Alexander Arkhipov

L-DACS1 physical layer laboratory demonstrator

In this paper, an L-DACS1 physical layer laboratory demonstrator is presented which has been implemented recently in FPGA technology by the German Aerospace Center (DLR). The main goal of this lab demonstrator is to perform first compatibility measurements between L-DACS1 and legacy L-band systems where interference from L-DACS1 towards the legacy systems as well as interference from the legacy systems onto the L-DACS1 receiver is considered. The lab demonstrator is already implemented up to the first intermediate frequency at 10.7 MHz. The radio frontend for up-/down-conversion to/from the L-band is scheduled to be delivered in May this year. First compatibility tests are planned to take place in the labs of the German ATC authority DFS this summer. Using real hardware equipment, these tests will give valuable insight into the L-band coexistence issue and possible deployment scenarios.

OFDMA-CDM performance enhancement by combining H-ARQ and interference cancellation

In this paper, superimposed packet allocation for orthogonal frequency-division multiple-access code-division multiplexing (OFDMA-CDM) is presented, where each transmitted packet is associated with one spreading code. An iterative algorithm which is a combination of parallel interference cancellation and hybrid automatic repeat request (H-ARQ) based on soft value combining (SVC) is proposed, and its performance is studied and compared with other existing H-ARQ schemes. The proposed algorithm exploits the reliability information of erroneously received copies of the same data packet to improve the performance of interference cancellation. The interference of correctly received packets is ideally reconstructed and subtracted. Thus, the overall system performance improves iteratively. As a result, the proposed algorithm outperforms conventional H-ARQ based on SVC, as well as H-ARQ based on maximum ratio combining.

Sampling Time Offset Estimation for IFDMA Uplink Systems

This article investigates the influence of sampling time offset on the performance of an interleaved frequency-division multiple-access (IFDMA) uplink scheme for mobile radio transmission. It is shown that sampling time offset causes signal-to-noise ratio (SNR) degradation and leads to inter-symbol interference (ISI). In addition, an efficient algorithm is proposed which is capable to estimate sampling time offset by applying oversampling at the receiver. The proposed algorithm requires only three-times oversampling, does not use any pilot symbols, and is independent from the transmission channel and modulation alphabet.

ESTIMATION OF SAMPLING TIME MISALIGNMENTS IN IFDMA UPLINK

This article investigates the influence of sampling time offset on the performance of an interleaved frequency-division multiple-access (IFDMA) uplink scheme for mobile radio transmission. It is shown that sampling time offset causes signal-to-noise ratio (SNR) degradation and leads to inter-symbol interference (ISI). In addition, an efficient algorithm is proposed which is capable to estimate sampling time offset by applying oversampling at the receiver. The proposed algorithm requires only three-times oversampling, does not use any pilot symbols, and is independent from the transmission channel and modulation alphabet.

Optimum equalizer for OFDM systems with windowing

Addition of a postfix in orthogonal frequency division multiplexing (OFDM) symbol makes it possible to apply at the receiver a window different from the rectangular one and change the subcarrier spectrum. In doing so, inter-carrier interference (ICI) caused by frequency offset can be minimized. In this letter, we propose a minimum mean square error (MMSE) algorithm for the OFDM equalization which allows to utilize the energy invested into the postfix. As a result, the bit-error rate (BER) of an OFDM system can be improved.

Suppression of ISI Caused by Sampling Time Offset in IFDMA Systems

In this article, we investigate the influence of sampling time offset on the bit error rate (BER) performance in interleaved frequency-division multiple-access (IFDMA) uplink systems. It is shown that even small sampling time offset results in inter-symbol interference (ISI) and in signal-to-noise ratio (SNR) degradation. To avoid this, a soft ISI suppression algorithm is proposed which allows to significantly improve the system performance. The proposed algorithm can be applied to both IFDMA uplink systems and to conventional orthogonal frequency-division multiple-access (OFDMA) uplink systems.

NETp1-12: Frequency Offset Estimation for IFDMA Uplink Systems

This paper proposes two frequency offset estimation algorithms for the uplink of an interleaved frequency-division multiple-access (IFDMA) system. One algorithm performs estimation in the frequency domain and the other in the time domain. Both algorithms are based on the maximum likelihood estimation (MLE) principle and use knowledge about pilot symbols. IFDMA utilizes a block-interleaved frequency allocation scheme to exploit the frequency diversity of the channel. In the presence of frequency offsets between users, multiple-access interference (MAI) appears, which has a negative impact on existing frequency offset estimation algorithms. The proposed algorithms are robust, since a special construction of pilot symbols allows to exclude a large amount of MAI in the presence of frequency offsets between users. As a result, the proposed time domain frequency estimation algorithm outperforms the frequency domain algorithm and all other known schemes.

COMBINATION OF H-ARQ AND ITERATIVE MULTI-USER DETECTION FOR OFDMA-CDM

In this paper, superimposed packet allocation for orthogonal frequency- division multiple-access code-division multiplexing (OFDMA-CDM) is presented, where each transmitted packet is associated with one spread- terference cancellation (PIC) and hybrid automatic repeat request (H- formance is studied and compared with other existing H-ARQ schemes. The interference of correctly received packets is ideally reconstructed and subtracted; thus, the overall system performance improves itera- tively .A s a result, the proposed algorithm outperform sc on ventional H-ARQ based on SV Ca swell as H-ARQ based on maximum ratio com- bining (MRC).

Advantages of superimposed packets allocation for OFDM-CDM

In this paper, superimposed packets allocation for orthogonal frequency-division multiplexing code-division multiplexing (OFDM-CDM) is presented. An iterative algorithm, which is a combination of parallel interference cancellation (PIC) and automatic repeat request (ARQ) based on soft value combining (SVC) is proposed and its performance studied and compared with other existing ARQ schemes. In the proposed algorithm, reliability information of erroneously received copies of the same data packet is exploited to improve the quality of interference cancellation. As a result, the proposed algorithm outperforms conventional ARQ based on a SVC as well as ARQ based on maximum ratio combining (MRC).