Multiplex communications – Communication techniques for information carried in plural... – Adaptive
Reexamination Certificate
1999-03-29
2001-06-05
Chin, Wellington (Department: 2664)
Multiplex communications
Communication techniques for information carried in plural...
Adaptive
C370S332000, C370S340000, C455S214000, C455S337000
Reexamination Certificate
active
06243393
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The invention relates to data transmissions and, in particular, to mobile data transmission systems using ALOHA as the multiple access protocol.
BACKGROUND OF THE INVENTION
A multiple access protocol is a communications protocol which allows multiple users access to a single channel (normally one at a time). In the case of a mobile data transmission system, the mobile system may have one radio channel which can be used by multiple mobile data terminals, one terminal at a time. Access control is determined by the particular multiple access protocol utilized. A number of different multiple access protocols may be utilized including Carrier Sense Multiple Access (“CSMA”), Time Division Multiple Access (“TDMA”), Code Division Multiple Access (“CDMA”), and ALOHA.
In a CSMA (Carrier Sense Multiple Access) protocol system, the mobile transmission device must check the status of the channel prior to transmitting. If the device senses carrier activity, it must wait until the channel is clear before transmitting.
In a TDMA (Time Division Multiple Access) protocol system, a single channel is divided into multiple time slots. Users are assigned time slots during which transmissions are allowed which segregate the users in time, thereby preventing overlap.
Users of a CDMA (Code Division Multiple Access) protocol system are assigned unique identification numbers which are multiplied by the user's transmission data. At the receiving end, the received signals are divided by the user's unique identification number in order to separate the individual transmissions.
In an ALOHA protocol system, users are permitted to transmit at will, regardless of whether the channel is busy or not.
Newton's Telecom Dictionary
(12th ed. 1997). Multiple simultaneous transmissions that result in colliding data streams can cause the transmissions to be nonrecoverable. When two messages collide and both are unrecoverable, both messages must be retransmitted, along with any new messages which have since queued. This situation can rapidly escalate to an avalanche condition which prematurely saturates an ALOHA-BASED data transmission system.
Mobile data transmissions typically start with a synchronization preamble which allows the modem to recognize the start of a data transmission. Most of these synchronization preambles consist of a known sequence of bits (i.e., 1100110011001100 . . . for some period of time) followed by the message itself, which is random in nature (i.e., 10011101000101 . . . ).
Capture effect is a phenomenon associated with the reception of frequency-modulated signals in which, if two signals are on or near the same frequency, only the stronger of the two will appear at the output. Complete suppression of the weaker signal occurs at the receiver limiter, where the weaker signal is treated as noise and rejected. Thus, FM capture is a phenomenon in which one radio frequency (“RF”) signal, if it is of sufficient amplitude relative to a simultaneously received weaker signal, will overpower the weaker signal and be received as if the weaker interfering signal were not present.
Thus, when a second-in-time transmission collides with a first-in-time transmission that is being decoded, if the second signal has sufficient amplitude to capture the receiver, the second signal will be decoded as if it were part of the first signal, resulting in an output which is an uncorrectable combination of the first and second transmissions. In this case, both transmissions must be retransmitted. Thus, there exists a need for an apparatus and method for reducing the need for retransmitting colliding signals.
SUMMARY OF THE INVENTION
In one embodiment, the present invention provides an apparatus for decoding an analog signal including an input terminal, a data bus, a first decoder and a second decoder. The first decoder is connected to the input terminal and data bus and configured to receive an analog signal, decode the signal into a first message and output the message to the data bus. The second decoder is also connected to the input terminal and data bus and configured to receive an analog signal, determine whether the analog signal includes a second message, decode the signal into a second message and out put the message to the data bus. In particular, the invention provides an apparatus and data transmission system for receiving and decoding data transmissions where an at will transmission protocol such as ALOHA is utilized.
A plurality of transmission units, and in particular mobile transmission units, transmit analog signals on a selected frequency on an at will basis. The signals are received by a receiver which is connected to a decoder for decoding and outputting the contents of the transmission. A second decoder is provided for detecting and decoding a colliding second signal having an amplitude sufficient to capture the receiver. A microprocessor operatively connected to the first and second decoders directs the second decoder to search for a colliding second transmission immediately after the first decoder has begun to decode a first transmission. Thus, in the event that a colliding second signal captures the receiver as the first transmission is decoded, the second decoder decodes and outputs the second message intact, even though the output from the first decoder will be an uncorrectable concatenated combination of the first and second transmissions. In one variation, a step change in the Received Signal Strength Indicator (“RSSI”) is used to trigger the command for the second decoder to search for a synchronization preamble of a second transmission.
The invention also provides a method for saving one of two colliding signals in a transmission system utilizing an at will transmission protocol such as ALOHA where the second signal is of sufficient amplitude or strength capture to capture the receiver as a first transmission is being decoded. The method includes the steps of: (1) using a receiver to receive signals transmitted by a plurality of transmission units; (2) using a first decoder to decode a first signal; (3) directing a second decoder to search for a second transmission as the first signal is decoded by the first decoder; and (4) decoding the second received signal, assuming that the second received signal has sufficient amplitude to capture the receiver. The second decoder may be directed to search for the synchronization preamble of a second colliding transmission immediately after the first decoder begins decoding the first signal. Alternatively, the second decoder may be prompted to search for the synchronization preamble of a second signal if a step change in the received signal strength indicator is detected. The method of the invention may also include the step of utilizing a microprocessor to prompt or direct the second decoder to search for a second signal as a first signal is being decoded.
REFERENCES:
patent: 5337310 (1994-08-01), Selyutin
patent: 5699405 (1997-12-01), Suzuki
patent: 5770927 (1998-06-01), Abe
patent: 5966385 (1999-10-01), Fujii et al.
Chin Wellington
Electrocom Communication Systems
Intellectual Property Law, P.C.
Jorgenson Edward I.
Meyers Philip G.
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