Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
2000-11-20
2004-09-14
Nguyen, Brian (Department: 2661)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C370S334000, C370S337000
Reexamination Certificate
active
06791967
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wireless base station that performs spatially multiplexed communication with a plurality of mobile stations by adjusting the signals to and from a plurality of antennas to form directivity patterns during reception and transmission. In particular, the invention relates to an improvement in the formation of directivity patterns.
2. Description of the Related Art
In recent years, increasing attention has been given to mobile communication methods that use adaptive arrays to make efficient use of frequencies and improve the quality of communication. In such methods, a plurality of antennas are used, with the signal amplitude and phase being controlled separately for each antenna to adaptively control the direction in which signals propagate. A wireless base station that uses an adaptive array method controls a plurality of non-directional antennas and forms a directivity pattern where the reception sensitivity or transmission intensity is high in a direction in which a signal has been received from a mobile station when communicating with that mobile station. Here, the expression “directivity pattern” refers to a representation of the intensity of signals that are transmitted or received in different directions by a group of antennas.
A wireless base station can form a directivity pattern that has high directivity in a direction in which a signal has been received from a mobile station and simultaneously has a “null” directivity,toward other mobile stations. Here, “null directivity” means that the pattern does not extend in a certain direction, or in other words, strong signals are not transmitted in that direction and reception is not sensitive to signals in that direction. By forming an optimal directivity pattern separately for each of a plurality of mobile stations, a wireless base station can simultaneously communicate with a plurality of mobile stations. This method is called “spatial multiplexing”.
FIG. 1A
shows two examples of directivity patterns that are formed when a wireless base station communicates with two mobile stations using spatial multiplexing. The wireless base station forms the directivity pattern
101
with a high directivity for the mobile station a and null directivity for the mobile station b and simultaneously forms the directivity pattern
102
with a high directivity for the mobile station b and null directivity for the mobile station a.
The wireless base station changes the directivity pattern to track the movement of the mobile stations. One way of tracking such movement is as follows. The wireless base station communicates with a mobile station using time-division multiplexing where each frame is only several milliseconds (such as five milliseconds) long. When performing reception within a frame, the wireless base station adjusts the directivity pattern so that a predetermined signal located at the start of each frame is received properly. The wireless base station receives the payload of the reception signal using this adjusted directivity pattern, and then transmits a signal to the mobile station using the same directivity pattern that was used during reception within the same frame.
FIG. 1B
shows the directivity patterns formed by a wireless base station, which is communicating via spatial multiplexing with the mobile stations a and b, for communication with the mobile station a. This drawing shows the respective positions of the mobile stations a and b at the times T
1
, T
2
, and T
3
. The times T
1
, T
2
, and T
3
correspond to the consecutive frames T
1
, T
2
, and T
3
, so that the wireless base station forms the directivity patterns
101
,
103
, and
104
from the predetermined signals received from the mobile station a as part of each frame. When performing transmission within frames, the wireless base station forms the same directivity patterns
101
,
103
, and
104
as those used during reception in the same frames. In this way, wireless base station transmits and receives signals by forming directivity patterns in frame units for the mobile station a. When these directivity patterns are optimally formed, there is directivity toward the mobile station b is definitely “null”, as shown in FIG.
1
B. The wireless base station also generates directivity patterns that track the movement of the mobile station b for communication with the mobile station b. These directivity patterns are formed in the same way as the directivity patterns produced for the mobile station a.
The wireless base station forms and adjusts directivity patterns by applying weightings to the transmission and reception signals of each antenna using a set of parameters and then combining the weighted signals. A set of such parameters is called a “weight vector”. The wireless base station sets a certain weight vector as an initial value. The wireless base station then receives a predetermined signal from a mobile station, such as a preamble or a synch word (or unique word), via each antenna and adds a weighting to the signal received by each antenna using the weight vector. A combined signal produced by combining these weighted signals is compared with a reference signal that is stored in advance, and the weight vector is adjusted so as to minimize the difference between the combined signal and the reference signal. By repeating this process of weighting and combining signals, comparing the combined signal with a reference signal, and adjusting the weight vector at predetermined intervals, the weight vector can be made to converge on an optimal vector around the time the payload of the signal is received from a mobile station.
The wireless base station receives the payload by using the converged weight vector to apply a weighting to each signal corresponding to the payload that is received by the antennas. During transmission, a transmission signal for the mobile station is distributed to each antenna. At this point, the transmission signal to be transmitted by each antenna is weighted using the weight vector calculated during reception in the same frame, and a weighted transmission signal is transmitted by each antenna.
In addition to using an adaptive array, the wireless base station is constructed so as to communicate using time-division multiplexing according to a TDMA/TDD (Time Division Multiple Access/Time Division Duplex) method where four channels are multiplexed together.
FIG. 2
shows one example of a TDMA/TDD frame when communication is performed using both spatial multiplexing and time-division multiplexing.
The period covered by the illustrated TDMA/TDD frame is divided into eight timeslots that are numbered one to eight. The first to fourth timeslots are upstream timeslots for transmission from mobile stations to the wireless base station and the fifth to eighth timeslots are downstream timeslots for transmission from the wireless base station to mobile stations.
In the second and sixth timeslots, the wireless base station performs transmission and reception for the mobile stations a and b using spatial multiplexing. In the third and seventh timeslots, the wireless base station performs transmission and reception to the mobile station c. In the fourth and eighth timeslots, the wireless base station performs transmission and reception to the mobile station d. By performing both time-division multiplexing and spatial multiplexing, the wireless base station can simultaneously communicate with a number of mobile stations given as the number of the mobile stations that can be simultaneously handled by time-division multiplexing multiplied by the number of stations that can be handled by spatial multiplexing.
Due to problems such as weak electrical fields, interference, and losses of synchronization, there are cases where the wireless base station cannot calculate a weight vector for generating an optimal directivity pattern from the received signals. This is because the weight vector does not converge to the optimal value. An inappropriate directivity pattern is formed u
Arent & Fox PLLC
Nguyen Brian
Sanyo Electric Co,. Ltd.
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