Optical communications – Transmitter and receiver system – Including feedback from receiver
Reexamination Certificate
2000-05-05
2003-11-04
Chan, Jason (Department: 2633)
Optical communications
Transmitter and receiver system
Including feedback from receiver
C398S123000, C398S124000, C375S222000, C375S225000, C370S311000, C340S007330, C455S343200
Reexamination Certificate
active
06643469
ABSTRACT:
TECHNICAL FIELD
The present invention relates to optical communication systems in general, and the link quality and link quality estimation between a transmitting station and a receiving station in particular.
BACKGROUND OF THE INVENTION
For infrared device communication the Infrared Data Association (IrDA) has published a series of specifications designed to allow data communication between devices using the Infrared (Ir) medium. The paper of F. Gfeller and W. Hirt, “Request for Comments on Advanced Infrared (Alr) Physical Layer Specification (IrPHY)”, presented at the IrDA Meeting in Toronto, Apr. 17, 1997, describes the concept of Robust Headers (RH) to support reliable collision avoidance with the Infrared Medium Access Control (IrMAC) protocol, which is responsible for coordinating the access to the infrared medium between infrared devices. The use of a RH in the physical layer header allows the coexistence of devices with different angular and range characteristics while maintaining the collision avoidance properties.
The International Patent Application PCT/IB96/00002 with publication number WO97/25788, filed on Jan. 3, 1996, describes an optical communication system enabling communication between several coexisting transmitting and receiving stations. In order to allow communication between coexisting stations, a robust physical layer header (herein referred to as Robust Header; RH) is employed which can be understood by all participating stations.
The two above-mentioned publications form the basis and the state of the art for the present invention and are incorporated by reference.
According to these references, infrared devices communicate in peer-to-peer mode where the transmitting station provides a Request-to-Send (RTS) frame to the receiving station to announce the transmission of a data packet, the receiving station provides a Clear-to-Send (CTS) frame in case of correct reception of the RTS frame, at least to the transmitting station, and the transmitting station subsequently sends the data frame to the receiving station. According to the general IrMAC frame format requirements, the RTS frame and the CTS frame comprise at least a Preamble field (PA), a Synchronization field (SYNC), and a Robust Header (RH) field. The PA field allows for reliable carrier sensing, symbol clock synchronization, and chip clock phase acquisition down to very low Signal-to-Noise Ratios (SNR). The PA field comprises symbols forming a periodic sequence of pulses, the number of slots per symbol (for example, L-slot Pulse Position Modulation; 4-slot (4-PPM) coding is proposed for AIrPHY) and the symbol content being known to all participating stations. The SYNC field, also a field with sequences of legal 4-PPM symbols, enables exact identification of the start of the RH field and consists of a certain predefined number of legal 4-PPM symbols. The RH field comprises according to the above referenced documents several fields of fixed length and known structure. By means of these fields, the receiving stations are informed about the signaling method used for data transmission. Further, the fields are used to provide other control information for the communication link or for the exchange of information to allow negotiation and/or adaptation of the data rate used for transmission in order to optimize the throughput depending on the quality of the channel.
The RTS frame further comprises, according to the above-mentioned IrDA Meeting paper, address fields called Source Address and Data Address (SA/DA) fields which follow the RH field. Further, this RH field is always robustly coded using repetition coding with a Rate Reduction of sixteen (RR=16) to provide maximum detection sensitivity. RR defines the level of repetition coding in order to ensure a correct data transmission; thus, every symbol is repeated RR times. The resulting redundancy in the symbol stream is intended to be exploited with suitable digital processing methods in the demodulator circuit and provide a SNR gain at the expense of a reduced data rate of 4/RR Mb/s. This is equivalent to individually matching the electrical receiver bandwidth to the reduced data rate. In contrast to this complex method the proposed method allows a virtual bandwidth reduction without having to physically change the receiver bandwidth and thus allowing a simple and optimal receiver implementation matched to the base rate of 4 Mb/s. The RR factor is defined to take on the values 1, 2, 4, 8, and 16, where RR=1 corresponds to the base rate of 4 Mb/s with 4-PPM and no repetition coding. The available data rates for the defined values of the RR factor are 4, 2, 1, 0.5, and 0.25 Mb/s. Every reduction step provides a SNR gain of nominally 3 dB electrical, corresponding to 1.5 dB gain in optical power. Thus, the nominal SNR gain increases from 0 dB to 12 dB electrical as RR changes from one to sixteen, respectively. Please note that for the remainder of this application relative power levels will be measured in the optical domain; a relative optical power level of×dB corresponds to 2×dB relative electrical power or 2×dB relative SNR change. The common data rate of 0.25 Mb/s with RR=16 must be supported by all systems within a communications cell, or within a subcell of a communication cell. This data rate is used in the RH field of every transmitted frame and serves to convey information relevant for the MAC and PHY layers. With increasing data rate reduction the communication system is capable of operating under successively worse SNR conditions. This can be used to gain a larger transmission range up to twice the transmission range at 0.25 Mb/s compared to the base rate of 4 Mb/s, or to maintain the link quality under hostile channel conditions such as high levels of background light. Further, repetition coding reduces the detrimental effect of interfering signals and can serve as power management by using a higher data rate for shorter distances which minimizes the energy consumption per transmitted frame.
In contrast to the RTS frame the CTS frame ends with the RH field. The RH field of the CTS frame comprises a RR* field instead of a RR field which is similar to the RR field in the RTS frame, except that it specifies recommendations for the RR to be used in the reverse direction based on an evaluation of the link quality or some other indication. Thus, it is a recommendation by the destination device that the source should use this parameter when communicating with it.
According to the IrDA Meeting paper, data frames comprise after the PA, SYNC, and RH fields a main body field which comprises a large data field which is followed by a Cyclic Redundancy Check (CRC) field. The CRC field is a variable repetition coded 32 bit field and is used in well-known manner for checking whether the transmission has been successfully accomplished.
The frames according to the prior art are transmitted with a single fixed optical power and use a fixed RR in the header. This does not allow to estimate the link quality for 4 Mb/s data rate (which corresponds to RR=1) with short packets, for example, RTS and CTS frames. Further, it does not provide sufficient SNR margins for expected parity mismatch conditions. Please note that the concept of parity for Ir systems has been introduced in the above-referenced IrDA Meeting publication.
The general object of the invention therefore is to provide absolution for improved collision avoidance properties and link quality estimation between the participating stations within the Alr concept.
It is thus a further object of the invention to provide a method which ensures only a connection between the stations if a certain prescribed SNR for a chosen RR is met.
It is another object of the invention to provide solutions for the introduction of special frames for link quality analysis.
It is a further object of this invention to propose an energy saving mode.
SUMMARY OF THE INVENTION
The invention as claimed is intended to meet these objectives. It provides a method for wireless optical comm
Gfeller Fritz R.
Hirt Walter
Chan Jason
Herzberg Louis P.
International Business Machines Corp.
Law Office of Charles W. Peterson, Jr.
Tran Dzung
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