Multiplex communications – Duplex – Communication over free space
Reexamination Certificate
1998-06-11
2001-01-16
Hsu, Alpus H. (Department: 2738)
Multiplex communications
Duplex
Communication over free space
C370S342000, C370S345000, C375S141000, C455S010000, C455S067700, C455S504000
Reexamination Certificate
active
06175555
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention involves improvements to communications systems and methods in a wireless discrete multitone spread spectrum communications system.
2. Description of Related Art
Wireless communications systems, such as cellular and personal communications systems, operate over limited spectral bandwidths. They must make highly efficient use of the scarce bandwidth resource to provide good service to a large population of users. Code Division Multiple Access (CDMA) protocol has been used by wireless communications systems to efficiently make use of limited bandwidths. The protocol uses a unique code to distinguish each user's data signal from other users' data signals. Knowledge of the unique code with which any specific information is transmitted, permits the separation and reconstruction of each user's message at the receiving end of the communication channel.
The personal wireless access network (PWAN) system described in the referenced Alamouti, et al. patent application, uses a form of the CDMA protocol known as discrete multitone spread spectrum (DMT-SS) to provide efficient communications between a base station and a plurality of remote units.
In this protocol, the user's data signal is modulated by a set of weighted discrete frequencies or tones. The weights are spreading codes that distribute the data signal over many discrete tones covering a broad range of frequencies. The weights are complex numbers with the real component acting to modulate the amplitude of a tone while the complex component of the weight acts to modulate the phase of the same tone. Each tone in the weighted tone set bears the same data signal. Plural users at the transmitting station can use the same tone set to transmit their data, but each of the users sharing the tone set has a different set of spreading codes. The weighted tone set for a particular user is transmitted to the receiving station where it is processed with despreading codes related to the user's spreading codes, to recover the user's data signal. For each of the spatially separated antennas at the receiver, the received multitone signals are transformed from time domain signals to frequency domain signals. Despreading weights are assigned to each frequency component of the signals received by each antenna element. The values of the despreading weights are combined with the received signals to obtain an optimized approximation of individual transmitted signals characterized by a particular multitone set and transmitting location. The PWAN system has a total of 2560 discrete tones (carriers) equally spaced in 8 MHZ of available bandwidth in the range of 1850 to 1990 MHZ. The spacing between the tones is 3.125 kHz. The total set of tones are numbered consecutively form 0 to 2559 starting from the lowest frequency tone. The tones are used to carry traffic messages and overhead messages between the base station and the plurality of remote units. The traffic tones are divided into 32 traffic partitions, with each traffic channel requiring at least one traffic partition of 72 tones.
In addition, the PWAN system uses overhead tones to establish synchronization and to pass control information between the base station and the remote units. A Common Link Channel (CLC) is used by the base to transmit control information to the Remote Units. A Common Access Channel (CAC) is used to transmit messages from the Remote Unit to the Base. There is one grouping of tones assigned to each channel. These overhead channels are used in common by all of the remote units when they are exchanging control messages with the base station.
In the PWAN system, Time Division Duplexing (TDD) is used by the base station and the remote unit to transmit data and control information in both directions over the same multi-tone frequency channel. Transmission from the base station to the remote unit is called forward transmission and transmission from the remote unit to the base station is called reverse transmission. The time between recurrent transmissions from either the remote unit or the base station is the TDD period. In every TDD period, there are four consecutive transmission bursts in each direction. Data is transmitted in each burst using multiple tones. The base station and each remote unit must synchronize and conform to the TDD timing structure and both the base station and the remote unit must synchronize to a framing structure. All remote units and base stations must be synchronized so that all remote units transmit at the same time and then all base stations transmit at the same time. When a remote unit initially powers up, it acquires synchronization from the base station so that it can exchange control and traffic messages within the prescribed TDD time format. The remote unit must also acquire frequency and phase synchronization for the DMT-SS signals so that the remote is operating at the same frequency and phase as the base station.
The PWAN wireless communications system, and other limited bandwidth communications systems, need to exploit new techniques to make the most efficient use of the scarce bandwidth resource to provide good service to a large population of users. When the characteristics of the transmit path and the receive path change with time, some means is required to compensate for tha drift without imposing additional overhead on the traffic bearing channels.
SUMMARY OF THE INVENTION
The invention disclosed herein is a new technique to make the most efficient use of the scarce spectral bandwidth. In a time division duplex (TDD) system, compensation measurements are made for the transmission circuitry during the receive portion of the TDD cycle and compensation measurements are made for the receive circuitry during the transmit portion of the TDD cycle. A base station has multiple antennas for spatial, as well as spectral, spreading and despreading of discrete multitone spread spectrum (DMT-SS) communications. Each antenna has its own transmission path components and receive path components. The transmit amplifier, for example, in the transmit path and the receive amplifier, for example, in the receive path tend to drift in their characteristics over time. The invention manages the sequential testing of each respective transmission path and receive path for each antenna. The invention measures the drift of the transmit path components and the receive path components and prepares compensating weights to be applied to signals processed in each path.
The base station's digital signal processor (DSP) applies the spreading and despreading weights for the DMT-SS signals for the transmit path and the receive path, respectively, for each antenna. In a first TDD interval, a test controller coupled to the DSP, uses the TDD timing signal from the DSP to first test the receive path of a first antenna (during the base station transmission period). To test the receive path, the test controller takes a multitone test signal output from the frequency modulator in the transmit path and applies it to a test transmitter that directs the multitone signal to the input of the receive amplifier in the receive path. The DSP processes the received test signal output by the receive amplifier and compiles receive path compensation weights that are stored in a receive path compensation buffer. The receive path compensation weights are then applied to the DMT-SS signals received in all later TDD receive periods, until the receive path test for that antenna are repeated. In one embodiment of the invention, a switch under the control of the test controller selectively directs the multitone test signal output from the test transmitter to the input of the receive amplifier in the receive path. In another, preferred embodiment of the invention, a probe antenna coupled to the output of the test transmitter directs the multitone test signal output from the test transmitter to the input of the receive amplifier in the receive path.
In the base receive period of the first TDD interval,
AT&T Wireless Svcs. Inc.
Hsu Alpus H.
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