Joachim Speidel

Quaternary optical ASK-DPSK and receivers with direct detection

We present a novel quaternary optical-modulation scheme based on the combination of amplitude and phase modulation. The modulator has a simple structure and can be realized by using standard components. We present two simple receivers, each one using photodiodes for direct detection. The performance of the modulation scheme is investigated.

A comparative study of iterative channel estimators for mobile OFDM systems

We investigate two iterative channel estimators for mobile orthogonal-frequency division multiplexing. The first estimator is based on iterative filtering and decoding whereas the second one uses an a posteriori probability (APP) algorithm. The first method consists of two cascaded one-dimensional Wiener filters, which interpolate the unknown time-varying two-dimensional frequency response in between the known pilot symbols. As shown, the performance can be increased by feeding back the likelihood values at the output of the APP-decoder to iteratively compute an improved estimate of the channel frequency response. The second method applies two APP estimators, one for the frequency and the other one for the time direction. The two estimators are embedded in an iterative loop similar to the turbo decoding principle. As shown in detail, this iterative estimator is superior and its performance is independent of whether the chosen time-frequency pilot grid satisfies the two-dimensional sampling theorem or not. The bit-error rate as a function of the signal-to-noise ratio is used as a performance measure. In addition, the convergence of the iterative decoding loop is studied with the extrinsic information transfer chart.

An adaptive two-dimensional channel estimator for wireless OFDM with application to mobile DVB-T

An adaptive channel estimator is proposed and investigated to improve the performance of the receiver for pilot aided wireless and mobile OFDM systems. The estimator consists of a two-dimensional Wiener filter which is implemented as a cascade of two one-dimensional filters. We propose an efficient algorithm for adaptation to time varying channels of the second filter. The method is applied to the Terrestrial Digital Video Broadcasting System (DVB-T), which was originally specified for fixed receivers and which is in the process of being extended for mobile reception. The results are shown after inner decoding. Depending on the channel conditions, the signal-to-noise ratio gain can be up to 1.6 dB. The method provides a compatible improvement to DVB-T receivers.

An efficient waterfilling algorithm for multiple access OFDM

In this paper we present a novel and computationally efficient waterfilling algorithm for multiuser OFDM. This algorithm is based on the multiuser waterfilling theorem and determines the subcarrier allocation for a multiple access OFDM system. This approach maximizes the total bitrate under the constraints of user-individual power budgets. Once the subcarrier allocation has been established, the bit and power allocation for each user can be determined with a single-user bitloading algorithm, for which several implementations can be found in the literature. The presented iterative algorithm performs well, also with a high number of subchannels.

BER Analysis and Optimization of Generalized Spatial Modulation in Correlated Fading Channels

We propose and analyze a generalized spatial mod- ulation scheme, where depending on the bits to be transmitted a certain data symbol as well as a certain beamforming vector is chosen. The receiver then has to estimate both the used beamforming vector and the transmitted data symbol for being able to reconstruct the originally transmitted bit sequence. We analyze our scheme theoretically by deriving a very tight upper bound on the average bit error rate (BER) for arbitrary symbol constellations and beamforming codebooks in correlated Rayleigh fading channels. Based on this bound, we determine a design criterion for optimizing the used beamforming codebook in presence of spatial correlation at the transmitter-side. Simulation results are shown to be in excellent agreement with our analytical calculated BERs and they illustrate the significant performance gains that can be obtained with our generalized scheme compared to conventional spatial modulation.

Short-term and long-term diagonalization of correlated MIMO channels with adaptive modulation

Theoretical results on MIMO capacity maximization suggest the decoupling of the MIMO channel into independent subchannels with optimum water-filling on these subchannels. Those results inspire the development of real systems that diagonalize the MIMO channel and adaptively control the modulation on each of the resulting subchannels. We study the design of a fully adaptive transmitter, where transmit filtering and adaptive modulation is controlled by short-term as well as long-term channel state information (channel correlation), where the focus is on channels with correlated fading at transmitter and receiver array. To this end, we are motivating the use of long-term channel eigenmodes by capacity-independent considerations. Simulation results confirm the potential of fully adaptive transmit processing.

Mutual information of MIMO channels in correlated Rayleigh fading environments - a general solution

We present a novel approach on the calculation of the moment generating function of mutual information, of MIMO channels with correlated Rayleigh fading. For the first time, a concise mathematical formulation of the moment generating function is given in terms of a hypergeometric function of matrix arguments. In contrast to existing literature, our approach is not based on eigenvalue probability density functions but uses a direct integration technique. In principle, via the moment generating function it is possible to calculate exact, i.e. non-asymptotic moments, including e.g. ergodic capacity, for arbitrary array sizes and arbitrary correlation properties at receiver as well as transmitter, thus unifying and completing existing partial solutions for special propagation scenarios. Monte-Carlo simulations of ergodic capacity verify the accuracy of the analysis.

Precoded Spatial Multiplexing MIMO for Inhome Power Line Communications

Inhome power line communications (PLC) enables new and highly convenient networking functions without any additional wires to mains-powered devices. Multiple input multiple output (MIMO) methods can significantly increase the PLC channel throughput and the link reliability. In this paper precoded spatial multiplexing is considered in more detail since it offers the best possible data throughput in the PLC channel. Precoded spatial multiplexing requires channel state information (CSI) at the transmitter. CSI feedback possibilities as well as appropriate quantization methods to reduce the amount of feedback data are investigated. We also present a new power allocation algorithm in conjunction with adaptive modulation.

Joint Base-Station Association, Channel Assignment, Beamforming and Power Control in Heterogeneous Networks

Heterogeneous cellular networks (HetNets), where low-power base-stations are overlaid with conventional macro base-stations, is a promising technique to improve coverage and capacity of the macro-only networks. To realize the potential benefits of HetNets, it is crucial to jointly optimize the user association, channel assignment, beamforming and power control to ensure that the inter- and intra-cell interference will not overwhelm the cell-splitting gains. This paper presents an iterative algorithm to solve the joint optimization problem with an objective of maximizing the network sum rate and simultaneously guaranteeing the individual user quality-of-service. The proposed algorithm is built on the so-called convex-concave procedure, and the feasibility issue is handled by L1-norm heuristic. Numerical results demonstrate the large gains over currently used methods for cellular networks.

Cooperative interference-aware joint scheduling for the 3GPP LTE uplink

In current cellular networks base stations (BSs) usually perform independent scheduling without coordinating the resource allocation among different cells. This, however, often leads to high interference levels in cellular networks operating with universal frequency reuse, such as the 3GPP UTRAN Long Term Evolution (LTE). Coordinated scheduling between different BSs may mitigate this problem by taking interference from and to nearby BSs into account in order to avoid high interference situations, especially for user equipments (UEs) located near the cell-edge. For that purpose, we propose in this paper a novel interference-aware joint scheduling scheme based on proportional fairness for the uplink of a 3GPP UTRAN LTE system, where different BSs cooperate with each other via a fast backhaul network in order to jointly allocate frequency resources to the various UEs, taking the caused inter-cell interference into account. Furthermore, we propose an efficient method for dynamic interference coordination based on the standardized high interference indicator and we compare the achievable performance with this approach to a system with interference-aware joint scheduling. It is shown that our joint scheduling scheme outperforms a dynamic interference coordination scheme and that it yields significant gains over a conventional LTE Release 8 system without any interference coordination.