Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1997-11-17
2001-02-13
Chin, Stephen (Department: 2734)
Pulse or digital communications
Spread spectrum
Direct sequence
Reexamination Certificate
active
06188717
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of simultaneous radio transmission of digital data between a plurality of subscriber stations and a base station.
REVIEW OF THE RELATED TECHNOLOGY
In modern radio-transmission systems, particularly in cellular mobile-radio systems, a transmission method for digital signal transmission is required to be able to supply a large number of active subscriber stations with variable data rates for multimedia services (audio, video, text, data, etc.) must be used for digitally transmitting signals (data). Current and future mobile-radio systems should include mobile subscriber stations that are small, flexible, reliable and robust, and consume little energy to permit long battery operation.
In particular, future cellular mobile-radio systems must be designed to possess a high spectral-efficiency with respect to the transmission method used, despite mobile-radio channel disturbances and the intended high number of subscribers, so a plurality of active subscriber stations can transmit on the available mobile-radio channel. To avoid extreme complexity, and to use identical components, the transmission method should also be applicable for both an uplink, i.e., in the direction from a subscriber station to the base station, and a downlink, i.e., in the direction from a base station to the subscriber station.
The modulation method of future mobile-radio systems should permit a coherent detection with the use of channel-state information, at low cost and with high reliability, and prevent interference (MAI, Multiple Access Interference) between the subscriber stations of a base station.
The detection method of a future cellular mobile-radio system should further permit a low-cost maximum-likelihood sequence estimation (optimum estimation) for common detection of the data of a subscriber station.
The different data streams (audio, video, text, data, etc.) must be transmitted virtually error-free at a variable data rate (a few kbit/s to about 2 Mbit/s) on the available mobile-radio channel.
Different known digital radio-transmission methods already include measures for meeting these requirements, which are expected to be especially required in future cellular mobile-radio systems. The multi-carrier modulation method OFDM (Orthogonal Frequency-Division Multiplexing) with a guard interval can be used as a modulation method having a high spectral efficiency; this method is known from the essay by S.
Weinstein and P. M. Ebert, “Data Transmission by Frequency-Division Multiplexing using the Discrete Fourier Transform” in IEEE Trans. Commun. Tech., Vol. COM-19, pp. 628-634, October 1971.
With a given signal-to-noise ratio and the respective channel properties, channel encoding permits the reception of the signal with the desired biterror probability. Moreover, it has been proposed to combine the multi-carrier modulation method OFDM with the CDMA (Code-Division Multiple Access) method (refer to K. Fazel and L. Papke, “On the performance of convolutionally-coded CDMA/OFDM for mobile communication system” in Proc. IEEE Int. Symp. on Personal, Indoor and Mobile Radio Commun. (PIMRC'93), pp. D3.2.1-D3.2.5, September 1993).
It is known that signal-level fluctuations frequently occur during transmission on radio channels, particularly on mobile-radio channels; these fluctuations can be caused by multi-path propagation, temporal change in the channel-transmission behavior and, particularly in mobile radio, by movement of a subscriber station. The time-varying multi-path propagation causes intersymbol interferences (ISI) in the received signal because of different signal arrival times over the individual reflection paths and, consequently, signal fading due to destructive signal superposition. The channel fading has correlations in the time and frequency range. The errors on such channels are therefore frequently bundled and statistically-dependent.
A number of options for eliminating channel disturbances are known. A narrow-band system having equalization can be used. A system of this type with the TDMA (Time-Division Multiple Access) method is already in use in cellular mobile-radio standard GSM (Global System for Mobile Communication). In GSM, the entire transmission bandwidth of 25 MHz is subdivided into 125 channels of 200 kHz. In each 200 kHz-wide channel, the TDMA method is used with GMSK (Gaussian Minimum Shift Keying) modulation. The maximum number of active subscribers per channel is 8 (at a data rate of 13 kbit/s). As a result, each channel has a spectral efficiency of about 0.52 bit/s/Hz.
A second known option for eliminating channel disturbances consists of using a broadband system with spread spectrum and rake receivers based on the CDMA method. This system is included in US Mobile-Radio Standard IS-95, in which all subscriber stations of a cell use the entire channel bandwidth of 1.25 MHz. Each subscriber station has its own code. QPSK (Quadrature Phase Shift Keying) and offset QPSK are used as modulation method. The maximum number of active subscriber stations in a cell, that is, in the region of a base station, is less than 60 (at a data rate of 9.6 kbit/s). Consequently, each channel as a spectral efficiency of less than 0.46 bit/s/Hz.
A third known option for eliminating channel disturbances consists of using a broadband system with Orthogonal Frequency-Division Multiplexing (OFDM). A system this type, having a guard interval, has already been selected for digital audio and terrestrial video broadcasting standards DAB (Digital Audio Broadcasting) and DVB-T (Digital Video Broadcasting Terrestrial)in Europe. The DQPSK (Differentially-Encoded Quadrature Phase Shift Keying) modulation method with robust channel encoding is used in DAB. The spectral efficiency in DAB is about 0.75 bit/s/Hz per channel. In DVB-T, a multi-resolution QAM (Quadrature Amplitude Modulation). is used, with which a flexible selection of up to a 64-QAM constellation can be made. A spectral efficiency of up to 4.5 bit/s/Hz per channel results.
A fourth known option for eliminating channel interference lies in the combination of the broadband spread-spectrum system with the OFDM and COMA methods, as mentioned in the literature references. The spectral efficiency of this method, with BPSK (Binary Phase Shift Keying) modulation, is about half the spectral efficiency of DAB. With QPSK the same spectral efficiency as in DAB can be obtained.
Of the four aforementioned options for eliminating channel disturbances, the narrow-band system with equalization (GSM) and the broadband system with spread spectrum and rake receivers (IS-95) having the drawback of requiring an extremely complex estimation of the channel-state information in the case of coherent detection. To reduce costs, coherent estimation was only used for the downlink in IS-95. For the uplink, a higher transmission power must be used. The spectral efficiency of these two systems is fairly low, namely 0.52 bit/s/Hz and lower.
The advantage of combining the broadband spread-spectrum system with multi-carrier modulation having a guard interval is that no intersymbol interference (ISI) equalization and no rake receiver are required, and the embodiment is relatively simple. The known spread-spectrum multiple-access systems employing the OFDM multi-carrier modulation method and a guard interval are nevertheless only designed for the downlink of a cellular mobile-radio system, because the channel estimation is not problematic in this case.
At this time there is no known method or apparatus that allows the use of synchronous spread-spectrum multiple-access systems having multi-carrier modulation and a guard interval in the uplink with coherent detection, because it is very difficult to estimate the channel-state information of the different transmission channels of all active mobile subscribers simultaneously in the base station.
SUMMARY OF THE INVENTION
Accordingly, the present invention has an object, among others, to overcome deficiencies in the prior art such as noted above.
It is an object of the.invention to
Fazel Khaled
Kaiser Stefan
Browdy & Neimark
Chin Stephen
Deutsche Forschungsanstalt fur Luft-und Raumfahrt e.V.
McKiernan Thomas E
LandOfFree
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