Circular Polarization Modulation for digital communication systems

In this paper, a framework of a novel communication system based on circular polarization modulation called Offset Circular Polarization Modulation (OCPM) is proposed which not only increases the channel capacity but also provides data security against eavesdropping. In conventional circular polarization modulation (CPM), only one bit of data (1 or 0) can be transmitted at one time by using either left hand circular polarization (LHCP) or right hand circular polarization (RHCP), limiting it to only 2-states polarization modulation scheme. However, the proposed OCPM scheme provides eight-fold increase in the polarization states. In OCPM, typical CPM constellations (LHCP or RHCP) can be shifted to different quadrants using an offset vector in order to increase the number of polarization states. In addition to switching the swing of circular polarization (CP) in OCPM, quasi-dc offset vectors are added to the horizontal and/or vertical components of the CP wave with different combinations. The reason why quasi-dc offset vectors have been used is because pure dc offsets cannot be wirelessly transmitted. The quadrant of the modulation is determined by the offset vectors. For instance, in order to shift a CP constellation (LHCP or RHCP) in the first quadrant, two positive offset vectors are required: one along horizontal-axis and the other along vertical-axis of the EM wave. Thus, OCPM is a dual dimension CPM scheme with polarization state as well as offset state. The eavesdropper would require both dimensions (i.e. Polarization state and offset state) to correctly determine the overall state of the OCPM scheme.

The efficiency of a retarder for conversion of linear polarization into circular polarization is defined. The average efficiency is calculated for the cases in which either the polarization plane of incident radiation or the phase retardation is modulated. Three different modulation functions are considered.

In this paper, the error performance of the Alamouti scheme with transmit antenna selection is investigated in the context of imperfect subset selection. The asymptotic bit error performance is derived for binary phase-shift keying (BPSK) modulation in flat Rayleigh fading channels. It is shown that the transmit diversity order is equal to the larger ordinal number of the antenna within the selected antenna subset, while the smaller ordinal number only determines horizontal location of the error performance curve without an impact on asymptotic diversity order. Simulation results are provided to substantiate the theoretical analysis

Publication in the conference proceedings of EUSIPCO, Toulouse, France, 2002

We consider data transmission through a time-selective (correlated) flat Rayleigh fading channel under an average power constraint. The channel is estimated at the receiver with a pilot signal, and the estimate is fed back to the transmitter. The estimate is used for coherent demodulation, and to adapt the data and pilot powers. We start with a block fading channel in which the channel gain changes according to a Gauss-Markov process. The channel estimate is updated during each coherence block with a Kalman filter, and optimizing the data and pilot powers is formulated as a dynamic program. We then study a continuous limit in which the coherence time tends to zero, and the correlation between successive channel gains tends to one, so that the channel process becomes a diffusion process. In this limit it is shown that the optimal pilot power control policy is "bang-bang", i.e., depending on the current system state (channel estimate and associated error variance) the pilot power is either the maximum allowable, or zero. The associated regions of the state space are illustrated numerically for specific system values. This example shows that the achievable rate with the optimized training policy provides substantial gains relative to constant training power at low SNRs

In this paper a simplified differential amplitude demodulation scheme is proposed for 64-DAPSK (differential amplitude and phase shift keying), which needs less quantity of decision thresholds compared with the conventional schemes. The arithmetic mean threshold (AlMT) scheme is proposed and analyzed with the simplified system in AWGN and flat Rayleigh fading channels. By these proposed schemes, system complexity is reduced considerably, but no performance degradation occurs. In the case of channel coding, the concatenated code consisting of an inner convolutional code and an outer Reed-Solomon code can be applied to 64-DAPSK. Compared with coherent 64-QAM system which employs pilot symbols for channel estimation, 64- DAPSK can achieve the performance close to or even better than that of the former by utilizing the additional redundancy for outer coding, provided that the channel SNR is high enough.

The authors derive the true Cramér–Rao lower bound (CRLB) for a non-data-aided (NDA) carrier phase estimation from general M-ary phase-shift keying (M-PSK) modulation signal over flat Rayleigh fading channels (FRFC). The numerical Monte Carlo method is used to compute the true CRLB from general M-PSK modulation signals over FRFC with complex additive white Gaussian noise. The efficiency of the proposed method is made obvious through the assessment of the true CRLBs from both symmetric and asymmetric 8-PSK modulation signals for various observation window sizes. The simulation results show that optimal NDA carrier phase recovery is harder to acquire from symmetric modulation signals for signal-to-noise ratio (SNR) range up to 5 dB, but depends on the constellation density, only for SNR range superior to 5 dB. For both constellations the CRLB decreases as long as the observation window size increases. The derived bound provides an efficient standard for evaluating the performance of any unbiased NDA carrier phase estimator from general M-PSK modulation signals over FRFC. The method presented here stands useful for general M-ary quadrature amplitude modulation signals as well.

Cutoff rates for (perfect) coordinate interleaving over flat Rayleigh fading channels are computed for some representative two-dimensional (2-D) and four-dimensional (4-D) quadrature amplitude modulation (QAM) schemes and compared to the standard symbol interleaved approaches. It is shown that for optimized coordinate interleaving [i.e., using optimum rotation parameters that maximize the cutoff rate for a given signal-to-noise ratio (SNR)], coding gains can be achieved vis-a-vis symbol interleaving that increases with the dimensionality of the signal constellation. Perfect coherent detection, as well as perfect channel state information are assumed.

In this paper, we propose and analyze a multihop transmission scheme based on detect-and-forward (DF) relay protocol and network coding (NC) for two-way relay channels. In this scheme, the odd relays perform hard detection and then forward the detected signals to the next hop, whereas the even relays perform NC on the detected signals from the two adjacent nodes and broadcast them to the next hop. By separating the network into multiple two-hop subsystems, we develop closed-form expressions for bit error rate (BER) in flat Rayleigh fading channels. Our results are given as both lower bound and asymptotic expression based on an accurate upper bound of the end-to-end signal-to-noise ratio (SNR). It is shown that the proposed scheme has the same asymptotic BER performance and a much higher throughput compared with the conventional bidirectional relay scheme based on four transmission phases. Therefore, the proposed scheme is efficient for practical wireless applications. Simulation results are provided to validate the analysis.

Performance Analysis of Wireless Adhoc Networks in Flat Fading and Frequency Selective Fading channels

In this letter, we propose a downlink nonorthogonal multiple access network with hybrid long-packet communications (for a central user) and short-packet communications (for a cell-edge user). The outage probability of the central user and average block error rate (BLER) of the cell-edge user in flat Rayleigh fading channels are theoretically derived. Furthermore, we provide the asymptotic outage probability and average BLER when the signal-to-noise ratio (SNR) is high. Numerical results illustrate that both the analytical outage probability of the central user and the analytical average BLER of the cell-edge user match the simulation results.

A two-user downlink cooperative non-orthogonal multiple access (NOMA) short-packet communication network is investigated in flat Rayleigh fading channels, where a central user cooperatively relays the signals intended to a cell-edge user. To derive the average block error rate (BLER) of the central user, unlike the work in the literature where the product term is neglected, a new method that works well for the whole range of power allocation factor is proposed. Furthermore, the average BLER of the cell-edge user is theoretically derived. Numerical results demonstrate that while remaining almost the same average BLER for the central user, the cell-edge user in our proposed cooperative NOMA network achieves lower average BLER than those in the conventional NOMA network and the cooperative orthogonal multiple access network.

Determining the effectiveness of any modulation scheme in mobile radio environment comprises reliability and error performance evaluation. The later involves determining the increase in Eb/No required to maintain a fixed BER. Thus, error performance plays a vital role in the selection of modulation scheme for specific applications. This paper begins with review of the conceptual difference between SNR and Eb/No performance metrics of communication system. The significance of BER for error performance analysis is verified by comparing the Eb/No characteristic plots for digital modulation schemes: M-PSK, M-QAM and M- FSK. The comparison parameters includes modulation order and simulation environment. The simulation environment considered are additive white Gaussian noise (AWGN) channel model and Rayleigh fading channel model. There are two important issues targeted in this paper. Firstly, the significance of the concept of multi-carrier modulation (MCM) used in wireless LAN standard IEEE802.11a is verified. The technology resolves the problem of multi-path in a piecewise manner, i.e. the linear relationship between error probability and Eb/No in case of wide-band Rayleigh fading channel is transformed into a number of narrow-band sub-channels with exponential relationship. Then, the sub-channel modulation schemes are compared by observing the effect of modulation order on BER, spectral usage, noise tolerance and trade-off issues. Finally, the inferences drawn out of the comparative analysis justifies the use of low-order PSK modulations for control and higher-order QAM for payload transmissions respectively, for OFDM implementation as in IEEE802.11a.

We analyze the error performance of minimum selection generalized selection combining (MS-GSC), in which the minimum number of diversity branches are selected such that their combined signal-to-noise ratio (SNR) is above a given threshold. A flat Rayleigh fading channel with independent and distinctly distributed branch SNRs is considered. By transforming the ordered instantaneous branch SNRs to their differences, we derive the distribution of the number of selected branches in closed form. We then modify the derivation of this distribution to get the characteristic function (cf.) of the combiner output SNR. This cf. is used to obtain the symbol error probability for different coherent digital modulation schemes.

In a wireless communication system, a cooperative communication technique is a communication method for improving performance. In the conventional scheme, when a destination user is located in the cell boundary, one base station communicates with a destination user. At a same time, a destination user communicates with other base stations which cooperate with base station. If a distance between a base station and a destination user is far, the quality of communication is degraded by the effect of channel attenuation. Therefore, this paper proposes an enhanced cooperative transmission scheme using the orthogonal frequency multiplexing in the wireless communication system. The proposed scheme uses extra relays between a base station and a destination user. When the channel state of a destination user is bad, other base stations are used. In order to obtain better performance than the conventional scheme, the cooperative decode and forward (DF), and space time cyclic delay diversity (STCDD) and cyclic delay diversity (CDD) are applied in the extra relays. In simulation results, the proposed scheme has higher error and throughput performances than the conventional scheme for obtaining diversity gain when the used modulation scheme is increased. Also, the proposed scheme which uses cooperative DF has slightly better error and throughput performances than the proposed scheme which uses cooperative STCDD and CDD due to less error propagations and resultant higher diversity gain.