Multiplex communications – Communication techniques for information carried in plural... – Combined time division and frequency division
Reexamination Certificate
1998-01-07
2001-08-14
Vu, Huy D. (Department: 2664)
Multiplex communications
Communication techniques for information carried in plural...
Combined time division and frequency division
C370S459000, C455S455000, C455S516000
Reexamination Certificate
active
06275506
ABSTRACT:
The invention relates to a radio transmission method in accordance with the preamble of Patent claim
1
.
The transmission of information between the strongly increasing number of multimedia terminals, for example television sets (portable or fixed TV sets), video recorders, computers (fixed personal computers or portable laptops), personal digital assistants (PDA), cordless telephones, alarm systems etc., in a local network (LAN, Local Area Network), for example in an indoor network, and the execution and organization of said transmission result in novel local networks.
Furthermore, in order to avoid network cabling it is desirable for these local networks to operate in a wireless manner via radio frequency (RF) radio connections. Furthermore, each of these terminals also receive from outside different types of signals which in turn originate from various networks. As a result, a local network has to cope with a very wide variety of data rates. Thus, for example in the case of digital television (DVB, Digital Video Broadcasting) the data rate per program can translate up to 6 Mbit/s or, in the case of high-resolution television HDTV (High Definition Television) even up to 24 Mbit/s. In contrast, the data rate of the ISDN service which operates via a telephone line is only 128 kbit/s. For a data transfer between two computers, 2 Mbit/s are required, for example.
An example of digital services with various data rates within a building or within a flat, that is to say a grouping (so-called cluster) of a wide variety of subscriber stations is given in Table 1. The overall data rate to be coped with, which is made up of the various data rates of a plurality of digital services with different data rates, is also given in the following Table 1.
TABLE 1
Digital service
Mbit/s
2 Digital television stations
6 + 6
1 Digital video recorder
6
2 × data transfer
2 + 2
2 × DAB channels
1.5 + 1.5
ISDN
0.128
Overall data rate of the services
25.128
The overall data rate within a cluster can therefore be approximately 25 Mbit/s. A future local indoor radio network system should therefore be capable of transmitting such high data rates within a cluster.
The RF connection can comprise, for example, the so-called ISM (Industrial Scientific and Medical) bands which lie at frequencies in the 2.4/5.8 GHz ranges. The ISM bands with a carrier frequency of 2.4 to 2.4835 GHz (or 5.725 to 5.875 GHz) have a bandwidth of 83.5 MHz (or 150 MHz). Their main advantage is to be seen in the fact that an adequate indoor coverage range with low outlay on RF components of the transmitters/receivers can be obtained. Also, any type of modulation technology and any desired multiple access schemes may be applied in the ISM bands. There are also no specific recommendations or regulations with respect to the channel spacing to be maintained. The transmit antenna may be of integrated design; however, if reception is to be at a fixed location, the directional antenna may also be used for transmission.
However, the ISM bands used for indoor communication suffer, like other similar frequencies, from multipath propagation (owing to refraction, scatter and diffraction), from Doppler shifts and from interference. The lower range of the first band at 2.4 GHz is also used for amateur radio purposes, which can cause strong interference in a local indoor network. Furthermore, microwave ovens can also cause additional interference in the transmission channel.
The propagation time period of indoor channels in medium-sized rooms is between 20 and 65 ns. Owing to movement in the surroundings, Doppler shifts occur which are of different sizes at the various radio frequencies (for example approximately 10 to 17 Hz at 2.4 GHz and approximately 20 to 35 Hz at 5.8 GHz). Apart from multi-path propagation and fading, strong interference may also occur in the frequency bands under consideration. This interference mainly originates—as already mentioned—from amateur radio up to approximately 50 dBm transmitting power and from microwave equipment (microwave ovens). In addition, co-channel interference (CCI) originating from adjacent networks operating in the same frequency band can also reduce the efficiency of the system.
In order to overcome the aforesaid channel interference, broadband systems with band spread spectrum technology and rake receivers or else narrowband systems with equalizers can be used in local networks, for example indoor networks. A broadband system with spread spectrum technology requires a very large bandwidth. Each information symbol is spread with a PN (pseudonoise) code. The spread spectrum can be based, in a known manner, either on the direct sequence method (DS-SS, Direct Sequence-Spread Spectrum) or on the frequency hopping method (FH-SS, Frequency Hopping-Spread Spectrum).
In the case of the direct sequence method, the receiver must break up all the received signal paths by means of a rake receiver. The multiple access can be carried out by using various spread codes, each assigned to a subscriber in the network. In the known US Standard IEEE.802.11 a spread spectrum with DS-SS or FH-SS is used.
In the case of DS-SS, the available ISM bandwidth is initially divided up into equidistant channels of 10 MHz bandwidth. Each information bit is spread by means of a PN code with a spread factor of 10. For the modulation, the differential BPSK (Binary Phase Shift Keying) modulation or the differential QPSK (Quadrature Phase Shift Keying) modulation is used.
However, in the case of FH-SS, 79 frequencies with a bandwidth of 1 MHz each are used. The minimum frequency hopping rate (frequency change) is 2.5 hops/s. In total, there are 22 PN codes for changing. The modulation is based on 2- or 4-state GFSK (Gaussian Frequency Shift Keying). The maximum data rate is only 2 Mbit/s for both systems. Neither DS-SS nor FH-SS systems use channel coding in the physical layer of the U.S. Standard IEEE.802.11.
The second known possible way of combating interference in an indoor channel is to use narrowband channels with optional equalization. The bandwidth of each channel has to be kept proportional here to the data rate to be transmitted. The multiple access can be based on FDMA (Frequency Division Multiple Access, frequency-division multiplexing), TDMA (Time Division Multiple Access, time division multiplexing) or a combination of TDMA and FDMA.
Such a concept is used in the so-called DECT (Digital European Cordless Telecommunication) Standard and also in the Hiperlan Standard (ETS 300 652). The DECT system uses carrier frequencies of 1.88 to 1.9 GHz with a channel spacing of 1.728 MHz employing TDMA multiplex technology. A combination of FDMA/TDMA is used as multiple access method, the time division multiplex system being used between the cordless receiver and the base station. Only the CRC code is used for channel coding. The audio channel with ADPCM (Adaptive Differential Pulse Code Modulation) compression has a data rate of 32 kbit/s (voice coding). The modulation is based on the so-called GFSK modulation method, which produces an overall data rate of 1.152 Mbit/s. As a result of the transmission with a low data rate, equalization is optional for the DECT system.
In the case of the Hiperlan system, carrier frequencies of 5.15 to 5.29 GHz are used. The available bandwidth is divided into five bands of approximately 23.5 MHz. In this system, transmission is provided at a high data rate (23.5 Mbit/s) and at an centre data rate (1.47 Mbit/s). The multiple access is based on the TDMA method. For transmission at a high data rate GMSK (Gaussian Minimum Shift Keying) is used as the modulation method and for transmission at the centre data rate FSK (Frequency Shift Keying) is used as the modulation method. For channel encoding, the BCH(31,26,3) code with time multiplexing is used, which makes it possible to correct an individual bit (or to detect two bit errors. Because of the high data rate, an equalizer is used in this system to minimize the effects of intersymbol interference (ISI).
With the exception of th
Fazel Khaled
Klank Otto
Robertson Patrick
Harper Kevin C.
Kurdyla Ronald H.
Laks Joseph J.
Thomson Licensing S.A.
Tripoli Joseph S.
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