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ABSTRACT: The recently emerging high speed downlink packet access (HSDPA) enhances conventional WCDMA systems according to the UMTS standard with data rates of up to 14 MBit/s in the downlink direction. This is achieved by using adaptive modulation and coding as well as a fast hybrid automatic repeat request (HARQ) mechanism. This functionality is implemented close to the air interface in the Node B. In addition to the data buffer in the RNC, this requires a second data buffer in the Node B. Consequently, a flow control mechanism is needed which controls the amount of data to be transmitted from the RNC buffer to the Node B's buffer. The spatial separation of RNC and Node B imposes significant signaling constraints and control dead time limitations to the flow control mechanism. Additionally, due to the time-varying nature of the radio channel, the data rate towards a particular user may be highly variable. In this paper, we study the impact of the flow control on system performance. We will show that it is essential to jointly consider scheduling and flow control in an HSDPA system as the constraints imposed by the flow control may dominate the system performance
Personal, Indoor and Mobile Radio Communications, 2005. PIMRC 2005. IEEE 16th International Symposium on; 10/2005
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ABSTRACT: When simulating radio access networks (RANs) on the network level, the representation of the physical layer is on the MAC-layer can have a significant impact on the system performance. This paper focuses on the distribution of frame errors in WCDMA systems. We first study the statistical properties of the frame-error distribution as obtained by physical layer simulation. We then compare the statistical properties to those obtained from statistical channel models for both cases with activated and deactivated closed-loop power control. Finally, an envelope based approach to decide on the successful transmission or the loss of a MAC frame is presented and the applicability of this new approach is verified
Wireless Communication Systems, 2005. 2nd International Symposium on; 10/2005
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ABSTRACT: The recently emerging high speed downlink packet access (HSDPA) enhances conventional WCDMA systems according to the UMTS standard with data rates of up to 14 MBit/s in the downlink direction. This is achieved by using adaptive modulation and coding as well as a fast hybrid automatic repeat request (HARQ) mechanism. This functionality is implemented close to the air interface in the node B. In addition to the data buffer in the RNC, this requires a second data buffer in the node B. Consequently, a flow control mechanism is needed which controls the amount of data to be transmitted from the RNC's buffer to the node B's buffer. The spatial separation of RNC and node B imposes significant signaling constraints and control dead time limitations to the flow control mechanism. Additionally, due to the time-varying nature of the radio channel, the data rate towards a particular user may be highly variable. In this paper, we study the impact of the flow control on system performance. In particular, we consider the parameter choice for a previously presented algorithm and highlight some inherent tradeoffs
Wireless Communication Systems, 2005. 2nd International Symposium on; 10/2005
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ABSTRACT: A new blind channel estimation scheme for orthogonal frequency division multiplexing systems is proposed based on the maximum likelihood principle. By avoiding the use of second- and higher-order statistics, a very fast convergence rate is achieved. A novel approach is also proposed for resolving the phase ambiguity of the blind channel estimate without the need for any reference symbols. The approach combines different modulation schemes on adjacent subcarriers, such as 3-phase shift keying (PSK) and quarternary PSK (QPSK), to resolve phase ambiguity. Simulations were performed for mobile radio environments with high Doppler frequencies and short-to-medium delay spreads. The achieved performance is comparable to that of pilot-based channel estimation for the case of QPSK-modulation.
IEEE Transactions on Wireless Communications 10/2004; · 2.59 Impact Factor
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ABSTRACT: Channel estimation in OFDM systems can conveniently be done by inserting a stream of pilot symbols at the transmitter and by using FIR interpolation filters at the receiver. The drawback of this method is the decrease in spectral efficiency due to the pilot symbols. Alternatively, blind channel estimation makes pilot symbols unnecessary. Most blind channel estimation approaches are based on higher order statistics and converge slowly, making them unsuitable for mobile environments. Moreover, the channel estimate suffers from a phase blindness, which can only be resolved by pilot symbols. The concept of totally blind channel estimation makes pilots completely unnecessary, and even works in rapidly time varying environments. This is achieved by using two different modulation schemes, such as QPSK and 3-PSK, on adjacent subcarriers. We further develop the concept of totally blind channel estimation by applying a regular 8-PSK and a generalized 8-PSK to achieve totally blind channel estimation without the need for any pilot symbols. We enhance the original receiver design by applying a two-dimensional a posteriori probability (APP) calculation algorithm. We evaluate our system at high Doppler frequencies with COST 207 channels on the basis of extrinsic information transfer (EXIT) and BER charts.
Vehicular Technology Conference, 2004. VTC 2004-Spring. 2004 IEEE 59th; 06/2004
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ABSTRACT: A new two-dimensional blind channel estimation scheme for coherent detection of orthogonal frequency-division multiplexing (OFDM) signals in a mobile environment is presented. The channel estimation is based on the a posteriori probability (APP) calculation algorithm. The time-variant channel transfer function is completely recovered without phase ambiguity with no need for any pilot or reference symbols, thus maximizing the spectral efficiency of the underlying OFDM system. The phase ambiguity problem is solved by using a 4-QAM (quadrature amplitude modulation) scheme with asymmetrical arrangement. The results clearly indicate that totally blind channel estimation is possible for virtually any realistic time-variant mobile channel.
IEEE Communications Letters 12/2003; · 0.98 Impact Factor
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ABSTRACT: A new two-dimensional blind channel estimation scheme for coherent detection of OFDM signals in a mobile environment is presented. The channel estimation is based on the a posteriori probability (APP) calculation algorithm. The time-variant channel transfer function is completely recovered without phase ambiguity with no need for any pilot or reference symbols. The two-dimensional channel estimation is performed by applying a concatenation of two one-dimensional APP estimators for frequency and time direction in combination with an iterative estimation and decoding loop. The phase ambiguity problem is solved by using higher order modulation schemes with asymmetrical arrangement. The proposed approach maximizes the spectral efficiency by avoiding any reference or pilot symbols and minimizes the BER by using coherent demodulation. We investigate the performance of our algorithm with respect to the BER and study the convergence of the iterative estimation and decoding loop using extrinsic information transfer (EXIT) charts.
Vehicular Technology Conference, 2003. VTC 2003-Fall. 2003 IEEE 58th; 11/2003
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ABSTRACT: A blind channel estimation scheme for OFDM is proposed. The
algorithm is based on the ML-principle and features a very fast
convergence rate. No reference symbols are needed to resolve the phase
ambiguity. Simulations show the performance in mobile environments at
high Doppler frequencies and short to medium delay spreads. The proposed
approach maximizes the spectral efficiency by avoiding any reference
symbols or pilots, and it minimizes the BER by using a non-differential
modulation scheme
Communications, 2002. ICC 2002. IEEE International Conference on; 02/2002
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ABSTRACT: The transmission of coded DPSK signals over a time-varying channel is considered. Coded DPSK is similar to a serially concatenated coding scheme where the inner encoder is replaced by a differential modulator. At the receiver side, the a posteriori probability (APP) calculation algorithm is applied for differential demodulation, followed by an outer APP channel decoder. The likelihood values at the output of the channel decoder are fed back to the differential demodulator in an iterative decoding loop. Such a system shows large coding gain for transmission over a time-varying flat fading channel if the receiver has perfect channel state information. However, perfect channel knowledge is normally not available at the receiver side. Therefore, joint channel estimation and demodulation has to be applied, leading to a dramatic performance degradation for conventional DPSK. In order to maintain the large coding gain even without any channel knowledge, we propose a novel concept for DPSK by applying regular and generalized PSK symbols in an alternating manner. We evaluate the proposed system on the basis of extrinsic information transfer (EXIT) and bit error ratio (BER) charts.
Vehicular Technology Conference, 2005. VTC 2005-Spring. 2005 IEEE 61st;
