Uncoded Modulation Research Articles

On the Spectral Efficiency of Energy Constrained Short-Range Communicating Systems

Recently the energy efficiency (EE) in wireless communication is becoming one of the key performance metric for wireless communicating systems. For battery driven system like wireless sensor networks and ad-hoc networks, the energy conservation is a critical factor for node life. In addition, the spectral efficiency (SE) has been traditionally used as a performance index for wireless transmission. This paper investigates the optimum spectral to energy efficiency tradeoff especially for short range communications. The analysis starts with the Shannon case in additive white gaussian noise channel where some theoretic results are developed when considering both transmit and circuit energy. Then, point to point communication system with uncoded M-ary quadrature amplitude modulation (MQAM) is considered. At the device layer, a system level power consumption model is proposed. At the physical layer, a novel and accurate approximation of the bit error rate (BER) function of the signal to noise ratio is made. The total energy per bit is formulated and the link between the Shannon limit capacity and MQAM based communication is established. The impact of distance, bandwidth, power consumption and BER as a quality-of-service parameter on EE–SE tradeoff is analyzed. It is shown that, varying distance, bandwidth and circuit power consumption induce more impact in the low SE regime whereas, the BER has more impact on the high SE regime. Moreover, the energy optimal spectral efficiency for MQAM is obtained in closed-form and confirmed by numerical results.

On the Use of Extrinsic Probabilities in the Computation of Non-Bayesian Cramér–Rao Bounds for Coded Linearly Modulated Signals

This contribution considers the non-Bayesian Cramer–Rao bound (CRB) related to parameter estimation from a linearly modulated signal observed in additive white Gaussian noise. We compare the exact CRB expression for coded modulation with two ad hoc CRB (ACRB) expressions; the first ACRB is obtained by substituting in the exact CRB expression for uncoded modulation the a priori symbol probabilities by the extrinsic symbol probabilities; the second ACRB, which has received some attention in recent scientific publications, additionally assumes that the real and imaginary parts of the symbols are independent. Our exposition focuses on the particular case of phase shift estimation. By means of examples we show that although for some coded modulation schemes the exact and ACRBs yield virtually the same numerical result, for other coded modulation schemes the ACRBs differ considerably among themselves and from the exact CRB. We provide some explanations for this behavior. We also argue that both ACRBs are expected to virtually coincide with the exact CRB in the case of bit-interleaved coded modulation combined with a rectangular constellations with independent in-phase and quadrature mapping and a binary code for which the factor graph does not contain short cycles.

Duality Between the Improved Rate Regions of Gaussian Multiple Access and Broadcast Channel for Discrete Constellations

In this paper, we first look at the achievable rate regions of the Gaussian Multiple Access Channel (G-MAC) and Gaussian Broadcast Channel (G-BC) when discrete constellations are transmitted. Focusing on the two-user case and assuming uncoded Pulse Amplitude Modulation (PAM), we show that the rate regions of G-MAC and G-BC can be improved by power conservation at the transmitter, a factor ignored in the earlier derived rate regions. We then further investigate duality between the G-MAC and the G-BC and show that a rate pair achieved in the G-MAC can be translated to a rate pair in the dual G-BC, such that the equal sum power constraint be satisfied. Due to the similarity of the rate expressions to Shannon's capacity formula, for an appropriate choice of signal to noise ratio (SNR) gap, we show that when finite constellations are used for transmission, rate regions of these two channels also have a dual relationship (the known for Gaussian alphabets). The rate region of the G-BC can therefore be characterized from the rate region of the dual G-MAC and vice versa.

The robustness of coding and modulation for body-area networks

We consider transmission over body area networks. Due to the difficulty in assessing an accurate statistical model valid for multiple scenarios, we advocate a system design technique favoring robustness. Our approach, which is based on results in [12] and generalizes them, examines the variation of a performance metric when the nominal statistical distribution of fading is replaced by the worst distribution within a given Kullback-Leibler divergence from it. The sensitivity of the performance metric to the divergence from the nominal distribution can be used as an indication of the design robustness. This concept is applied by evaluating the error probability of binary uncoded modulation and the outage probability ? the first parameter is useful to assess system performance with no error-control coding, while the second reflects the performance when a near-optimal code is used. The usefulness of channel coding can be assessed by comparing its robustness with that of uncoded transmission.

Generalized maximum likelihood noncoherent block detection for decode-and-forward relay systems

This paper considers noncoherent cooperative decode-and-forward (DF) half-duplex multi-branch relay systems. Each relay branch is modeled as a probabilistic transition system at the last hop, and thus it can be considered as a relaying chain comprising multi-hop relays. An approximation to the generalized maximum likelihood (ML) noncoherent block detection is derived for uncoded M-ary modulation in a faded noisy environment. In particular, the derived noncoherent block detection in a noiseless case is equivalent to a multichannel reception with full diversity. Furthermore, the generalized detection is extended specifically to block coded M-ary phase shift keying (MPSK) modulation. For a DF three node relay system using block coded quadrature phase shift keying (QPSK), simulation results are provided to examine the end-to-end error performance of the noncoherent detection with considering the effects of network geometry and power allocation, respectively. It is shown that under a fixed power allocation, a proper relay placement can yield near full diversity for large signal-to-noise ratio.

Multiuser MIMO Achievable Rates With Downlink Training and Channel State Feedback

We consider a MIMO fading broadcast channel and compute achievable ergodic rates when channel state information is acquired at the receivers via downlink training and it is provided to the transmitter by channel state feedback. Unquantized (analog) and quantized (digital) channel state feedback schemes are analyzed and compared under various assumptions. Digital feedback is shown to be potentially superior when the feedback channel uses per channel state coefficient is larger than 1. Also, we show that by proper design of the digital feedback link, errors in the feedback have a minor effect even if simple uncoded modulation is used on the feedback channel. We discuss first the case of an unfaded AWGN feedback channel with orthogonal access and then the case of fading MIMO multi-access (MIMO-MAC). We show that by exploiting the MIMO-MAC nature of the uplink channel, a much better scaling of the feedback channel resource with the number of base station antennas can be achieved. Finally, for the case of delayed feedback, we show that in the realistic case where the fading process has (normalized) maximum Doppler frequency shift 0 < F < 1/2, a fraction 1 - 2F of the optimal multiplexing gain is achievable. The general conclusion of this work is that very significant downlink throughput is achievable with simple and efficient channel state feedback, provided that the feedback link is properly designed.

Energy Efficient Physical Layer Design for Passive RFID System

In forward link,to cooperate with ASK modulation,a new data coding method for carrying out more energy to passive tag is presented for passive RFID system in this paper. The basic idea of the coding is to set adjustable and asymmetric high and low level duration within a symbol cycle,and a longer duration for high level. The performance analysis results show the energy of baseband data transmitted from reader to tags increased by 84% at most,compared with that of Manchester coding at typical 40kbps. The system performance can be increased by using this coding. The same results are showed by the testbed and theoretical analysis. In return link,the scheme of coded OOK and coded BPSK is proposed. The cutoff rate is used to study the influence of encoding on system performances. Analysis results show that coded modulation is more energy efficient than uncoded modulation in Passive RFID return link and the system performance is improved with almost no increase in chip complexity. The gains of encoding in system information rates and communication range for different cutoff rate is analyzed and evaluated.

Adaptive Modulation Based on Finite-Rate Feedback in Broadcast Channels

In a single-input single-output (SISO) downlink with multiple users, to obtain the multiuser diversity gain, the full channel state information (CSI) of all users is required for selecting the desired user and transmission mode. However, feedback channels are often capable of carrying only a limited number of bits. With such rate limited feedback links, we intend to maximize the average throughput of each user by designing transmit power levels, signal constellations and feedback thresholds jointly. Both un-coded adaptive M-ary quadrature amplitude modulation (MQAM) and adaptive trellis-coded modulation (TCM) are investigated. A nested iterative algorithm is proposed to obtain a finite number of transmission modes and thresholds. Optimization in the heterogeneous network is also considered. Simulation results demonstrate a small number of feedback bits make the throughput approach to that with perfect CSI. As the number of users increases, the multiuser diversity gain is achieved and the average feedback load of each user is reduced.

A nakagami fading phase difference distribution and its impact on BER performance

We propose a new model for investigating the impact of complex imperfect channel state information (CSI) over Nakagami-m fading channels. The phase difference between two correlated complex Nakagami random variables (RVs) is proposed and its statistical properties are investigated. A closed-form expression for the distribution is obtained using an approach from [1] and depends on the Nakagami-m parameter and the power correlation coefficient (p) between two correlated complex RVs. A simple approximation is given when p is close to unity. The proposed phase difference is shown to accurately approximate that of the Ricean. We derive the joint density function of this phase difference and its associated correlated envelopes. This is given in a closed form in terms of a generalized hypergeometric function, but the RVs can be easily generated for simulation. We analyze the impact of imperfect CSI on the bit error rate (BER) of uncoded and coded modulation. For uncoded modulation, the average BER and a closed-form expression of the asymptotic BER or error floor are obtained. For coded modulation, we determine the error free feedback bound of bit-interleaved coded modulation with iterative decoding for both perfect and imperfect CSI. We show that imperfect phase estimation is the major cause of the error floor.

Secure data compression with sphere packing

A system that accomplishes compression using sphere packing with the support of a hyper-elliptic curve providing encryption is presented. In addition, with use of channel code algorithms, high consistency is achieved and the expansion factor is calculated. Results of simulation are also presented where the hyper-elliptic curve encryption is concatenated with sphere packing, followed by a non-binary low density parity check encoder and sphere packing modulation. A gain of 8 dB over uncoded QPSK modulation for BER=10−4 is achieved on a two tap fading channel. Finally, system complexity is presented for a system based on symbols and a system based on bits.

Large penalty of hybrid diversity with uncoded modulation in slow rayleigh fading

In hybrid selection/maximal ratio combining (H-S/MRC) the receiver selects the L branches with the largest signal-to-noise ratios (SNRs) from N available diversity branches and performs maximal ratio combining (MRC). A penalty is incurred with respect to MRC which can be defined as the increase in SNR required for H-S/MRC to achieve the same symbol error probability as MRC. Recently, this penalty was investigated specifically for two-dimensional modulations. In this paper, we derive the asymptotic SNR penalties in slow Rayleigh fading for small and large values of SNR for general uncoded signaling constellations.

Low-complexity block coded multi-carrier modulation for line-of-sight microwave systems

The authors present low-complexity block coded multi-carrier modulation (BC-MCM) schemes that more are efficient to overcome the deep frequency-selective fading that appears on high-data-rate transmission line-of-sight digital radio channels. These BC-MCM schemes are based on time-limited orthogonal multi-carrier modulation with waveform shaping (ITLO-MCM) systems associated to an interleaving process and an array of very simple constituent binary block codes. The performance is evaluated in terms of signature areas for fixed BER thresholds, on severe frequency-selective fading channels. Results show that the proposed low-complexity BC-MCM schemes provide significant improvement in performance over equivalent uncoded multi-carrier modulation (MCM) and practical single-carrier TCM systems.

Simple Rate Control for Fluctuating Channels in Ad Hoc Wireless Networks

In the presence of channel fluctuation, rate adaptation is one way to maintain the quality of the link at a desired level. This is especially important in ad hoc wireless networks, where temporary channel fluctuations might create frequent needs for rerouting that would result in severe overhead and adversely affect the performance. We study an unusual method of passive rate adaptation in which some bits are dropped at the receiver end of a link. The symbol-error probability decreases as some bits are dropped. In terms of the distortion for a realtime analog signal, the tradeoff is between more reliable detection of fewer bits and less reliable detection of more bits. Our scheme achieves smaller distortion for a certain region of signal-to-noise ratio (SNR) values when compared with the original scheme without rate adaptation. Two examples, uniformly spaced, uncoded pulse amplitude modulation and quadrature amplitude modulation, are studied and compared for both a Gaussian channel and a Rayleigh fading channel. We conclude that our scheme is more suitable to use in a fading channel than in a Gaussian channel. We also verify that our scheme has a larger applicable region of SNR values when a nonuniform constellation is used, since the important bits are given additional protection.

Combined Huffman Code and Generalized Array Codes Employing Phase/Frequency Modulation

The authors present a simple technique to combine source coding, channel coding and modulation. This technique employs the Huffman code for source coding, Generalized Array Codes (GAC) for channel coding and Phase/Frequency (BFSK/MPSK) for modulation. The combined scheme has the advantage of reducing the redundancy, providing error protection and simultaneously optimizing on channel bandwidth. It is shown the combined system gives the same performance as its separate counterparts but has lower complexity. The algorithm used to construct the combined trellis decoder employing phase/frequency for a given source text file is designed and implemented in software. The error performances for the combined schemes using BFSK/MPSK modulation are compared with those employing coded as well as Uncoded MPSK modulation. Such a Block Coded Modulation (BCM) scheme is suitable for both information storage and digital transmission systems.

Diversity Combining and Equalization with TCM for Wireless Communications

This paper presents an equalization structure in which antenna diversity, adaptive decision feedback equalization (DFE), interleaving and trellis-coded modulation (TCM) can be effectively combined to combat both ISI and cochannel interference in cellular mobile radio environments. The feedback filter of the DFE can use either tentative or final decision symbols of the TCM Viterbi decoding to cancel tail ISI with the square root Kalman algorithm. A performance bound on the average pairwise error probability for TCM under perfect interleaving and equalization is obtained by analysis. Some simulation results which illustrate the potential of the proposed system will also be given. In particular, a performance comparison between the proposed method and uncoded QPSK modulation will be undertaken.

Multirate trellis coded modulation schemes in multimedia communications

Multirate multimedia information with different quality of service may be required at the same time in future communication systems. We propose a new class of trellis coded modulation (TCM) schemes, which can carry two different rates of multimedia information using only the same signal constellation. The transmitted information bit number per symbol in the proposed TCM is not an integer and is greater than that of the conventional TCM. An interesting character of the proposed multirate TCM is that it can provide different error protection for the two information rates. We also present the design criterion and the existence of noncatastrophic multirate TCM. Some theoretical and simulation results have shown that it can provide a certain coding gain compared with the corresponding uncoded modulation schemes.

Increasing storage capacity in multilevel memory cells by means of communications and signal processing techniques

Multilevel and analogue memory cells have the capability of storing considerably more than one bit of information per cell. The number of levels the memory cells can store is limited by noise and bit error probability. The authors propose using error correction coding and digital modulation techniques used in communication systems to increase the storage capacity of multilevel and analogue memory cells. They evaluate these techniques and find that compared to the methods that do not use coded modulation techniques, typically, one extra bit per cell (i.e. effectively doubling the number of levels per cell) in storage capacity can be achieved. The techniques can also be applied to improve readout bit-error probability and provide unequal error protection for the bits to be stored. The latter technique is important for the efficient storage of digitised multimedia signals such as speech, audio, image and video signals. One- and two-dimensional coded modulation schemes as well as uncoded multilevel digital modulation schemes are presented in detail. Complexity and speed issues are also considered.

Coded and uncoded CPM schemes over rings of integersforAWGN and Rayleigh fading channels.

Coded and uncoded modulation schemes based on continuous phase modulation (CPM) over rings of integers are investigated. A baseband modulator utilising a decomposed model of CPM is used at the transmitter and a Viterbi soft-decision decoder is employed at the receiver. The performances of a number of partial-response CPM schemes are investigated when operating over an additive white Gaussian noise (AWGN) channel and Rayleigh flat fading (RFF) channel. The simulation results obtained are shown to compare favourably with theoretical bounds.

Multilevel concatenated-coded M-DPSK modulation schemes for the shadowed mobile satellite communication channel

This paper presents a bandwidth-efficient multilevel concatenated-coded modulation scheme for reliable data transmission over the shadowed mobile satellite communication (MSAT) channel. In this scheme, bandwidth-efficient block modulation codes are used as the inner codes, and Reed-Solomon codes of various error correcting capabilities are used as the outer codes. The inner and outer codes are concatenated in multiple levels. A general method for constructing multilevel concatenated modulation codes is presented, and a multistage closest coset decoding for these codes is proposed. Specific multilevel concatenated 8-PSK modulation codes have been constructed. These codes are designed to lower the bit error rate (BER) of the error floor caused by the large Doppler frequency shift due to the motion of vehicles. Simulation results show that these codes perform very well and achieve large coding gains over the uncoded reference modulation systems.

Impact of diversity reception on fading channels with coded modulation. II. Differential block detection

For pt. I see ibid., vol.45, no.6, p.563-572, 1997. We study coded modulation with block differential detection in an arbitrarily correlated Rician fading channel with space diversity. Coded differential q-PSK is included in our analysis as a special case. A metric is chosen that is optimum for perfect interleaving, slow fading, and independent diversity branches. For slow fading, we compare the the cutoff rates of the channels resulting from different choices of block length N and diversity index M. Specifically, we show that block detection with diversity may or may not generate a better coding channel than usual differential detection, according to the code selected and the combination of values of M and N. In particular, for low-diversity orders (M=1,2) and for low-to-medium code rates, differential detection is still an optimal or near-optimal solution, while for high-diversity orders (M/spl ges/2) and medium-to-high code rates (up to uncoded modulation) block detection with N>2 can provide a significant gain. An error floor always exists when fading is fast. It decreases exponentially with the product of code diversity and space diversity, so that the latter emerges as a very effective technique for lowering the error floor of a system affected by fast fading. Performance examples based on actual coding schemes are also shown.