Communications: directive radio wave systems and devices (e.g. – Directive – Beacon or receiver
Reexamination Certificate
2000-02-22
2001-09-11
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Beacon or receiver
C342S357490, C342S357490
Reexamination Certificate
active
06288674
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to communication systems in general and more particularly to a wireless communication system utilizing Doppler diversity.
BACKGROUND OF THE INVENTION
Cellular telephones are commonplace in today's society. As a result, there is a desire to be able to accommodate more users at higher data rates in a given geographic area. Wireless communications are also being implemented for services besides voice. For example, wireless communications are now being planned and implemented to allow access to the Internet and other networks from mobile devices.
As the uses for wireless communications continue to expand, the reliability of the communication must be continually improved. One problem that greatly degrades the performance of mobile communications systems is multipath fading. This phenomenon is caused by the reflection of the signal from multiple scatterers. Moreover, this environment is constantly changing due to the motion of mobile receivers and transmitters.
One technique that has been used in practice to combat fading is diversity. Diversity implies the exploitation of multiple copies of the transmitted signal at the receiver. Common examples of diversity includes, time diversity, frequency (Doppler), and spatial diversity. The mobile wireless channel provides inherent diversity in the form of multiple paths (time diversity) which is exploited in current cellular systems through the use of rake receivers.
In current systems, rake receivers exploit multipath diversity by combining the various multipath components. The use of rake receivers, however, assumes that the channel characteristics vary slowly over time and is suitable for slow-fading situations. As a result, the performance degrades in the presence of Doppler induced by rapid channel variations due to fast relative motion between the transmitter and the receiver.
Recently a new technique to exploit rapid channel variations to improve performance through joint multipath-Doppler diversity processing has been proposed, in the research community, for CDMA systems. This technique facilitates maximal exploitation of channel diversity in slow as well as fast fading scenarios, thus combating fading in both situations.
SUMMARY OF THE INVENTION
In one aspect, the present invention utilizes the fact that future cellular handsets sold in the United States will incorporate GPS receivers in order to accommodate a government mandated requirement to support emergency notification services (e.g., E911) in mobile environments. This GPS unit can be utilized to determine the current vector of motion of a mobile terminal, which will also be communicated to the base station. The GPS information can be used to estimate the Doppler profile of the wireless channel between the mobile and the base station.
The current vector of motion (the relative velocity of the mobile with respect to the base station) translates into knowledge of the Doppler spread which in turn translates to the number of Doppler components that can be exploited at the receiver. The combined knowledge of Doppler and multipath profiles of the channel can be used to apply diversity exploiting schemes such as the rake, but in a two-dimensional time-frequency framework. In the preferred embodiment, the Doppler as well as multipath components are combined using maximal ratio combining to enhance the performance of the system.
As an example, a method of communicating between a mobile communication unit and a second communication unit (either stationary or mobile) would include determining the location and velocity of the mobile unit at a particular time. This information is preferably determined using global positioning system (GPS) information. A velocity vector of the mobile communication unit relative to the stationary communication unit can be calculated from GPS information. From this velocity vector, a Doppler profile can be calculated. The system could then use the Doppler profile in communicating between the mobile communication unit and the stationary communication unit.
The present invention can be used with either a mobile unit or a stationary unit. The communication unit of this aspect of the invention utilizes global positioning system (GPS) information within a Doppler profile generator. For example, in a mobile unit, the GPS information would typically be generated at the device. A receiver, e.g., a rake receiver, is coupled to the Doppler profile generator. The receiver utilizes Doppler profile information in translating a received wireless signal into user information.
The present invention provides a number of benefits relative to the prior art. In some aspects, these benefits are in terms of improved performance of the cellular system in a fast-fading scenario, which is a scenario where the conventional rake receiver suffers drastically. Exploiting Doppler diversity to improve performance will be possible in any situation where the effective processing time (typically, the symbol period) is long enough compared to the rate of variation of the channel. In addition, the underlying channel representation for joint multipath-Doppler processing facilitates more accurate modeling of the signal structure of the desired as well as the interfering users, which will lead to improved interference suppression.
REFERENCES:
patent: 4754283 (1988-06-01), Fowler
patent: 5920284 (1999-07-01), Victor
patent: 6017959 (2000-08-01), Levanon
Panasik Carl
Sengupta Chaitali
Brady III W. James
Hernandez Pedro P.
Phan Dao
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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