Multiplex communications – Communication over free space – Combining or distributing information via time channels
Patent
1997-08-20
2000-07-04
Vu, Huy D.
Multiplex communications
Communication over free space
Combining or distributing information via time channels
370203, 370320, 370335, 370342, 370503, H04L 708, H04B 726
Patent
active
060848715
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to synchronization of MC/DS-CDMA-systems. The invention accordingly relates to mobile radio systems for multicarrier/direct sequence code divided multiple access, MC/DS-CDMA. The technology is a combination of OFDM, orthogonal frequency division multiplexing, and traditional DS-CDMA, direct sequence CDMA.
PRIOR ART
It is previously known to transmit information in a data channel where the digital information is divided into a number of different blocks. The blocks are separated by guard spaces. The task of the guard spaces is to deal with the time dispersion on the channel. Further, it is known to arrange a pilot channel with the same block division and guard spaces as the data channel. Transmission units transmits a cyclic pilot sequence which is unique within the range possible to listen in to.
A base station always transmits a unique pilot sequence. This causes that a recently turned on mobile station identifies the base station/stations which are located within the range possible to listen in to. A mobile station transmits one of two different types of pilot sequences. A so called random access sequence is transmitted when a mobile station wants to make a connection to a base station. By the random access sequence being used only for this purpose, and being unique for each mobile station, the base station knows that a mobile station wants to establish a connection to it. The other type of pilot sequence the mobile station transmits during an established connection. The mobile station has been given both uplink sequences from the base station.
In the German patent document DE 3401727 is described a TDM-system for transmission of digital information which uses a synchronization channel. The synchronization channel also can contain other information, as for instance information about other performing channels.
In the European patent document EP 606941 is described a device for synchronization of a local oscillator in a OFDM-system.
The document WO 92-5646 describes reception of OFDM-signals. To achieve synchronization, a multiplication with the complex conjugate of a known reference is performed.
In the document WO 92-16063 is described reception and synchronization in an OFDM-system. According to the document, reference symbols are placed at least one carrier from the edge of the frequency raster, and at least two carriers from other reference symbols.
The European document EP 218966 describes a TDMA-system intended for radio transmission. To achieve a synchronization as good as possible, a special signal is introduced into each free time slot.
DESCRIPTION OF THE INVENTION
At transmission of packet switched digital signals between a transmitting and a receiving equipment in a communications system it is necessary to have the possibility to synchronize the signal transmission to obtain usable information in the receiving equipment. For the message transport between the transmitter and the receiver a number of access methods have been developed, of which MC/DS-CDMA is one. The method is a combination of OFDM and traditional DS-CDMA.
In an OFDM-system the pilot frequencies often become long, for instance N=512 och 1024. Such long correlations are possible but very voluminous to implement by means of digital hardware for falting. Since one in an OFDM-system always has the signals available both in the time and frequence plane, it is attractive to implement the correlation by means of multiplication in the frequency plane. At that, one exchanges a process which demands N*N complex multiplications per frame for another which demands N complex multiplications per frame. This is, however, not easy to perform since the OFDM-system operates with guard spaces (.DELTA.). The Fouriertransform of the data channel is performed with the block length N. If the pilot channel is synchronous with the data channel, the pilot channel is cyclic with the repeating distance N+.DELTA.. In order to perform the correlation in the frequency plane, one has to use a block length whi
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Engstrom Bo
Isaksson Mikael
Larsson Roger
Olofsson Sven-Rune
Olsson Lennart
Telia AB
Trinh Dung
Vu Huy D.
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