Markku Pukkila

Constant modulus single antenna interference cancellation for GSM

In cellular systems, interference suppression at the mobile receiver can be used to improve the downlink performance and capacity. We investigate single antenna interference cancellation (SAIC) at the receiver for a GSM/EDGE system with GMSK modulation. An interference suppression technique, which exploits the constant envelope property of the GMSK modulated signal, was proposed by Berangi et al in (Wireless Pets. Commun., Kluwer Acad. Pub., no.19, p.37-55, 2001). We observed that such technique is very sensitive to channel estimation error and hardly provides any gain when least square (LS) channel estimation is employed. Therefore, we study the performance of constant modulus single antenna interference cancellation (CM-SAIC) with multi channel estimation and show that significant performance improvement can be achieved. In the presence of a single dominant interfering signal, we obtain up to 7 dB gain in the TU3 channel.

Iterative data detection and channel estimation for advanced TDMA systems

This letter presents a new receiver for Q-ary transmission, where all receiver blocks are embedded in an iterative structure. Packet data transmission in Global Systems for Mobile communications (GSM) and Enhanced Data rates for Global Evolution (EDGE) are considered as examples. A low-complexity soft-in-soft-out detector for EDGE is introduced and its modification suitable for iterative detection is derived. Application of iterative detection and channel estimation techniques in GSM/EDGE shows a significant performance enhancement. Additional improvement may be obtained if the iterative processing is applied to packet retransmission schemes.

Iterative channel estimation for GPRS

In this paper we consider iterative estimation and equalization techniques and present a simple method of updating channel estimates that includes decoder outputs into the iteration process. To clarify performance-complexity trade-off we evaluated iterative estimation and turbo equalization techniques in the General Packet Radio System (GPRS). It is found that turbo estimation is more beneficial for GPRS, showing 1 dB gain after one channel estimate update, while turbo equalization rounds provide only a slight improvement on the top of that.

Cochannel interference suppression for constant modulus signals

In this paper we investigate the single antenna interference cancellation (SAIC) for the GSM/EDGE system with the GMSK modulation. If the received signal is a superposition of the desired signal and constant modulus (CM) interference, the semi-blind method proposed by Berangi and Leung in 2001 can be used to suppress interference and thereby improve the receiver performance. However, they assume an ideal channel state information which is not available in practical cellular systems. Furthermore, the CM-SAIC algorithm is rather sensitive for channel estimation error and the suppression gain can be lost due to inaccurate estimation. For instance, the CM-SAIC with least squares (LS) channel estimation provides hardly any gain. Therefore we introduce a novel iterative channel estimation method based on the CM criterion. When combined with the CM-SAIC detector significant performance improvement can be achieved. In the presence of single dominant interfering signal we obtain up to 5 dB gain in TU3 channel.

Turbo equalisation for the Enhanced GPRS system

The iterative receiver structures can often improve the performance with a reasonable level of complexity. We study an iterative equalisation technique, i.e., turbo equalisation (TE), which enables to improve the performance by signal processing means in the receiver-without altering any other part of the system. TE is based on the feedback information from the channel decoder into the equaliser. That information is used as a priori information in the equaliser to start a new iteration, which provides more reliable decisions. The performance evaluation for Enhanced General Packet Radio System (EGPRS) shows that TE is beneficial for 8-PSK modulation in particular giving a 2 dB gain with one extra iteration. However, TE requires a soft-in-soft-out (SISO) decoder to be used, since the feedback information from the decoder should contain reliability information.

Iterative receiver concept for TDMA packet data systems

In this paper we present an iterative receiver concept that improves the radio link performance of TDMA mobile communication systems. We consider sub-optimal receiver structures comprising of channel estimator, detector and channel decoder, where the performance is improved by iterative data processing among the receiver blocks. As a practical example, we consider packet data transmission in GSM and Enhanced Data rates for Global Evolution (EDGE). Low-complexity soft-in-soft-out (SISO) equalizers for EDGE are introduced and its modifications suitable for iterative detection in EDGE are derived. Application of iterative detection and channel estimation techniques in GSM/EDGE shows a significant performance improvement. Furthermore, we show that retransmission schemes specified for EDGE also benefit from iterative data processing. Copyright

Turbo equalization with low complexity decoder

Turbo equalization (TE) is an iterative receiver algorithm repeating equalization and channel decoding to improve receiver performance. As equalization and decoding are performed several times, the complexity of the TE receiver increases respectively. Furthermore, the decoder needs to provide probability information as output, which increases the computational burden even more. In this paper we introduce a new method to reduce the decoding complexity of the TE receiver. This soft trellis decoding (STD) technique utilises information obtained from the previous decoding step to select those trellis metrics that need to be recalculated. Hence, the most unlikely trellis branches are eliminated. We evaluate STD performance in the Enhanced General Packet Radio System (EGPRS) platform by simulations. We show that STD reaches performance close to full trellis decoding, but requires only a fraction of the computational power.

Interference suppression by joint demodulation of cochannel signals

Inter-cell cochannel interference (ICCI) is an inherent problem in all cellular systems due the necessity to reuse the same frequencies after a certain reuse distance. In GSM, the fact that the number of nearby cochannel interferers is relatively small leads to a high probability of a dominant interferer (DI). Hence, suppression of DI alone provides substantial capacity improvement for GSM. The paper summarises different aspects of interference suppression by joint demodulation of cochannel signals in the GSM system. The probability of DI is investigated by network simulations. Moreover, receiver algorithms are described and receiver performance analysis is provided. In addition, requirements that the application of the technique poses for the GSM systems are explained.

Space-time trellis coding with turbo equalisation for the EGPRS system

Transmit diversity techniques like delay diversity (DD) or space-time trellis coding (STTC) utilise several transmission antennas and thereby protect mobile radio systems against fast fading. The more complex STTC can recover bursts with a few errors, but on the other hand it increases bit errors for bad bursts. Therefore the concatenation of an outer convolutional code and STTC is not superior to the simple DD scheme. The outer coding is commonly used in mobile packet data applications to detect and correct transmission errors. We are interested in further improving STTC performance by the iterative turbo equalisation (TE) technique, which performs space-time decoding-equalisation and outer decoding in an iterative fashion. Our objective is to analyse whether TE can exploit the more complex STTC structure by comparing the iterative gains of STTC and DD. The performance evaluation is done by simulations in the Enhanced General Packet Radio System (EGPRS) platform.