Telecommunications – Transmitter – Diversity
Reexamination Certificate
1997-01-31
2001-06-05
Urban, Edward F. (Department: 2746)
Telecommunications
Transmitter
Diversity
C455S561000, C375S299000
Reexamination Certificate
active
06243565
ABSTRACT:
The present invention relates generally to apparatus and methods for overcoming multi-path fading of communication signals transmitted upon a communication channel susceptible to multi-path fading. More particularly, the present invention relates to a transmitter diversity assembly, and an associated method for creating signal diversity, such as transmission space diversity, polarization diversity, or frequency diversity, for a transmitter which transmits the communication signals upon the communication channel. The signal diversity created through operation of the assembly mitigates the effects of fading of the communication signals transmitted upon the communication channel.
The transmitter diversity assembly, and its associated method of creating signal diversity, is advantageously utilized, for example, at a base station of a cellular communication system. The transmitter diversity assembly, and its associated method, is advantageously utilized to create signal diversity both in a conventional cellular communication system which transmits analog communication signals and in a digital cellular communication system. When used at a base station of a digital cellular communication system, or at a transmitter of another digital communication system to create transmission space diversity, communication signal bursts are transmitted from different antennas of an antenna assembly, thereby to create the transmission space diversity. The transmitter diversity assembly, and its associated method, is also advantageously utilized to create frequency diversity in conjunction with a communication system which utilizes a frequency hopping scheme. And, the transmitter diversity assembly, and its associated method, is further advantageously utilized to create polarization diversity in conjunction with a communication system which utilizes a polarization hopping scheme. Use of the transmitter diversity assembly to create a combination of frequency, transmission space, and/or polarization diversity further mitigates the effects of fading of the communication signals transmitted upon the multi-path channel.
BACKGROUND OF THE INVENTION
A communication system is formed, at a minimum, of a transmitter and a receiver interconnected by a communication channel. Communication signals generated by the transmitter are transmitted upon the communication channel to be received by the receiver.
A radio communication system is a type of communication system in which the communication channel is formed of a radio frequency channel. A radio communication system is advantageous for the reason that a fixed, or hardwired, connection is not required to form the communication channel extending between the transmitter and receiver. Communication can be effectuated between remotely-positioned transmitters and receivers without the need to form the hardwired or other fixed connections therebetween.
A cellular communication system is a type of radio communication system. When the infrastructure, hereinafter referred to as the network, of the cellular communication system is installed in a geographical area, a subscriber to the cellular system is generally able to communicate telephonically in the system when positioned at any location within the geographical area encompassed by the cellular network.
Technological advancements and economies of scale have contributed to lowered costs of communicating pursuant to a cellular communication system. Concomitant with the decreased costs of communicating pursuant to a cellular communication system has been an increase in usage of such systems. In some instances, conventional cellular communication systems have been operated at their maximum capacities. When operated at their maximum capacities, access is sometimes denied to additional users attempting to communicate pursuant to such systems. Ongoing calls are sometimes also adversely affected.
To avoid capacity problems and to permit an increase in the number of users permitted to utilize a cellular communication system, attempts have been made to increase the communication capabilities of such systems. To increase communication capacities, some conventional, cellular communication systems using conventional, analog technologies have been converted to digital, cellular communications systems which utilize digital coding and modulation technologies. Other radiotelephonic, and other communication systems, have been similarly converted.
Because digital communication systems generally utilize the radio frequency transmission channels upon which the communication signals are transmitted more efficiently, increased numbers of communication signals can be transmitted upon the radio frequency channels allocated for such communication systems.
By digitizing an information signal, which is modulated to form a communication signal, signal redundancies can be removed out of the information signal without affecting the amount of information transmitted in a communication signal formed therefrom. Also, once an information signal is digitized, a communication signal formed therefrom can be transmitted in discrete, discontinuous bursts. Two or more communication signals can thereby be multiplexed together and transmitted sequentially upon a single frequency channel. A twofold, or greater, increase in capacity is thereby possible when the communication system is converted into a digital communication system.
The communication signal transmitted upon a radio frequency channel is susceptible to scattering, diffraction, reflection, and attenuation. Signal reflection of the transmitted signal causes the signal actually received by a receiver to be the summation of signal components transmitted by the transmitter by way of, and some instances, many different paths, in addition to a direct, line-of-sight path.
The communication channel is sometimes referred to as a “multi-path channel,” as the signal actually received by the receiver is the summation of a plurality of signal components transmitted to the receiver on a plurality of different paths. Values of the signal components transmitted upon the multiple numbers of paths are dependent, in part, upon their relative phases. Therefore, the value of the summation of the plurality of signal components received by the receiver is dependent upon the position at which the receiver is located when the signal is received. The receiver might be positioned such that signals transmitted on the plurality of signal paths add together destructively. Signal “fading” occurs when the signals add together destructively, and fading “dips” or “nulls” occur when the summation of the received signals results in cancellation when such destructive addition makes difficult, or prevents, accurate determination of the informational content of the received signal.
Because fading deleteriously effects the quality of communications, attempts are sometimes made to mitigate the effects of fading. The deleterious effects of fading are particularly problematical in a cellular communication system when a subscriber unit operable to communicate therein is maintained in a stationary, or slowly moving, position. If positioned at an area in which a fading dip is significant, a significant amount of sequential information might be lost.
More particularly, various kinds of diversity are created at selected locations of the digital communication system to mitigate the effects of the multi-path fading. Time diversity, frequency diversity, receiver space diversity, transmission space diversity, and polarization diversity are all types of diversity which can be created to mitigate the effects of multi-path fading.
When time diversity is created, signal bits of an informational signal are spread out, or interleaved together with other signal bits, over time, thereby to spread the bits over a time period. When interleaved or otherwise spread out over time, the likelihood that all of the bits are received at a receiver at a fading null is reduced. To be effective, the bit spreading requires that the fading nulls of the multi-path channel not
Almgren Knut Magnus
Olofsson Håkan Gunnar
Smith David Anthony
Jenkens & Gilchrist
Telefonaktiebolaget LM Ericsson (publ)
Urban Edward F.
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