Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
1998-06-24
2002-06-25
Trost, William (Department: 2683)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C455S562100, C455S101000, C370S320000, C342S361000
Reexamination Certificate
active
06411824
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of wireless communications, and more particularly, to polarization-diversity systems for wireless communications.
2. Description of the Related Art
It can be fairly said that the age of wireless communications began in 1898 when Guglielmo Marconi broadcast the first paid radio program from the Isle of Wight. The system used by Marconi was a one-way wireless communication system comprising a transmitter that sent messages, carried by electromagnetic waves, to one or more receivers. One-way communications systems, such as broadcast radio, television, etc., are still widely used today.
In contrast to one-way systems that can only send messages from one person to another, duplex (two-way) wireless communications systems, such as cellular telephones, cordless telephones, etc., allow two-way communication between two or more parties. In its simplest form, a duplex communication system is the combination of two one-way systems. In a duplex communication system, each party is equipped with a transceiver (a transmitter combined with a receiver) so that each party can both send and receive messages. Communication is two-way because each transceiver uses its transmitter to send messages to the other transceivers, and each transceiver uses its receiver to receive messages from the other transceivers.
As with normal conversation between people, duplex communication systems typically use some technique to minimize the interference that occurs when two parties try to transmit (i.e., talk) at the same time. As with normal conversation, many duplex systems use some form of a Time Division Duplexing (TDD) algorithm, wherein only one party at a time is allowed to transmit. Each party transmits only during its allotted time interval, and during that time interval, all other parties are expected to receive the transmission (i.e., listen). Other division techniques, such as, for example, frequency division, code division, etc., are also used to separate transmissions between parties.
TDD systems include the Digital European Cordless Telephone (DECT), the Personal Handy phone System (PHS), the Personal ACcess System (PACS), and the Personal Wireless Telecommunications (PWT) system. DECT is a 2nd generation cordless telephone standard, designed to be capable of supporting very high traffic densities at 1895-1906 MHz (private) and 1906-1918 MHz (public), with a proposed extension to a 300 MHz frequency band. DECT uses a TDMA/TDD access technique and a GMSK modulation technique, making it suitable for low mobility-high capacity concentrated usage environments such as city center offices and transport hubs. PHS, developed in Japan, operates at 1880-1900 MHz, uses a TDMA/TDD access technique and a &pgr;/4 QPSK modulation technique. PACS, developed by Bellcore, uses both TDMA/FDD (Frequency Division Duplex) and TDMA/TDD. PWT is the new name for the licensed DT1900 as well as the unlicensed WCPE cordless technologies found in the United States.
In both one-way and duplex communication systems, the transmitter provides Radio Frequency (RF) signals to a transmitting antenna that converts the RF signals into ElectroMagnetic (EM) waves. The EM waves propagate to a receiving antenna where the EM waves are converted back into RF signals that are provided to the receiver. Ideally, the EM waves travel in a single path directly from the transmitting antenna to the receiving antenna, without any external influences or perturbations, and without taking multiple paths. Unfortunately, ideal conditions are rarely found in the real-world and thus the EM waves that propagate from the transmitting antenna to the receiving antenna are often disturbed by external influences. These disturbances often reduce the strength of the EM waves that reach the receiving antenna, and thus impair the performance of the communications system. Fluctuation in the strength of the received signal is known as signal fading. The impairment caused by signal fading can include reduced range, higher noise, higher error rates, etc. Fading is usually caused by destructive interference of multipath waves. In theory, the reduction in signal strength at the receiving antenna can be offset by increasing the strength of the EM wave produced by the transmitting antenna. However, the strength of the EM wave produced by the transmitting antenna is usually limited by various factors, including, government regulations, the size/cost/weight of the transmitter, the size/cost/weight of the transmitting antenna, and the power available to operate the transmitter. The power available to the transmitter is particularly important in battery operated devices, such as handheld cellular telephones, where battery life is an important aspect of overall system performance.
Two common types of signal fading are multipath fading and polarization mismatch fading. Multipath fading occurs when the EM waves take two or more paths to travel from the transmitting antenna to the receiving antenna. The waves arriving at the receiving antenna along different paths will often interfere with each other, such that a wave arriving from a first path will tend to cancel a wave arriving from a second path. Receive-antenna position-diversity is a method often used to mitigate the effects of multipath fading. In systems with receive-antenna position-diversity, several receiving antennas are positioned such that the phase centers (i.e., positions) of the antennas are physically separated by a few wavelengths. The receiving antennas are used to receive the EM waves, and the output from each receiving antenna is provided to the receiver for special processing. Receive-antenna position-diversity works because the destructive interference is typically a localized phenomenon. Even if one of the receiving antennas is experiencing multipath fading, it is likely that another receiving antenna located several wavelengths away will not experience fading. The separation between the antennas is desirable because the probability of having all of the received signals for all of the receiving antennas faded at one time becomes increasingly small as the number of antennas are increased.
Receive-antenna position-diversity is commonly used in wireless base stations where antenna size, weight, and cost are less important than in handheld units. Antenna position diversity is rarely used in handheld units because of the size, weight, and cost associated with multiple receiving antennas spaced several wavelengths apart. For example, conventional analog cellular telephones operate using EM waves having a frequency of approximately 1 GigaHertz (GHz). A 1 GHz EM wave in air has a wavelength of approximately 1 foot. Thus, an effective position-diversity antenna system would be several feet across. This is clearly impractical for a handheld telephone, but very practical for a base station antenna mounted on a large tower.
Various techniques are used to process the antenna outputs, including, for example, Antenna Switching Diversity, and Maximal Ratio Combining. Antenna Switching Diversity systems simply pick the receiving antenna that is currently receiving the strongest EM wave and use that antenna as the receiving antenna.
Maximal Ratio Combining systems combine the outputs of one or more receiving antennas into a single output signal. The outputs of the antennas are coherently phased and weighted to provide maximum power in the output signal. Maximal Ratio Combining typically offers better performance than Antenna Switching Diversity because it combines the antenna outputs, thus bringing in more signal while tending to average out the noise. This results in a higher Signal-to-Noise Ratio (SNR).
The combination of antenna-position diversity and maximal ratio combining is closely related to the technique of antenna-pattern diversity. In antenna pattern diversity, the antenna typically comprises several antenna elements. The transmitter provides RF signal to each antenna element such that the EM radiation from the antenna
Gesesse Tilahun
Knobbe Martens Olson & Bear LLP
Trost William
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