Robust method and apparatus enabling multi-mode wireless optical communication

• US 6,442,145 B1
• Filed: 07/01/1998
• Issued: 08/27/2002
• Est. Priority Date: 01/03/1996
• Status: Expired due to Term

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: De Lange, Martin, Gfeller, Fritz R., Hirt, Walter
• Primary Examiner(s): Ton, Dang
• Assistant Examiner(s): NGUYEN, BRIAN D
• Application Number:
  US09/101,104
• Time in Patent Office: 1,518 Days
• Field of Search: 370/310, 370/204, 370/205, 370/213, 370/215, 370/229, 370/233, 370/234, 370/337, 370/343, 370/389, 370/395.1, 370/503, 370/509, 370/520, 370/321, 370/203, 370/401, 375/239, 399/158
• US Class Current: 370/310
• CPC Class Codes: H04B 10/1141 One-way transmission
• CPC Class Codes: H04B 10/1149 Arrangements for indoor wireless networking of information
• CPC Class Codes: H04L 1/0003 by switching between different modulation schemes
• CPC Class Codes: H04L 1/0025 Transmission of mode-switching indication
• CPC Class Codes: H04L 7/042 Detectors therefor, e.g. correlators, state machines digital correlators in general G06F17/15
• CPC Class Codes: H04L 7/046 using a dotting sequence
• CPC Class Codes: Y02D 30/50 in wire-line communication networks, e.g. low power modes or reduced link rate

Abstract

Disclosed is an optical communication system enabling communication between several co-existing transmitting and receiving stations. In order to allow communication between the co-existing stations, a robust physical layer header (RPLH; 50) is employed which can be understood by all participating stations. This robust header (50) at least comprises a preamble (52) consisting of frames forming a periodic sequence of pulses, the number of slots per frame and the frame content being known to all participating stations. The preamble (52) serves for relative synchronization and carrier detection of the receiving stations. The robust header (50) further comprises a unique synchronization word (53) used for absolute synchronization of the receiving stations. This synchronization word (53) is followed by a control field (59) of fixed length and known structure. By means of this control field (59) the receiving stations are informed which modulation method will be used for the transmission of data. Under certain circumstances it is also useful to provide other control information in said control field (59) for communication link and network control. Furthermore, information may be exchanged to allow negotiation and/or adaptation of the data rate used for transmission. This allows to optimize the throughput depending on the conditions (quality) on the channel.

Claims

1. A method of enabling wireless optical communication between a transmitting station and a first receiving station capable of receiving data which are modulated using a first modulation method and a second receiving station capable of receiving data which are modulated using a second modulation method, the method comprising:

optically transmitting a preamble comprising frames forming a periodic sequence of pulses with defined period, the number of slots (L) per frame and the frame content being known to said receiving stations,

performing carrier detection based on said sequence of pulses received, where

each of said receiving stations determines said period of the sequence of pulses to obtain relative synchronization, and

each of said receiving stations adjusts its clock to the phase of slots of the received sequence of pulses, and clocks said incoming sequence of pulses through a shift register,

transmitting a unique synchronization word aligned to said period, said unique synchronization word being known to said receiving stations,

correlating said sequence of pulses in said shift register with said unique synchronization word known to it in order to achieve absolute synchronization with said transmitting station upon recognition of said synchronization word, by each receiving station

indicating in a control field of predefined length and structure, whether the first or second modulation method will be used for transmission of the data in a data/subsystem field, such that all receiving stations which are able to support the respective modulation method wait for said data.

2. The method of claim 1, whereby a receiving station capable to support different modulation methods further comprising the step of:

switching to the respective modulation method indicated in said control field.

3. The method of claim 1, whereby either said control field, or said data/subsystem field contains a list of addressees for said data.

4. The method of claim 1, whereby pulse-position modulation (PPM) is used as modulation method for the transmission.

5. The method of claim 4, whereby said control field comprises a rate reduction field (RR) which indicates a receiving station how often each pulse-position modulation (PPM) symbol of said data/subsystem field will be repeated.

6. The method of claim 1, whereby said control field comprises a recommended rate reduction field (RR*) for transmission of a recommended rate reduction (RR*) which is employed to negotiate a data rate best suited for communication.

7. The method of claim 6, whereby said recommended rate reduction (RR*) is determined based on pre-defined rules taking into account the actual error rate which occurred during the communication between transmitting station and receiving station.

8. The method of claim 1, whereby said control field comprises a block size field (BS) indicating the number of data units which will be transmitted in said data/subsystem field.

9. The method of claim 1, whereby said control field comprises a mode field which carries the information used to indicate to said receiving station which modulation method will be used.

10. The method of claim 9, whereby said modulation method is either

4-slot pulse-position modulation (4-PPM),

16-slot pulse-position modulation (16-PPM), or

the modulation methods defined in the IrDA standard.

11. The method of claim 1, whereby said control field comprises information signaling to a receiving station whether said data shall be forwarded by said receiving station.

12. The method of claim 1, whereby said control field comprises information which allows any receiving station not being addressed, or not being able to support the modulation scheme indicated in said control field, to determine how long the transmission of said data will take to ensure that these receiving stations remain silent during this transmission.

13. The method of claim 1, further comprising the step of recognizing said synchronization word using an algorithm even in the presence of potential errors caused by corrupted optical communications by each receiving station.

14. The method of claim 1, whereby said synchronization word consists of two synchronizations words such that the recognition at the receiving station can be split into two stages.

15. The method of claim 1, whereby said receiving station and/or transmitting station determine the data rate at which said data are to be transmitted.

16. The method of claim 1, whereby a receiving station determines a recommended data rate to be used for transmission of said data, taking into account the current quality of the communication channel between transmitting station and receiving station, said recommended data rate being determined based on the knowledge when exactly the control field ends and thus said data/subsystem field starts.

17. The method of claim 15 whereby said date rate and/or recommended data rate information is made available to an application program or an end user.

18. The method of claim 1, whereby said control field contains priority information allowing at least two co-existing communication subcells (pico-cells) within a communication cell.

19. The method of claim 1, whereby said control field contains information supporting encryption of said data.

20. A multi-mode packet for wireless optical communication between a transmitting station and a first receiving station capable of receiving data which are modulated using a first modulation method and a second receiving station capable of receiving data which are modulated using a second modulation method, said multi-mode packet comprising:

a pulse-position modulated (PPM) data/subsystem field which carries data to be transmitted, and

a preceding robust physical layer header (RPLH) which can be decoded by all receiving stations, said robust physical layer header (RPLH) having;

a. a preamble with frames forming a periodic sequence of pulses with defined period, the number of slots per frame and the frame contents being known to said receiving stations,

b. a unique synchronization word being known to said receiving stations, and

c. a control field of predefined length and structure comprising mode information indicating which modulation method is used for the transmission of said data, and a rate reduction field (RR) which indicates to said receiving stations how often each pulse-position modulation (PPM) symbol of said data/subsystem field will be repeated.

21. The multi-mode packet of claim 20, whereby either said control field, or said data/subsystem field comprises a list of addressees for said data.

22. The multi-mode packet of claim 20, whereby said control field comprises a recommended rate reduction field (RR*;

) for transmission of recommended rate reduction (RR*) information which is employed to negotiate a date rate best suited for communication.

23. The multi-mode packet of claim 20, whereby said control field comprises a block size field (BS) indicating the number of data units which will be transmitted in said data/subsystem field.

24. The multi-mode packet of claim 20, whereby said control field comprises information signaling to a receiving station whether said data shall be forwarded by said receiving station.

25. The multi-mode packet of claim 20, whereby said control field contains information which allows any receiving station not being addressed, or not being able to support the modulation scheme indicated in said control field, to determine how long the transmission of said data will take to ensure that these receiving stations remain silent during this transmission.

26. The multi-mode packet of claim 20, whereby said control field comprises information supporting encryption of said data.

27. A transmitter for wireless optical communication with a first receiving station capable of receiving data which are modulated using a first modulation method and a second receiving station capable of receiving data which are modulated using a second modulation method, said communication being initiated using a robust physical layer header (RPLH) which can be decoded by all receiving stations, said transmitter comprising:

a header generator providing a preamble, being part of said robust physical layer header (RPLH), with frames forming a periodic sequence of pulses with defined period, the number of slots per frame and the frame content being known to said receiving stations,

a synchronizer for providing a unique synchronization word, being part of said robust physical layer header (RPLH), and being known to said receiving stations,

a controller for providing a control field of fixed length and known structure, being part of said robust physical layer header (RPLH), said control field indicating the respective modulation method which will be used for transmission of data,

a modulator for modulating said data to be transmitted, and

a data transmitter for transmitting said sequence of pulses in a data/subsystem field, said unique synchronization word being aligned to said sequence of pulses, said control field, and said modulated data.

28. The transmitter of claim 27, further comprising means to provide a list of addressees for said data to be transmitted, said list of addressees being either provided within said control field or said data/subsystem field.

29. The transmitter of claim 27, further comprising means to generate a data field (DSF) using the previously received recommended rate reduction (RR*) for transmission of data, and transmits in said control field its own recommended rate reduction (RR*) being used to negotiate a data rate best suited for communication.

30. The transmitter of claim 27, further comprising means for indicating the number/size of said data to be transmitted.

31. The transmitter of claim 27, further comprising means for signaling to a receiving station whether said data shall be forwarded by said receiving station.

32. A receiver for wireless optical communication in a multi-mode communications cell with a transmitting station which transmits data utilizing one of at least two different modulation methods and which transmits said data with a robust physical layer header (RPLH) which can be decoded by all receivers in the multi-mode communications cell and which RPLH comprises:

a preamble with frames forming a periodic sequence of pulses with defined period, the number of slots per frame and the frame content being known to said receiver,

a unique synchronization word known to said receiver, and

a control field of fixed length and known structure, indicating the respective modulation method which will be used for transmission of data,

said receiver comprising;

means for determining said period of the sequence of pulses, based on the number of slots per frame and the frame content being known to it, in order to obtain relative synchronization,

means for carrier detection based on said sequence of pulses,

means for adjusting said receiver'"'"'s clock to the phase of the slots of said sequence of pulses received,

means for clocking said sequence of pulses received through a shift register the length of which is defined by said unique synchronization word,

means for correlating said sequence of pulses in said shift register with said unique synchronization word known to it in order to achieve absolute synchronization with said transmitting station upon recognition of said unique synchronization word,

means to determine from said control field whether said receiver is able to support the respective modulation method which will be used for the transmission of said data,

means which determine from the information received whether the receiver is the right recipient for said data, and

means enabling the receiver to receive said data right after the end of said control field.

33. The receiver of claim 32, further comprising means to switch between one, two or more modulation methods.

34. The receiver of claim 32, wherein said means to achieve absolute synchronization comprise a first and a second correlator such that a synchronization word generated using two synchronization words can be recognized in two stages.

35. The receiver of claim 32, further comprising means to determine a recommended rate reduction (RR*), derived from channel quality estimation, being used to negotiate a data rate which is best suited for the transmission of said data.

36. The receiver of claim 32, further comprising means for determining the number/size of said data to be expected, analyzing information transmitted in said control field.

37. The receiver of claim 32, further comprising means for retransmission of said data if the received information indicates that retransmission is required.

38. A multi-mode wireless optical communication system comprising:

at least one transmitter comprising;

a header generator providing a preamble, being part of a robust physical layer header (RPLH), with frames forming a periodic sequence of pulses with defined period, the number of slots per frame and the frame content being known to a receiving station,

a synchronizer for providing a unique synchronization word, being part of said robust physical layer header (RPLH), and being known to said receiving station,

a controller for providing a control field of fixed length and known structure, being part of said robust physical layer header (RPLH), said control field indicating the respective modulation method which will be used for transmission of data,

a modulator for modulating said data to be transmitted, and

a data transmitter for transmitting said sequence of pulses in a data/subsystem field, said unique synchronization word being aligned to said sequence of pulses, said control field, and said modulated data, and at least one receiver comprising;

means for determining said period of the sequence of pulses, based on the number of slots per frame and the frame content being known to it, in order to obtain relative synchronization,

means for carrier detection based on said sequence of pulses,

means for adjusting said receiver'"'"'s clock to the phase of the slots of said sequence of pulses received,

means for clocking said sequence of pulses received through a shift register the length of which is defined by said unique synchronization word,

means for correlating said sequence of pulses in said shift register with said unique synchronization word known to it in order to achieve absolute synchronization with said transmitter upon recognition of said unique synchronization word,

means to determine from said control field whether said receiver is able to support the respective modulation method which will be used for the transmission of said data,

means which determine from the information received whether the receiver is the right recipient for said data, and

means enabling the receiver to receive said data right after the end of said control field.

39. The communications system of claim 38 wherein the at least one transmitter further comprises a list generator for providing a list of addresses to which said data is to be transmitted, said list being provided either within said control field or said data/subsystem field.

40. The communications system of claim 38 wherein:

the at least one transmitter further comprising means to generate a data field using the previously received recommended rate reduction for transmission of data, and transmits in said control field its own recommended rate reduction being used to negotiate a data rate best suited for communication; and

the at least one receiver further comprising means to determine a recommended rate reduction, derived from channel quality estimation, being used to negotiate a data rate which is best suited for the transmission of said data.

41. The communications system of claim 38 wherein:

the at least one transmitter further comprising means for indicating the number/size of said data to be transmitted; and

the at least one receiver further comprising means for determining the number/size of said data to be expected by analyzing information transmitted in said control field.

42. The communications system of claim 38 wherein:

the at least one transmitter further comprising means for signaling to said at least one receiver whether said data should be forwarded by said at least one receiver; and

the at least one receiver further comprising means for retransmission of said data if the received information indicates that retransmission is required.