Scanning method for receiving a transmission in a communication system with slow frequency hopping and coding

• US 5,471,503 A
• Filed: 12/29/1992
• Issued: 11/28/1995
• Est. Priority Date: 12/29/1992
• Status: Expired due to Term

• Current Assignee: Apple Inc.
• Original Assignee: Apple Computer Incorporated (Apple Inc.)
• Inventors: Potrebic, Peter J., Altmaier, Paulette R.
• Primary Examiner(s): CANGIALOSI, SALVATORE A
• Application Number:
  US07/997,880
• Time in Patent Office: 1,064 Days
• Field of Search: 375/1, 375/202, 370/95.1
• US Class Current: 375/133
• CPC Class Codes: H04B 1/713 using frequency hopping
• CPC Class Codes: H04B 2001/71563 Acquisition

Abstract

A scanning method for receiving a signal in a frequency hopped communication system that transmits a packet having N segments that are encoded with an error correction code that allows recovery of a packet even if e segments are erasures. A receiving station continuously scans the first p (1≦p≦e+1) channels, testing each channel for the existence of a transmission. If detected, a segment is received, and then the receiving station sequences through the remaining channels, receiving the segments if possible, and then applying error correction to the complete transmission. This technique of scanning the first several channels, receiving the first unimpaired segment, and thereby gaining the ability to recover all unimpaired segments enables the full theoretical power of a coded, slow frequency hopped system to be realized.

Claims

1. In a decentralized communication system for communicating between two or more of a plurality of stations each of which includes a transmitter and a receiver, a method for detecting and receiving a packet of data transmitted by an arbitrary transmitting station of said plurality of stations that first encodes and arranges the packet into a plurality of N segments, and then transmits the packets using frequency hopping in a predetermined channel sequence of N radio frequencies, said method comprising the steps of:

(a) in an arbitrary receiving station that is initially unsynchronized with said transmitting station, scanning a subset of frequencies of said N radio frequencies, said subset including a first plurality p (p<

N) of frequencies in said predetermined channel sequence wherein a scan time for each frequency of said subset is substantially less than the time necessary to receive a segment;

(b) testing for the presence of a transmission on each of said p scanned frequencies;

(c) if a transmission is detected on one of said p scanned frequencies, then performing a receiving step (d), otherwise returning to step (a); and

(d) if a transmission is detected in the step (c), then synchronizing the receiving station with the transmitted packet, receiving the segment, hopping to the next frequency in the predetermined channel sequence and receiving a segment on that channel and continuing to hop and receive until the N^th segment has been received, otherwise returning to step (a).

2. The method of claim 1, further comprising a step (e) of arranging and decoding said received segments to recover said packet.

3. The method of claim 1 wherein each segment includes a preamble having a preamble time length t₁, and said seeps (a) and (b), scanning and testing are completed within a scanning and testing time period t₂ that is less than said preamble time length t₁.

4. The method of claim 2, wherein said packet is arranged by said transmitting station according to a predetermined protocol having N segments including a predetermined encoding in which the data in said packet is recoverable even if a number e of said N segments are erased, and wherein in said step (a) the number p is bounded on its upper limit by e+1.

5. The method of claim 4 wherein each segment includes a preamble having a preamble time length t₁, and said steps (a) and (b), scanning and testing are completed within a scanning and testing time period t₂ that is less than said preamble time length t₁.

6. The method of claim 1, wherein in the step (d), a destination address field of a received segment is tested to determine if the destination address matches that of the receiving station, and if the addresses do not match, then returning to step (a), otherwise continuing the receiving step (c).

7. The method of claim 1 wherein the receiving steps (a) through (d) described therein are performed by at least two of said plurality of stations.

13. The method of claim 5 wherein the step (b) is performed by at least two of said plurality of stations.

8. A decentralized communication method for communicating a packet of data between a plurality of stations using frequency hopping in a predetermined channel sequence of N radio frequencies, said communication method comprising the steps of:

(a) in an arbitrary transmitting station, transmitting a packet including the steps of

(a)(i) encoding and arranging said packet into a plurality of N segments according to a predetermined protocol and a predetermined encoding in which the data in said packet is recoverable even if a number e of said N segments are erased, and

(a)(ii) transmitting said N segments sequentially by hopping through said predetermined channel sequence of N radio frequencies, so that a first segment is transmitted on a first channel, a second segment is transmitted on a second channel, and so forth until the N^th sediment is transmitted on an N^th channel; and

(b) in a plurality of receiving stations that are unsynchronized with said transmitting station, performing the steps of

(b)(i) scanning a subset of frequencies of said N radio frequencies, said subset including a first plurality p (p<

N) of frequencies in said predetermined channel sequence wherein a scan time for each frequency of said subset is substantially less than the time necessary to receive a segment,

(b)(ii) testing for the presence of a transmission on one of said p scanned frequencies,

(b)(iii) if a transmission is detected on one of said p scanned frequencies, then synchronizing the receiving station with the transmitted packet, then receiving a segment in the frequency in which the transmission is first detected, hopping to the next frequency in the predetermined channel sequence and receiving a segment on that channel, and continuing until the N^th segment has been received, and

(b)(iv) decoding said received segments to recover the data packet.

9. The method of claim 8, wherein

the step (a)(i) includes adding a preamble to each of the segments corresponding to the p frequencies, said preamble having a preamble time length t₁ ;

and

in the scanning step (b)(i) and the testing step (b)(ii), scanning and testing all p frequencies occurs in a scanning and testing time length t₂ that is less than the preamble time length t₁.

10. The method of claim 8, wherein

the step (a)(i) includes adding a preamble and a header to each of the segments, said preamble having a first time length t₁, said header information being indicative of the length of each segment and a destination address for said packet;

and

in the scanning step (b)(i) and the testing step (b)(ii), scanning and testing all p frequencies occurs in a second time length t₂ that is less than the first time length

11. The method of claim 8 wherein the step (a)(i) includes adding a header to each of the segments, said header including information indicative of the length of each segment and a destination address that is intended for the packet.

12. The method of claim 11, wherein in the step (b)(iii), a destination address field of a received segment is tested to determine if the destination address matches that of the receiving station, and if the addresses do not match, then returning to step (a), otherwise continuing the receiving step (b)(iii).

14. The method of claim 8 wherein before the transmitting step (a)(ii), the first channel is monitored, and if energy on said first channel is detected in a first amount sufficient to interfere with transmission, then waiting before performing step (a)(ii), otherwise continuing with step (a)(ii).

15. A scanning and receiving circuit for receiving a packet that has been arranged by a transmitting station according to a predetermined protocol having N segments including a predetermined encoding in which the data in said packet is recoverable even if a number e of said N segments are erased, said segments being transmitted using N frequencies in a predetermined channel sequence, said scanning and receiving circuit comprising:

an antenna for receiving said plurality of frequencies;

a radio frequency (RF) modem coupled to said antenna, including

a frequency synthesizer for producing a plurality of RF frequencies,

a RF control unit coupled to the frequency synthesizer for selecting a radio frequency, said control unit including means for controlling the frequency synthesizer so that it serially scans p (1≦

p≦

N) frequencies of the N frequencies in said predetermined channel sequence in substantially less time than the time necessary to receive a segment; and

a RF receiver including a detection circuit for testing each frequency as it is scanned for the existence of a transmission and a circuit responsive to said detection circuit for controlling the RF receiver and the RF frequency synthesizer to synchronize with the transmitted packet and to receive the detected segment and following transmitted segments.

16. The receiving circuit of claim 15 further comprising

control logic coupled to the RF control unit for supplying control information thereto and for receiving status information therefrom,

a protocol engine coupled to the control logic and the receiver, said protocol engine including program means for arranging received segments and decoding said received segments to recover the packet.

17. A frequency hopping communication unit for an unsynchronized station in a Local Area Network (LAN) that receives data and commands on a system bus, said frequency hopping communication unit comprising:

an antenna;

a radio frequency (RF) modem coupled to said antenna, including

a frequency synthesizer for producing a plurality of RF frequencies,

a RF transmitter,

a RF receiver, and

a RF control unit coupled to said RF transmitter and RF receiver, said RF control unit also coupled to the frequency synthesizer for selecting a radio frequency, said RF control unit including control means for controlling the frequency synthesizer so that it serially scans a selected number of frequencies in substantially less time than the time necessary to receive a segment;

a data bus coupled to the RF modem for supplying data to the transmitter and receiving data front the receiver; and

a LAN controller coupled to the RF modem and the data bus, said LAN controller comprising

control logic coupled to the RF control unit for supplying control information thereto and for receiving status information therefrom, and

a protocol engine coupled to receive data and commands from the system bus, said protocol engine also coupled to the control logic and the data bus, said protocol engine for arranging said packet according to a predetermined protocol having N segments and a predetermined protocol having N segments including a predetermined encoding in which the data in said packet is recoverable even if a number e of said N segments are erased, said protocol engine also for supplying the segments to the RF transmitter through the data bus and commands to the control logic.

18. The communication unit of claim 17 wherein the selected number of scanned frequencies is p, (1≦

p≦

e+1), and the receiver includes means for testing each frequency as it is scanned for the existence of a transmission.

19. In a decentralized communication system for communicating between two or more of a plurality of unsynchronized stations, a method for detecting and receiving a packet of data transmitted by an arbitrary transmitting station of said plurality of stations using frequency hopping in a predetermined channel sequence of N radio frequencies including a first p (p<

N) frequencies, in which said transmitting station encodes and arranges the packet into a plurality of N segments including a first p (p<

N) segments each having a corresponding preamble time length t₁, comprising the steps of;

(a) in an arbitrary receiving station that is unsynchronized with said transmitting station, scanning and testing said first p frequencies in a second time period t₂ that is less than the preamble time length t₁ ;

(b) if a transmission is detected on a first one of said p frequencies, then performing a receiving step (c), otherwise returning to step (a); and

(c) if a transmission is detected in the step (a), receiving the segment associated with said first one of said p frequencies, hopping to the next frequency in the predetermined channel sequence and receiving a segment on that channel, and continuing to hop and receive until the N^th segment has been received.

20. The method of claim 19, further comprising a step:

(d) arranging and decoding said received segments to recover said packet.

21. The method of claim 20, wherein said packet is arranged by said transmitting station according to a predetermined protocol having N segments including a predetermined encoding in which the data in said packet is recoverable even if the number e of said N segments are erased, and wherein the number p is bounded on its upper limit by e+1.

22. The method of claim 19, wherein at least one of said N segments includes a header including information indicative of the destination address for said packet, and further comprising a step in which the destination address of a received segment is tested to determine if the destination address matches that of the receiving station, and if the addresses do not match, then returning to step (a), otherwise continuing the receiving step (c).

23. The method of claim 19, wherein at least one or said p segments includes information indicative of the length of each segment, and step (c) further includes detecting said segment length and utilizing it to receive each segment.

24. The method of claim 19 wherein the steps (a) through (c) described therein are performed by at least two of said plurality of stations.