCs, the authors' approach estimates the channel matrix in a closed form and in a fully blind fashion. This channel estimate is then used in the maximum-likelihood (ML) receiver to decode the information symbols. The proposed estimation technique provides consistent channel estimates, and, as a result, the performance of the authors' blind ML receiver approaches that of the coherent ML receiver, which exploits the exact channel state information (CSI). Simulation results demonstrate the performance improvements achieved by the proposed blind decoding algorithm relative to the popular differential space-time modulation scheme.

Differential modulation based on quasi-orthogonal codes

This paper proposes a differential modulation scheme based on quasi-orthogonal space-time block codes (STBCs) when neither the transmitter nor the receiver has knowledge of the channel. A system with four transmit antennas is considered and divided into two subsystems. Each subsystem is equivalent to a system with two transmit antennas and uses differential STBC modulation. Based on the encoding and decoding for the two subsystems, the overall encoding and decoding algorithm for the original system is derived. We also compare the performance of our differential detection with that of the corresponding coherent detection in terms of achieved signal-to-noise ratio (SNR) for a given transmission power and propose a quantitative measure to estimate the performance difference between the coherent quasi-orthogonal code and our differential detection. Furthermore, we show that our differential modulation scheme can be extended to a system with more than four transmit antennas. Compared with the existing differential modulation schemes for four and eight transmit antennas, the proposed scheme has a lower bit-error rate (BER) and provides full diversity and pair-wise decoding complexity.

Virtually Lossless Compression of Astrophysical Images

We describe an image compression strategy potentially capable of preserving the scientific quality of astrophysical data, simultaneously allowing a consistent bandwidth reduction to be achieved. Unlike strictly lossless techniques, by which moderate compression ratios are attainable, and conventional lossy techniques, in which the mean square error of the decoded data is globally controlled by users, near-lossless methods are capable of locally constraining the maximum absolute error, based on user's requirements. An advanced lossless/near-lossless differential pulse code modulation (DPCM) scheme, recently introduced by the authors and relying on a causal spatial prediction, is adjusted to the specific characteristics of astrophysical image data (high radiometric resolution, generally low noise, etc.). The background noise is preliminarily estimated to drive the quantization stage for high quality, which is the primary concern in most of astrophysical applications. Extensive experimental results of lossless, near-lossless, and lossy compression of astrophysical images acquired by the Hubble space telescope show the advantages of the proposed method compared to standard techniques like JPEG-LS and JPEG2000. Eventually, the rationale of virtually lossless compression, that is, a noise-adjusted lossles/near-lossless compression, is highlighted and found to be in accordance with concepts well established for the astronomers' community.