Raimund Meyer

On prefilter computation for reduced-state equalization

In advanced time-division multiple-access (TDMA) mobile communications systems, reduced-state equalization algorithms have to be employed because high-level modulation is used in order to improve spectral efficiency. Reduced-state equalizers yield only high performance, if the overall discrete-time system to be equalized is minimum-phase. Therefore, in general, a discrete-time prefilter has to be inserted in front of equalization. For prefilter computation, several approaches are investigated in this paper. For the finite impulse response (FIR) prefilter case, which seems to be more relevant for practical applications than the in finite impulse response case, we discuss a method based on minimum mean-squared error decision-feedback equalization and a novel approach based on linear prediction (LP). The LP method seems to be very robust and requires an only moderate amount of computational complexity. Here, the prefilter consists of the cascade of a channel-matched filter and a prediction-error filter, which may be viewed as a finite-length approximation to the noise whitening part of the ideal prefilter transfer function. A key observation of the paper is that the proposed cascaded structure enables a very efficient prefilter computation because a prediction-error filter can be calculated via the Levinson-Durbin algorithm. Simulation results are given, which demonstrate that the performance of reduced-state equalization with proper FIR prefiltering is close to that of equalization combined with ideal all-pass prefiltering. Furthermore, it is shown that high performance can be obtained for TDMA mobile communications systems, if the LP scheme is employed for prefiltering.

An efficient method for prefilter computation for reduced-state equalization

In advanced TDMA mobile communications systems, reduced-state equalization algorithms have to be employed because a high-level modulation is used in order to improve the spectral efficiency. Such equalizers only have a high performance, if the overall discrete-time system to be equalized is minimum-phase. Therefore, in general, a discrete-time prefilter has to be inserted in front of equalization. In the literature, several approaches have been proposed for computation of a suitable FIR or IIR prefilter. We present an approach for FIR prefilter computation, which is quite robust and requires an only moderate computational complexity. The prefilter consists of the cascade of a channel-matched filter and a prediction-error filter, which can be calculated via the Levinson-Durbin algorithm. Simulation results are given, which demonstrate that the performance of the proposed approach is essentially equivalent to the case of reduced-state equalization combined with ideal allpass prefiltering.

A single antenna interference cancellation algorithm for increased gsm capacity

In mobile communications networks, system capacity is often limited by cochannel interference. Therefore, receiver algorithms for cancellation of cochannel interference have recently attracted much interest. At the mobile terminal, algorithms can usually rely only on one received signal delivered by a single receive antenna. In this letter, a low-complexity single antenna interference cancellation (SAIC) algorithm for real-valued modulation formats referred to as mono interference cancellation (MIC) is introduced which is well suited for practical applications. Field trials in commercial GSM networks using prototype terminals with the proposed MIC algorithm have demonstrated that the novel concept may yield capacity improvements of up to 80%. The underlying principle is also beneficial for adjacent channel interference and receivers with multiple antennas. Furthermore, in coverage-limited scenarios, there is no performance degradation compared with conventional receivers

A single antenna interference cancellation algorithm for GSM

In mobile communication networks, system capacity is often limited by cochannel interference. Therefore, receiver algorithms for cancellation of cochannel interference have recently attracted much interest. At the mobile terminal, algorithms can usually rely only on one received signal delivered by a single receive antenna. In this paper, a novel low-complexity single antenna interference cancellation (SAIC) algorithm for real-valued modulation formats referred to as mono interference cancellation (MlC) is introduced which is well suited for practical applications. By using this algorithm in the mobile terminals, capacity of GSM networks can be improved by up to 40-60 %.

Efficient receivers for GSM MUROS downlink transmission

Currently, Multiple Users Reusing One Slot (MUROS) is discussed in 3GPP GERAN as an extension of the GSM standard. In MUROS, two overlaid GMSK signals are transmitted in the same time slot and at the same frequency resource. By this, capacity of existing GSM networks in principle can be doubled and up to four half rate voice users can share one time slot. In this paper, channel estimation and detection is investigated for the MUROS downlink. Two novel channel estimation algorithms are presented, taking into account the specifics of the MUROS downlink. For detection, a joint MLSE of both user signals can be applied in case of noise limited scenarios. For interference limited environments, it turns out that approaches based on the mono interference cancellation (MIC) algorithm for single antenna interference cancellation (SAIC) are more favorable. It is shown that the standard MIC algorithm performs sufficiently well and could be used for a fast introduction of MUROS in existing GSM networks. For enhanced performance, a novel algorithm based on MIC along with successive interference cancellation is proposed. The presented results demonstrate that an even better performance as the GSM reference performance before introduction of SAIC can be obtained for MUROS if well-designed receivers are used.

Receiver Concepts and Resource Allocation for OSC Downlink Transmission

Voice services over Adaptive Multi-user channels on One Slot (VAMOS) has been standardized as an extension to the Global System for Mobile Communications (GSM). The aim of VAMOS is to increase the capacity of GSM, while maintaining backward compatibility with the legacy system. To this end, the Orthogonal Sub-channels (OSC) concept is employed, where two Gaussian minimum-shift keying (GMSK) signals are transmitted in the same time slot and with the same carrier frequency. To fully exploit the possible capacity gain of OSC, new receiver concepts are necessary. In contrast to the base station, where multiple antennas can be employed, the mobile station is typically equipped with only one receive antenna. Therefore, the downlink receiver design is a very challenging task. Different concepts for channel estimation, user separation, and equalization at the receiver of an OSC downlink transmission are introduced in this paper. Furthermore, the system capacity must be improved by suitable downlink power and resource allocation algorithms. Making realistic assumptions on the information available at the base station, an algorithm for joint power and radio resource allocation is proposed. Simulation results show the excellent performance of the proposed channel estimation algorithms, equalization schemes, and joint radio resource and power allocation algorithms in realistic VAMOS environments.

Radio resource allocation for OSC downlink channels

In Voice services over Adaptive Multi-user channels on One Slot (VAMOS), which has been standardized as an extension to the Global System for Mobile Communications (GSM), the concept of Orthogonal Sub-channels (OSC) is employed. Two overlaid Gaussian minimum-shift keying (GMSK) signals are transmitted in the same time slot and on the same frequency. The aim of VAMOS is to approximately double the cell capacity compared to the legacy GSM system. This work studies the downlink radio resource allocation problem. The base station (BS) can decide which users should share the same time and frequency resource within a cell. Additionally, the power allocation for each user must be considered along with a limitation of the subchannel power imbalance ratio (SCPIR) of the users in each pair. Due to frequency hopping (FH) an estimation of the co-channel interference level is not possible. Therefore, this paper proposes a radio resource allocation scheme that allocates resources without the exact knowledge of the actual interference situation. To reduce the energy consumption of the BS, the resources are allocated for a minimum sum transmit power taking into account a prescribed target frame error rate (FER) at the receiver. Simulation results show that with the proposed resource allocation scheme a significant reduction of the required transmit power is possible. Furthermore, the influence of the equalizers on the FER performance, and therefore also the energy savings are evaluated, considering different VAMOS receiver algorithms.