The present invention relates to a device for receiving and/or emitting electromagnetic signals with radiation diversity, it being possible for the said device to be used in the field of wireless transmissions, especially within the context of transmissions in a closed or semiclosed environment such as domestic environments, gymnasia, television studios, theatres or the like.
In the known high-speed wireless transmission systems, the signals transmitted by the emitter reach the receiver along a plurality of paths. When they are combined at the receiver, the phase differences between the different rays that have travelled paths of different lengths give rise to an interference pattern capable of causing significant signal fading or detorioration.
Furthermore, the location of the fading changes over time, depending on modifications to the environment such as the presence of new objects or the movement of people. The fading due to the multiple paths may lead to significant detorioration both in the quality of the signal received and in system performance. To combat these fading phenomena, the technique most often used is a technique implementing spatial diversity.
As shown in FIG. 1, this technique consists, inter alia, in using a pair of antenna with wide spatial coverage such as two antennas 1, 2 of the slot type which are connected by supply lines 4, 5 to a switch 3. The two antennas 1, 2 consisting of annular slots are separated by a distance which must be greater than or equal to λ0/2 where λ0 is the wavelength in a vacuum corresponding to the operating frequency of the antenna. Furthermore, the supply lines 4, 5 are dimensioned such that they cut the slot of the slot antenna at a length kλm/4 from its end and that they are connected to the switch 3 by a length of line equal to k′λm/2 where λm is the wavelength guided by the line at the central operating frequency with λm=λ0/√εreff where λ0 is the wavelength in a vacuum and εreff is the effective permittivity of the line, k and k′ being integers and k an odd integer.
Slot-type antennas may also be replaced by patch-type antennas. With this type of device, it is possible to demonstrate that the probability of the two antennas simultaneously fading is very small. The demonstration results in particular from the description given in “Wireless digital communications” by Dr Kamilo Feher, chapter 7: “Diversity techniques for mobile wireless radio systems”. It is also possible to demonstrate it by means of a pure probability calculation on the assumption that the levels received by each antenna are completely independent. By virtue of the switch 3, it is possible to select the branch connected to the antenna having the highest level by examining the signal received by means of a control circuit (not shown). In fact, since the two antennas 1, 2 are sufficiently separated, two uncorrelated channel responses are obtained. It is thus possible to switch to the better of the two antennas and thus reduce considerably the probability of fading.