Uplink-Noise Limited Satellite Channels

Many applications, current and emerging, are faced with a relatively new and interesting channel model. Systems which transmit data through a nonlinear relay, such as a satellite, must deal with a composite channel that can be separated into two distinct channels - the uplink channel between the user and the relay, and the downlink channel between the relay and the final destination. If the system has a strict power limitation and high data rate demands, such as a small satellite transmitting through NASA\u27s TDRSS Network, the dominant noise is present on the uplink rather than the downlink channel. Such a system is deemed to be uplink-noise limited and presents the designer with a number of problems not encountered in a more typical downlink-noise limited channel. Whereas the transmitted signal constellation can be pre-distorted to take into account the effect of the nonlinearity in the down-link limited channel, no amount of pre-distortion will solve the problems encountered when the majority of the noise is present before the nonlinearity. Instead, the receiver must be modified to reflect the non- Gaussian noise due to the operation of the nonlinearity on Gaussian noise. Under three assumptions - there is no downlink-noise present, the downlink channel is wideband relative to the data, and the filter proceeding the nonlinearity meets both matched filter and Nyquist requirements - such modifications can be made based on the nature of the nonlinearity. By mapping the ideal decision region through the nonlinearity, performance almost identical to that of a linear-wideband AWGN channel can be achieved. This paper will develop the theoretical performance of the receiver described for a nonlinearity typical of a satellite channel. Performance curves will be presented for QPSK, SPSK, 16PSK and 16QAM modulation schemes

Symbol-Level Multiuser MISO Precoding for Multi-level Adaptive Modulation

Symbol-level precoding is a new paradigm for multiuser downlink systems which aims at creating constructive interference among the transmitted data streams. This can be enabled by designing the precoded signal of the multiantenna transmitter on a symbol level, taking into account both channel state information and data symbols. Previous literature has studied this paradigm for MPSK modulations by addressing various performance metrics, such as power minimization and maximization of the minimum rate. In this paper, we extend this to generic multi-level modulations i.e. MQAM and APSK by establishing connection to PHY layer multicasting with phase constraints. Furthermore, we address adaptive modulation schemes which are crucial in enabling the throughput scaling of symbol-level precoded systems. In this direction, we design signal processing algorithms for minimizing the required power under per-user SINR or goodput constraints. Extensive numerical results show that the proposed algorithm provides considerable power and energy efficiency gains, while adapting the employed modulation scheme to match the requested data rate

Communications over fading channels with partial channel information : performance and design criteria

The effects of system parameters upon the performance are quantified under the assumption that some statistical information of the wireless fading channels is available. These results are useful in determining the optimal design of system parameters. Suboptimal receivers are designed for systems that are constrained in terms of implementation complexity. The achievable rates are investigated for a wireless communication system when neither the transmitter nor the receiver has prior knowledge of the channel state information (CSI). Quantitative results are provided for independent and identically distributed (i.i.d.) Gaussian signals. A simple, low-duty-cycle signaling scheme is proposed to improve the information rates for low signal-to-noise ratio (SNR), and the optimal duty cycle is expressed as a function of the fading rate and SNR. It is demonstrated that the resource allocations and duty cycles developed for Gaussian signals can also be applied to systems using other signaling formats. The average SNR and outage probabilities are examined for amplify-and-forward cooperative relaying schemes in Rayleigh fading channels. Simple power allocation strategies are determined by using knowledge of the mean strengths of the channels. Suboptimal algorithms are proposed for cases that optimal receivers are difficult to implement. For systems with multiple transmit antennas, an iterative method is used to avoid the inversion of a data-dependent matrix in decision-directed channel estimation. When CSI is not available, two noncoherent detection algorithms are formulated based on the generalized likelihood ratio test (GLRT). Numerical results are presented to demonstrate the use of GLRT-based detectors in systems with cooperative diversity

Novel Channel Estimation Techniques for Next Generation MIMO-OFDM Wireless Communication Systems: Design and Performance Analysis

During the last decade, major evolutions were introduced for Multiple Input Multiple Output (MIMO) wireless communication systems. To reap the full benefits of MIMO systems, the Base Station (BS) and user equipments require accurate Channel State Information (CSI), which can be acquired using one of the two major approaches for Channel Estimation (CE): pilot-based or blind-based CE. In this thesis, a pilot-based lower complexity channel estimator for Cell-Specific Reference Signals (C-RS) and User Equipment RS (UE-RS) in LTE-A Downlink (DL) system is proposed based on using a hybrid Wiener filter. The proposed system is a sub-optimum scheme that requires 8.8% and 74.5% of the number of computations required by the optimum system and other sub-optimum systems. Moreover, a less computationally complex CE scheme based on Fast Fourier Transform (FFT) is proposed. The presented pilot-based system is validated in end-to-end LTE-A system in terms of throughput, which confirms that the proposed system is suitable for practical implementation. Next, a new blind-based CE technique based on a hybrid OFDM symbol structure for SIMO and MIMO systems is presented. It is shown that the developed system, with enough receive antennas, performs as good as pilot-based system, with similar complexity and better spectral efficiency. Finally, new Resource Grid (RG) configurations that serve the blind-based CE scheme developed for MIMO-OFDM system are presented, with the aim to improve the Mean Squared Error (MSE) performance, while minimizing the number of required receive antennas. Results show that the proposed RG configurations provide superior MSE performance, from the perspective of the blind-based CE scheme under investigation, compared to the LTE-A RG configuration. Throughout the thesis, performances of linear receivers is presented in terms of spectral efficiency as a function of Signal-to-Noise Ratio (SNR), and number of BS antennas. CE techniques are evaluated in terms of MSE as a function of SNR for different channel condi- tions. Analytical results wherever possible and, in general, simulation results are presented

Error performance analysis of cross QAM and space-time labeling diversity for cross QAM.

Doctoral Degrees. University of KwaZulu-Natal, Durban.Abstract available in the PD

On space-time trellis codes over rapid fading channels with channel estimation

Ph.DDOCTOR OF PHILOSOPH