Stefan Kaiser

Multi-Carrier and Spread Spectrum Systems

From the Publisher: Frequency spectrum is a limited and valuable resource for wireless communications. A good example can be observed among network operators in Europe for the prices to pay for UMTS-frequency bands. Therefore, the first goal when designing future wireless communication systems (e.g. 4G - fourth generation) has to be the increase in spectral efficiency. The development in digital communications in the past years has enabled efficient modulation and coding techniques for robust and spectral efficient data, speech, audio and video transmission. These are the multi-carrier modulation (e.g. OFDM) and the spread spectrum technique (e.g. DS-CDMA), where OFDM was chosen for broadcast applications (DVB, DAB) as well as for broadband wireless indoor standards (ETSI HIPERLAN-II, IEEE-802.11) and the DS-CDMA was selected in mobile communications (IS-95, third generation mobile radio systems world wide, UMTS/IMT 2000). Since 1993 various combinations of multi-carrier (MC) modulation and the spread spectrum (SS) technique have been introduced and the field of MC-SS communications has become an independent and important research topic with increasing activities. New application fields have been proposed such as high rate cellular mobile, high rate wireless indoor and LMDS. It has been shown that MC-SS offers the high spectral efficiency, robustness and flexibility that is required for the next generation systems. Meanwhile, different alternative hybrid schemes such as OFDM/OFDMA, MC-TDMA, etc. have been deeply analysed and adopted in different international standards (ETSI-BRAN, IEEE-802 & MMAC). Multi-Carrier & Spread-Spectrum: Analysis of Hybrid Air Interfaces draws together all ofthe above mentioned hybrid schemes therefore providing a greatly needed resource for system engineers, telecommunication designers and researchers in order to enable them to develop, build and deploy several schemes based on MC-transmission for the next generation systems (which will be an integration of broadband multimedia services covering both 4G mobile and fixed wireless systems). Offers a complete treatment of multi-carrier, spread-spectrum (SS) and time division multiplexing (TDM) techniquesProvides an in-depth insight into hybrid multiple access techniques based on multi-carrier (MC) transmissionPresents numerous hybrid multiple access and air interface architectures including OFDM/CDMA, MC-CDMA, MC-DS-CDMA and MT-CDMACovers new techniques such as space-time coding and software radio Telecommunications engineers, hardware & software system designers and researchers as well as students, lecturers and technicians will all find this an invaluable addition to their bookshelf.

Two-dimensional pilot-symbol-aided channel estimation by Wiener filtering

The potential of pilot-symbol-aided channel estimation in two dimensions are explored. In order to procure this goal, the discrete shift-variant 2-D Wiener filter is derived and analyzed given an arbitrary sampling grid, an arbitrary (but possibly optimized) selection of observations, and the possibility of model mismatch. Filtering in two dimensions is revealed to outperform filtering in just one dimension with respect to overhead and mean-square error performance. However, two cascaded orthogonal 1-D filters are simpler to implement and shown to be virtually as good as true 2-D filters.

Pilot-symbol-aided channel estimation in time and frequency

The potentials of pilot-symbol-aided channel estimation in two dimensions are explored for mobile radio and broadcasting applications. In order to procure this goal, the discrete shift-variant 2-D Wiener filter is analyzed given an arbitrary sampling grid, an arbitrary (but possibly optimized) selection of observations, and the possibility of model mismatch. Filtering in two dimensions is revealed to outperform filtering in just one dimension with respect to overhead, mean-square error performance and latency. Conceptually, the discrete shiftvariant 2-D Wiener filter is the optimal linear estimator for the given problem, however, two cascaded orthogonal 1-D filters are simpler to implement and virtually as good as true 2-D filters. Analytical results are presented, verified by Monte-Carlo simulations.

Standard conformable antenna diversity techniques for OFDM and its application to the DVB-T system

In this paper, we investigate different antenna diversity concepts, which can be easily applied to orthogonal frequency division multiplexing (OFDM) systems. The focus is to provide diversity schemes, which can be implemented to already existing OFDM systems without changing the standards. It is also possible to combine these techniques in a suitable manner. The introduced diversity techniques are applied to the DVB-T system for error performance investigations, which were done by simulation.

Analysis of the effects of phase-noise in orthogonal frequency division multiplex (OFDM) systems

In OFDM transmission schemes, phase-noise from oscillator instabilities in the receiver is a potentially serious problem, especially when bandwidth efficient, high order signal constellations are employed. The paper analyses the two effects of phase-noise: inter-carrier interference (ICI) and a phase error common to all OFDM sub-carriers. Through numerical integration, the ICI power can be evaluated and is shown as a function of the number of OFDM sub-carriers and various parameters of the phase-noise model. Increasing the number of sub-carriers causes an increase in the ICI power, which our analysis indeed shows to become a potential problem, since it can lead to a BER floor. The analysis allows the design of low-cost tuners through specifying the required phase-noise characteristics. A similar technique is applied to calculate the variance of the common phase error. After showing that the common phase error is essentially uncorrelated from symbol to symbol, we propose a simple feed-forward correction technique based on pilot cells, that dramatically reduces the degradation due to phase-noise. This is confirmed by BER simulations of a coded OFDM scheme (proposed for terrestrial transmission of digital television) with 64 QAM.

Spatial transmit diversity techniques for broadband OFDM systems

In this paper, we investigate different spatial transmit diversity concepts for orthogonal frequency division multiplexing (OFDM) systems. The idea is to artificially increase the frequency and time selectivity of the resulting channel transfer function at the receiver antenna by specific time-variant phase rotations of the signal at the transmit antennas. The achievable performance gains with the proposed transmit diversity concepts are presented for convolutionally coded OFDM systems in typical indoor and outdoor environments. The new concepts can be implemented in already standardized and existing OFDM systems like DAB and DVB-T or HIPERLAN/2, IEEE 802.11 and MMAC, without changing the standards and the receivers. Moreover, we show further performance improvements by combining the spatial transmit diversity concepts for OFDM with code division multiplexing (CDM).

Multi-carrier CDMA with iterative decoding and soft-interference cancellation

A novel multi-user detection scheme applying iterative soft-interference cancellation in multi-carrier code division multiple access (MC-CDMA) systems is proposed and investigated. The novel detection scheme cancels the interference from the other users by taking into account reliability information about the detected interference. For this purpose, we do not use the decisions from the inner decoder (despreader) but the more reliable soft-values from the outer soft-in/soft-out decoder to reduce error propagation. It can be shown that the iterative soft-interference cancellation scheme clearly outperforms an MC-CDMA system with inner joint detection based on maximum likelihood sequence estimation (MLSE) which is followed by an outer channel decoder. Furthermore, the bit error rate (BER) obtained with the proposed scheme closely approaches the single-user bound. Hence, iterative soft-interference cancellation is able to eliminate almost all multiple access interference.

A Flexible and High Performance Cellular Mobile Communications System Based on Orthogonal Multi-Carrier SSMA

The conventional Direct-Sequence Spread-Spectrum-Multiple-Access (DS-SSMA) system with RAKE-receiver is an interesting approach in mobile communications in order to combat the frequency selectivity of the channel. The main advantages of DS-SSMA are well known, but its capacity is limited by other-user interference. Another interesting SSMA technique that combats the frequency selectivity of the channel and achieves high spectral efficiency is based on Orthogonal-Frequency-Division-Multiplexing (OFDM). This new SSMA technique combines the principles of DS-SSMA with OFDM and is called Multi-Carrier Spread-Spectrum-Multiple-Access (MC-SSMA). By providing high frequency diversity, this combination enables the possibility to perform a maximum-likelihood detection (MLD) resulting in high performance/capacity, to use the spectrum in an efficient way and to retain many advantages of the conventional DS-SSMA system. In addition, it allows both simple cell-separation by using frequency hopping and simple hardware realization. An example for the downlink of a mobile communications system, i.e. the transmission from the base station to a mobile station, using MC-SSMA with Walsh-Hadamard code spreading is studied. Different coherent detection algorithms such as conventional detection, MLD, and iterative detection are analyzed. The analytical performance of MLD is evaluated. It is also shown analytically, and by simulations that the MC-SSMA system with MLD outperforms the DS-SSMA system with RAKE-receiver. Up to 64 active users can be transmitted in a 1.25 MHz bandwidth. Each user has a rate of 16.0 kbit/s by using BPSK modulation. At a signal to noise ratio (SNR) of 13 dB a bit error probability ofPb=10−3 can be guaranteed without channel coding. This results in a spectral efficiency of about 0.8 bit/s/Hz. Under the same conditions the conventional DS-SSMA system results in a spectral efficiency of about 0.15 bit/s/Hz. Hence, using MC-SSMA for the downlink of a cellular mobile communications system is a promising approach.

Analysis of the loss of orthogonality through Doppler spread in OFDM systems

The paper quantifies the significant effects of Doppler spread in a time variant mobile radio channel on the performance of OFDM multi-carrier systems. A suitable model for OFDM is introduced and serves as the basis for the derivation of the inter-carrier interference (ICI) due to the loss of sub-carrier orthogonality. Several Doppler spectra are analyzed and compared, such as the classical and uniform distributions, as well as the case of two distinct values for the Doppler frequency. We can thus employ the computed variance of the ICI process, together with the channel noise, in system analyses and designs, and avoid lengthy time-domain simulations. We also analyze optimal receiver frequency synchronisation in the case of Doppler spreads.

Transmit/Receive‐antenna diversity techniques for OFDM systems

In this paper, we investigate different antenna diversity concepts, which can be easily applied to orthogonal frequency division multiplexing (OFDM) systems. The focus is on standard compatibility, i.e. these schemes can be implemented to already existing OFDM systems without changing the standards. The introduced diversity techniques are applied exemplarily to the DVB-T system. Bit error performance investigations were done by simulation for different DVB-T and diversity parameter sets.