A rapid synchronization method is presented for an orthogonal frequency-division multiplexing (OFDM) system using either a continuous transmission or a burst operation over a frequency-selective channel. The presence of a signal can be detected upon the receipt of just one training sequence of two symbols. The start of the frame and the beginning of the symbol can be found, and carrier frequency offsets of many subchannels spacings can be corrected. The algorithms operate near the Cramer-Rao lower bound for the variance of the frequency offset estimate, and the inherent averaging over many subcarriers allows acquisition at very low signal-to-noise ratios (SNRs).

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Robust frequency and timing synchronization for OFDM

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.

/pdf/multi-carrier-and-spread-spectrum-systems-4jp27uw6t7.pdf

Multi-Carrier and Spread Spectrum Systems

The communication system includes a hybrid fiber/coax distribution network. A head end provides for downstream transmission of telephony and control data in a first frequency bandwidth over the hybrid fiber/coax distribution network and reception of upstream telephony and control data in a second frequency bandwidth over the hybrid fiber/coax distribution network. The head end includes a head end multicarrier modem for modulating downstream telephony information on a plurality of orthogonal carriers in the first frequency bandwidth and demodulating upstream telephony information modulated on a plurality of orthogonal carriers in the second frequency bandwidth. The system includes service units, operatively connected to the hybrid fiber/coax distribution network for upstream transmission of telephony and control data and for receipt of the downstream control data and telephony. Also provided is a method and apparatus for performing a Fast Fourier Transform (FFT).

Communication system with multicarrier telephony transport

In this paper, the problem of carrier synchronization of OFDM systems in the presence of a substantial frequency offset is considered. New frequency estimation algorithms for the data aided (DA) mode are presented. The resulting two stage structure is able to cope with frequency offsets in the order of multiples of the spacing between subchannels. Key features of the novel scheme-which are presented in terms of estimation error variances, the required amount of training symbols and the computational load-ensure high speed synchronization with negligible decoder performance degradation at a low implementation effort. >

/pdf/frequency-synchronization-algorithms-for-ofdm-systems-2te1fwhrzv.pdf

Frequency synchronization algorithms for OFDM systems suitable for communication over frequency selective fading channels

This paper describes a novel channel estimation scheme identified as coded decision directed demodulation (CD3) for coherent demodulation of orthogonal frequency division multiplex (OFDM) signals making use of any constellation format [e.g., quaternary phase shift keying (QPSK), 16-quadrature amplitude modulation (QAM), 64-QAM]. The structure of the CD3-OFDM demodulator is described, based on a new channel estimation loop exploiting the error correction capability of a forward error correction (FEC) decoder and frequency and time domain filtering to mitigate the effects of noise and residual errors. In contrast to the conventional coherent OFDM demodulation schemes, CD3-OFDM does not require the transmission of a comb of pilot tones for channel estimation and equalization, therefore yielding a significant improvement in spectrum efficiency (typically between 5-15%). The performance of the system with QPSK modulation is analyzed by computer simulations, on additive white Gaussian noise (AWGN) and frequency selective channels, under static and mobile reception conditions. For convolutional coding rate 1/2, the results indicate that CD3-OFDM allows one to achieve a very fast adaptation to the channel characteristics in a mobile environment (maximum tolerable Doppler shift of about 80 Hz for an OFDM symbol duration of 1 ms, as differential demodulation) and an E/sub b//N/sub 0/ performance similar to coherent demodulation (e.g., E/sub b//N/sub 0/=4.3 dB at bit-error rate (BER)=2/spl middot/10/sup -4/ on the AWGN channel). Therefore, CD3-OFDM can be suitable for digital sound and television broadcasting services over selective radio channels, addressed to fixed and vehicular receivers.

CD3-OFDM: a novel demodulation scheme for fixed and mobile receivers

An example of a modem for two-resolution digital terrestrial transmission, providing two programs in a single 8 MHz channel is described. For each program, it is desired to transmit 5 Mb/s to portable receivers and additionally 5 Mb/s to fixed receivers. In the example, orthogonal frequency division multiplexing (OFDM) is used with guard intervals as in digital audio broadcasting. The two-resolution transmission is achieved by using a two-resolution quadrature amplitude modulation (QAM) signal constellation. Reed-Solomon coding gives burst error correction capability to combat the frequency selective nature of the transmission channel. The two-resolution system can easily be reconfigured into a compatible system, providing high-definition television to fixed receivers. >

An example of a multi-resolution digital terrestrial TV modem

The RACE dTTb project (Digital Terrestrial Television Broadcasting), launched in spring 1992, finalized the definition of its first hardware demonstrator during the first quarter of 1994. Development tasks have been shared by dTTb partners in order to realize a complete equipment by beginning of 1995. The objective of the project is to offer a significant contribution for the standardization of the European digital terrestrial television system; this should be based on a well-established experience relying on field trials using real-time hardware. Therefore, this first demonstrator will provide a large flexibility in relation with service facilities and network operation facilities. Concerning service facilities, the chosen modulation and channel coding schemes should allow multiprogramming capabilities as well as hierarchical services, with downward and upward compatibilities. This means that fixed as well as portable reception should be optimally supported. Concerning network operation facilities, the system will be designed to handle the most stringent multipath propagation conditions. This opens the possibility to operate Single Frequency Networks (SFN), which allows for an optimized field strength distribution, thus avoiding the use of very high power transmitters and reducing the frequency re-use distances.

An overview of the modulation and hcannel coding schemes developed for digital terrestrial television broadcasting within the dTTb project

The paper describes measurements of indoor reception of a 7 MHz OFDM signal transmitted at a center frequency of 762 MHz. The channel impulse response and the selectivity characteristics in the frequency domain and the spatial domain are measured. Furthermore, an indication is given of the effect of building attenuation. Building penetration loss is due to attenuation or shielding of the radio signals by walls, floors and ceilings. Using over 300 measurement runs in different rooms at different floors in 40 residential houses, apartment buildings and office buildings in Eindhoven, the building penetration loss can be determined. From the measurements, it can for example be determined that the typical building penetration loss in 90% of the buildings (macro-scale) will be smaller than 15 dB. If one decreases the height of an outdoor receiving antenna, the received signal power will usually decrease as well. This is partially due to shielding by buildings or other obstacles in the line-of-sight path. From the measurements, it appears that the combined height/building penetration loss compared with a 10 m outdoor antenna, will be less than 22 dB in 90% of the buildings.

Propagation of broadcast signals into buildings at 762 MHz

This paper describes the channel conditions under which digital OFDM-based broadcasting systems should operate. Error correcting codes are used to improve the transmission reliability. For further improving the performance of the codes, the receiver needs knowledge about the channel conditions.

http://ieeexplore.ieee.org/iel3/1359/6936/00280186.pdf

P.G.M. de Bot

Papers

An example of a multi-resolution digital terrestrial TV modem

An overview of the modulation and hcannel coding schemes developed for digital terrestrial television broadcasting within the dTTb project

Propagation of broadcast signals into buildings at 762 MHz

Error correcting coding for OFDM broadcasting over frequency selective channels