Wireless local area network - Patent Review 5206881

Claims

We claim:

1. In a local area network comprising a plurality of stations each adapted to generate a PN code common to the stations, to transmit packets spread spectrum encoded with the PN code and to decode packets by combining the packets with the PN code, a method of determining phase values that synchronize the PN code of a first of the stations with a data packet transmitted through air by a second station to the first station, comprising:

transmitting from any one of the stations through air to the other stations a synchronizing packet direct sequence spread spectrum encoded with the PN code of the one station, the transmission of the synchronizing packet occurring before transmission of the data packet of the second station;

adjusting the phase of the PN code of each of the other stations prior to transmission of the data packet by the second station to a phase value synchronizing the phase of the PN code of the station with the phase of the transmitted synchronizing packet, the adjusting of the phase at each of the other stations comprising producing phase-shifted PN codes corresponding to the PN code of the station shifted in phase according to different phase values, combining the transmitted synchronizing packet with the phase-shifted PN codes to produce signals and detecting from the signals the synchronizing phase value;

searching at the first station for a phase value which synchronizes the PN code of the first station with the data packet, the searching at the first station comprising:

a. setting predetermined upper and lower bounds for a restricted set of phase values corresponding to the phase of the synchronizing packet,

b. selecting different phase values for the PN code of the first station from the restricted set,

c. producing phase-shifted PN codes corresponding to the PN code of the first station shifted in phase according to each of the phase values selected from the restricted set of phase values,

d. combining the phase-shifted PN codes with the data packet to produce signals each corresponding to a different phase value selected from the restricted set,

e. detecting from the signals a phase value which synchronizes the PN code with the data packet.

2. The method of claim 1 in which:

the synchronizing packet is a polling packet transmitted by the first station and addressed to the second station and the data packet is a response packet addressed to the first station;

the transmission of the polling packet by the first station comprises direct sequence spread spectrum encoding the polling packet with the PN code of the first station phase-shifted according to a predetermined phase value;

the transmission of the response packet by the second station includes spread spectrum encoding the response packet with the phase-adjusted PN code of the second station;

the setting of the upper and lower bounds of the restricted set of phase values at the first station comprises setting the lower bound to the predetermined phase value plus a first predetermined offset and setting the upper bound to the predetermined phase value plus a second predetermined offset.

3. The method of claim of claim 2 in which the step of adjusting the phase of the PN code of the other stations comprises repeatedly selecting the different phase values for production of phase-shifted PN codes from a relatively exhaustive set of phase values.

4. A method as claimed in claim 1 in which:

the synchronizing packet is transmitted by the second station;

the adjusting of the phase of the PN code of the first station to a phase value which synchronizes the PN code of the first station with the phase of the synchronizing packet comprises repeatedly selecting different phase values for production of phase-shifted PN codes at the first station from a relatively exhaustive set of phase values;

the setting of the upper and lower bounds of the restricted set comprises setting the lower bound to the detected synchronizing phase value which synchronized the phase of the PN code of the first station with the phase of the synchronizing packet plus a first predetermined offset and setting the upper bound to the detected synchronizing phase value which synchronized the phase of the PN code of the first station with the phase of the synchronizing packet plus a second predetermined offset.

5. The method of claim 4 in which:

the transmission of the synchronizing packet includes inserting into a trailing portion of the synchronizing packet an indicator identifying the packet as a synchronizing packet;

the detecting at each of the other stations of the synchronizing phase value synchronizing the phase of the PN code of the station with the phase of the synchronizing packet comprises spread spectrum decoding at least a portion of the synchronizing packet containing the indicator by combining the synchronizing packet with PN code of the station phase-shifted according to the synchronizing phase value and detecting the indicator in the decoded packet portion.

6. The method of claim 1 in which:

the one station transmitting the synchronizing packet is a station other than the first or second stations;

the transmission of the synchronizing packet includes inserting into a trailing portion of the synchronizing packet an indicator identifying the packet as a synchronizing packet;

the detecting at each of the other stations of the synchronizing phase value synchronizing the phase of the PN code of the station with the phase of the synchronizing packet comprises spread spectrum decoding at least a portion of the synchronizing packet by combining the synchronizing packet with PN code of the station phase-shifted according to the synchronizing phase value and detecting the indicator in the decoded packet portion.

7. A method of determining phase values that synchronize a PN code generated at a network station of a local area network with temporally spaced-apart packets transmitted through the air by a source station, the method comprising:

generating the PN code at the source station and spread spectrum encoding each of the temporally spaced-apart packets at the source station prior to transmission by combining each of the packets with the PN code of the source station phase-shifted to a substantially constant preset phase value;

performing a wide-range search for a phase value synchronizing the PN code of the network station with a first of the packets, the wide-range search comprising

a. repeatedly selecting different phase values for the PN code of the network station from a relatively exhaustive set of phase values;

b. producing phase-shifted PN codes corresponding to the PN code of the network station shifted in phase according to each of the selected phase values;

c. combining the phase-shifted PN codes with the first packet to produce signals each corresponding to a different phase value selected from the relatively exhaustive set;

d. detecting from the signals a phase value which synchronizes the PN code of the network station with the first received packet;

performing a narrow-range search for phase values synchronizing the PN code of the network station with each of the temporally spaced-apart packets succeeding the first packet, the narrow-range search comprising

a. selecting different phase values for the PN code of the network station from a restricted set of phase values with predetermined upper and lower bounds corresponding to a phase value which synchronized the PN code of the network station with an immediately preceding one of the packets;

b. producing phase-shifted PN codes corresponding to the PN code of the network station shifted in phase according to each of the phase values selected from the restricted set;

c. combining the phase-shifted PN codes with a current one of the packets to produce signals each corresponding to a different phase value selected from the restricted set;

d. detecting from the signals a phase value of the restricted set which synchronizes the PN code of the network station with the current data packet.

8. The method of claim 7 adapted for use in a polled local area network in which:

each of the temporally spaced-apart packets is a polling packet;

the method comprises inserting into a trailing portion of each of the packets prior to encoding and transmission by the source station an indicator identifying that the packet as a source station packet;

in each of the wide-range and narrow-range searches, the detecting of a phase value synchronizing the PN code of the network station with each of the temporally spaced-apart packets comprises spread spectrum decoding at least a portion of each of the packets by combining the packet packet with PN code of the station phase-shifted according to the synchronizing phase value and detecting the indicator in the decoded packet portion;

the method comprises switching from the wide-range search to the narrow-range search in response to detection of the indicator in a decoded portion of the first of the packets.

9. A method of determining phase values that synchronize a PN code generated at a network station of a local area network with temporally spaced-apart network packets transmitted through air and received by the network station, each network packet being direct sequence spread spectrum encoded with the PN code, the method comprising:

inserting a predetermined indicator into a trailing portion of each of the network packets prior to encoding and transmission of the network packet;

searching in either a wide-range search mode or a narrow-range search mode for phase values synchronizing the PN code of the network station with each of the network packets;

the wide-range search mode comprising

a. repeatedly selecting different phase values for the PN code of the network station from a relatively exhaustive set of phase values;

b. producing phase-shifted PN codes corresponding to the PN code shifted in phase according to each of the phase values selected from the relatively exhaustive set;

c. combining the phase-shifted PN codes with a currently received one of the network packets to produce signals each corresponding to a different phase value selected from the relatively exhaustive set;

d. detecting from the signals corresponding to phase values selected from the relatively exhaustive set a phase value which synchronizes the PN code of the network station with the currently received network packet including decoding at least a portion of the currently received network packet by combining the packet portion with the PN code phase shifted according to the synchronizing phase value and detecting presence of the indicator in the decoded packet portion;

the narrow-range search mode comprising

a. selecting different phases values for the PN code of the network station from a restricted set of phase values having predetermined upper and lower bounds corresponding to a phase value that synchronized the PN code of the network station with an immediately preceding one of the received network packets;

b. producing phase-shifted PN codes corresponding to the PN code of the network station shifted in phase according to each of the phase values selected from the restricted set;

c. combining the phase-shifted PN codes with a currently received one of the network packets to produce signals each corresponding to a different phase value selected from the restricted set;

d. detecting from the signals corresponding to phase values selected from the restricted set a phase value which synchronizes the PN code of the network station with the currently received network packet including decoding at least a portion of the currently received network packet by combining the packet portion with the PN code phase shifted according to the synchronizing phase value and detecting presence of the indicator in the decoded packet portion;

switching from the wide-range search mode to the narrow-range search mode in response to detection of the indicator in a received network packet;

switching from the narrow-range search mode to the wide-range search mode whenever a predetermined period of time has expired from detection of the indicator in a received network patent.

10. The method of claim 9 in which in at least the wide-range search mode:

the repeated selection of different phase values comprises repeatedly selecting different subsets of phase values, each subset comprising a plurality of different phase values;

the production of phase-shifted PN codes comprises producing a set of phase-shifted PN codes simultaneously in response to each currently selected subset of phase values, each phase-shifted PN code of the set of phase-shifted PN codes corresponding to a different phase value of the currently selected subset of phase values;

the combining of the phase-shifted PN codes with a currently received one of the packets to produce signals comprises combining each of the phase-shifted PN codes of the current set of phase-shifted PN codes with the currently received network packet to produce simultaneously a corresponding set of signals.

11. The method of claim 9 in which the network is a polled local area network, the temporally spaced-apart packets are polling packets transmitted by a base station comprised by the network and generating the PN code, and the predetermined indicator uniquely identifies each polling packet as a polling packet transmitted by the base station.

12. The method of claim 11 in combination with a method of determining phase values at the base station which synchronize the PN code of the base station with response packets transmitted through air by the network station to the base station in response to the polling packets, comprising:

direct sequence spread spectrum encoding each of the polling packet at the base station with the PN code of the base station phase-shifted according to a constant predetermined phase value of the base station;

encoding each of the response packets at the network station prior to transmission, the encoding of each of the response packets comprising

a. adjusting the phase of the PN code of the network station to the detected synchronizing phase value which synchronized the PN code of the network station with the corresponding polling packet;

b. spread spectrum encoding the response packet prior to transmission with the phase-adjusted PN code of the network station;

searching at the base station for phase values which synchronize the PN of the base station with each of the response packets, the searching at the base station comprising:

a. setting predetermined upper and lower bounds for a restricted set of phase values corresponding to the predetermined phase value of the base station;

b. selecting different phase values for the PN code of the base station from the restricted set;

c. producing phase-shifted PN codes corresponding to the PN code of the base station shifted in phase according to each of the selected phase values;

d. combining the phase-shifted PN codes of the base station with a currently received on of the response packet to produce signals each corresponding to a different selected phase value;

e. detecting from the signals a phase value which synchronizes the PN code of the base station with the currently received response packet.

13. The method of claim 11 in which the base station inserts a synchronizing header into a leading portion of each polling packet prior to encoding and transmission by the base station, the method comprising setting the length of the synchronizing header for the polling packets, the setting of the length of the synchronizing header comprising:

a. transmitting a series of temporally spaced-apart test polling packets from the base station addressed to the network station and containing a synchronizing header,

b. detecting whether a response packet is received from the network station in response to any one of the test polling packets,

c. discontinuing the transmission of the series of test polling packets if a response packet is detected,

d. setting an incrementally greater length for the synchronizing header if a response packet to the series of test polling packets is not detected,

e. repeating steps a to d until a response packet from the network station is detected.

14. The method of claim 13 comprising repeating steps a-e of claim 13 successively for any other polled network station such that a length for the synchronization header is determined which causes all polled network stations to respond to polling packets from the base station.

15. In a local area network, apparatus for use in processing temporally spaced-apart network packets transmitted through air and direct sequence spread spectrum encoded with a predetermined PN code, the apparatus comprising:

means for generating the PN code;

means for receiving and decoding the network packets, the receiving and decoding means comprising means for adjusting the phase of the PN code of the apparatus to a phase value which synchronizes the PN code of the apparatus with each of the network packets and means for combining the phase-adjusted PN code with each packet to spread spectrum decode the packet, the phase adjusting means comprising means for searching in either a wide-range search mode or a narrow-range search mode for phase values synchronizing the generated PN code with the packets;

the searching means comprising

a. means for selecting different phase values for the PN code of the apparatus from a set of phase values,

b. means for producing phase-shifted PN codes corresponding to the PN code of the apparatus shifted in phase according to each of the selected phases,

c. means for combining the phase-shifted PN codes with a currently received one of the packets to produce signals each corresponding to a different phase value selected from the set,

d. means for detecting from the signals a phase value which synchronizes the PN code with the currently received packet;

the searching means being adapted in the wide-range search mode repeatedly to select the different phases values for the PN code of the apparatus from a relatively exhaustive set of phase values;

the searching means being adapted in the narrow-range search mode to select the different phases for the PN code of the apparatus from a restricted set of phase values having predetermined upper and lower bounds corresponding to a phase value that synchronized the PN code of the apparatus with an immediately preceding one of the received packets.

16. The apparatus of claim 15 in which the searching means are adapted to switch from the wide-range search mode to the narrow-range search mode in response to detection of a phase value synchronizing the PN code of the apparatus with a currently received packet and to switch from the narrow-range search mode to the wide-range search mode whenever a predetermined period of time has expired following detection of a phase value synchronizing the PN code of the apparatus with a received packet.

17. The apparatus of claim 15 in which each of the packets comprises an predetermined indicator in a trailing portion of the packet and in which the searching means are adapted to confirm detection of the phase value synchronizing the PN code of the apparatus with the currently received packet in both the wide-range search and narrow-range search modes by decoding at least a portion of the currently received packet with the PN code phase-shifted to the synchronizing phase value and detecting the indicator in the decoded packet portion.

18. The apparatus of claim 17 in which the searching means are adapted to switch from the wide-range search mode to the narrow-range search mode in response to detection of the indicator in a received packet and to switch from the narrow-range search mode to the wide-range search mode whenever a predetermined period of time has expired from detection of the indicator.

19. The apparatus of claim 15 in which:

the means for selecting different phase values are adapted at least in the wide-range search mode to select different subsets of phase values, each subset comprising a plurality of different phase values;

the means for producing phase-shifted PN codes are adapted at least in the wide-range search mode to produce a set of phase-shifted PN codes simultaneously in response to each currently selected subset of phase values, each phase-shifted PN code of the set of phase-shifted PN codes corresponding to a different phase value of the currently selected subset of phase values;

the means for combining the phase-shifted PN codes with a currently received one of the packets to produce signals are adapted at least in the wide-range search mode to combine each of the phase-shifted PN codes of the current set of phase-shifted PN codes with the currently received network packet to produce simultaneously a corresponding set of signals.

20. A polled local area network comprising:

network stations including a base station and a plurality of polled stations polled by the base station;

each of the network stations comprising

a. means for generating a PN code common to the network stations,

b. means for encoding and transmitting packets through air, the encoding means including means for direct sequence spread spectrum encoding each packet prior to transmission with the PN code of the network station;

c. means for receiving and decoding each packet transmitted through air to the network station and direct sequence spread spectrum encoded with the PN code, the receiving and decoding means comprising means for adjusting the phase of the PN code of the network station to a phase value which synchronizes the PN code of the network station with the received packet and means for combining the phase-adjusted PN code with the received packet to spread spectrum decode the received packet, the phase adjusting means comprising means for searching for phase values synchronizing the PN code of the network station with the received packet;

the searching means of each of the polled stations being adapted to search in either a wide-range search mode or a narrow-range search mode for a phase value synchronizing the PN code of the polled station with polling packets transmitted by the base station and spread spectrum encoded with the PN code of the base station, the searching means of each of the polled stations comprising means for

a. selecting different phase values for the PN code of the polled station from a set of phase values;

b. producing phase-shifted PN codes corresponding to the PN code of the polled station shifted in phase according to each of the selected phases;

c. combining the phase-shifted PN codes with a currently received polling packet to produce signals each corresponding to a different phase value selected from the set of phase values;

d. detecting from the signals a phase value synchronizing the PN code of the polled station with the currently received polling packet;

the searching means of each of the polled stations being adapted in the wide-range search mode repeatedly to select the different phases values for the PN code of the polled station from a relatively exhaustive set of phase values;

the searching means of each of the polled stations being adapted in the narrow-range search mode to select the different phases values for the PN code of the polled station from a restricted set of phase values having predetermined upper and lower bounds corresponding to a phase value that synchronized the PN code of the apparatus with an immediately preceding one of the polling packets received by the polled station;

the searching means of each of the polled stations being adapted to switch from the wide-range search mode to the narrow-range search mode in response to detection of a phase value synchronizing the PN code of the polled station with a polling packet and to switch from the narrow-range search mode to the wide range search mode whenever a predetermined period of time has expired from detection of a phase value synchronizing the PN code of the polled station with a polling packet.

21. The local area network of claim 20 in which:

the base station is adapted to insert into a trailing portion of each polling packet prior to encoding and transmission by the base station an indicator identifying the packet as a polling packet;

the searching means of each polled station are adapted in both the wide-range and narrow-range search modes to confirm detection of the phase value synchronizing the PN code of the polled station with the currently received polling packet by decoding at least a portion of the currently received polling packet by combining the packet portion with the PN code of the polled station phase-shifted according to the synchronizing phase value and detecting the indicator in the decoded packet portion.

22. The local area network of claim of claim 20 in which:

the base station is adapted to direct sequence spread spectrum encode each polling packet with the PN code of the base station phase-shifted according to a predetermined constant phase value of the base station;

each polled station is adapted to direct sequence spread spectrum encode each response packet of the polled station with the PN code of the polled station set at the phase value which synchronized the PN code of the polled station with the corresponding polling packet;

the searching means of the base station are adapted to search for phase values synchronizing the PN of the base station with each response packet transmitted by the polled stations, the searchings means of the base station comprising means for:

a. setting predetermined upper and lower bounds for a restricted set of phase values corresponding to the predetermined phase value;

b. selecting different phase values for the PN code of the base from the restricted set;

c. producing phase-shifted PN codes corresponding to the PN code of the base station shifted in phase according to each of the selected phase values;

d. combining the phase-shifted PN codes of the base station with the response packet to produce signals each corresponding to a different selected phase value;

e. detecting from the signals a phase value which synchronizes the PN code with the response packet.

23. A polled local area network comprising:

network stations including a base station and a plurality of polled stations;

each of the network stations comprising

a. means for generating a PN code common to the network stations,

b. means for encoding and transmitting packets through air, the encoding means including means for direct sequence spread spectrum encoding each packet prior to transmission with the PN code of the network station;

c. means for receiving and decoding each packet transmitted through air to the network station and direct sequence spread spectrum encoded with the PN code, the receiving and decoding means comprising means for adjusting the phase of the PN code of the network station to a phase value which synchronizes the PN code of the network station with the received packet and means for combining the phase-adjusted PN code with the received packet to spread spectrum decode the received packet, the phase adjusting means comprising means for searching for phase values synchronizing the PN code of the network station with the packet;

the base station being adapted to direct sequence spread spectrum encode each polling packet transmitted by the base station with the PN code of the base station phase-shifted according to a predetermined phase value;

each polled station being adapted to direct sequence spread spectrum encode each response packet of the polled station with the PN code of the polled station set at a phase value which synchronized the PN code of the polled station with the corresponding polling packet received by the station;

the searching means of the base station being adapted to search for phase values synchronizing the PN code of the base station with response packets generated by the polled stations, the searching means of the base station comprising means for:

a. setting upper and lower bounds for a restricted set of phase values corresponding to the predetermined phase value of the base station used to encode the polling packet corresponding to the response packet;

b. selecting different phase values for the PN code of the base station from the restricted set;

c. producing phase-shifted PN codes corresponding to the PN code of the base station shifted in phase according to each of the selected phase values;

d. combining the phase-shifted PN codes of the base station with the response packet to produce signals each corresponding to a different selected phase value;

e. detecting from the signals a phase value which synchronizes the PN code of the base station with the response packet.

FIELD OF THE INVENTION

The invention relates to local area networks involving spread spectrum transmission of data packets, and more specifically, to methods and apparatus for determining phase values that synchronize pseudorandom noise codes (referred to in this disclosure and appended claims as "PN codes") with network packets.

BACKGROUND OF THE INVENTION

It is known to form a wireless local area network in which data packets are direct sequence spread spectrum encoded and transmitted through air. Techniques for decoding such packets, and directed more specifically to the problem of synchronizing local PN codes with received data packets, are described, for example, in prior U.S. Pat. No. 4,774,715 to Messenger, one of the co-inventors of the present invention. The packets transmitted in such systems include a synchronization header that permits a receiving station to lock onto a received packet and then spread spectrum decode the data following the header. Without a synchronization header of adequate length, the data is apt not to be decoded in a proper manner.

One shortcoming associated with such prior systems relates to the length of the synchronization header and time required to synchronize to a received packet before data can be decoded. The size of the synchronization header ultimately limits data transfer rates. This is most significant where a communications channel is time-shared by a large number of users. A requirement for large synchronization headers can make such systems impractical for digitized voice transmission.

It is common in decoding spread spectrum transmissions to select different phase values for the PN code of a receiving station from a relatively exhaustive set of values. Such a set will correspond in size to the number of "chips" contained in the PN code. Phase-shifted PN codes corresponding to each of the phase values are produced and combined with incoming signals to determine a phase value that will synchronize the station's PN code with an intended communication. The strength of the test signals may be detected in a known manner to determine whether any selected phase value synchronizes the station's PN code with the received data. Thereafter, the data is simply decoded by combining the data with the PN code of the station adjusted to the synchronizing phase value.

One method of producing more rapid synchronization is to provide a measure of parallelism in the search for a synchronizing phase value. More specifically, sets of possible phase values may be simultaneously tested to determine whether any one of the phase values is appropriate for decoding the incoming signal. If repeated selection and testing of phase values is to be entirely eliminated, the required phase-shifting and combining circuitry must be duplicated until all practical phase values of the receiving station's PN code are simultaneously tested. Given the typical chip size of a PN code (in excess of 100 chips), providing parallel processing circuitry at various stations in a local area network would be exceptionally costly. Other rapid synchronizing techniques, including matched filters, are known, but these too are very costly

BRIEF SUMMARY OF THE INVENTION

In prior spread spectrum communication systems, designers have largely assumed that a synchronizing mechanism can be provided to achieve whatever rate of synchronization is required. The present invention, however, takes an overall system approach to providing faster synchronization to direct sequence spread spectrum encoded packets in a local area network. What the inventors have recognized is that receipt of a packet by various network stations tends to produce a measure of synchronization among the PN codes of the stations. This can be exploited, as described more fully below, to restrict the range of phase values which must be tested at any given network station to determine synchronizing values for decoding of subsequent network transmissions.

In one aspect, in a local area network comprising a plurality of stations each adapted to generate a PN code common to the stations, to transmit packets spread spectrum encoded with a PN code and to decode packets by combining them with the PN code, the invention provides a method of determining phase values that synchronize the PN code of a first network station with a data packet direct sequence spread spectrum encoded with the PN code by a second network station. The method involves transmitting from one of the network stations (not necessarily the relevant first or second station), through air, a synchronizing packet encoded with the PN code. The phase of the PN codes of the other stations are adjusted to phase values which synchronize each station's PN code with the synchronizing packet. The initial phase adjusting may involve conventional searching through a relatively exhaustive set of phase values (repeatedly selecting different phase values, producing phase-shifted PN codes, combining the phase-shifted PN codes with the synchronizing packet to produce test signals, and detecting the synchronizing phase value from the test signals). The transmission of the synchronizing packet and the initial phase adjusting at the various network stations occur before the data packet in issue is transmitted to the first station from the second station. Thereafter, the searching at the first station for a phase value that synchronizes the PN code of the station with the data packet transmitted by the second station comprises setting upper and lower bounds for a restricted set of phase values corresponding to the phase of the synchronizing packet, selecting different phase values from the restricted set, producing phase-shifted PN codes corresponding to the PN code of the station shifted in phase according to each of the phase values selected from the restricted set, combining the phase-shifted PN codes with the data packet to produce test signals, and detecting from the test signals a phase value which synchronizes the PN code with the data packet. The term "PN code of the station" as used herein should be understood as referring to the PN code used by a station to spread spectrum encode or decode packets rather than intermediate test signals generated by searching for synchronizing phase values.

The synchronizing packet has a phase corresponding to that of the PN code of the source station at the time of its transmission. When the PN codes of the other stations are adjusted to synchronize to the packet, their phases then correspond to the current phase of the source station phase-delayed by an amount corresponding primarily to packet travel time. For example, the chip generation and transmission rate of each network station might typically be 10 megachips/second. Assuming a maximum separation between any two network stations of 500 feet, the transmission delay has a maximum value of 0.5 microseconds (1 nanosecond per foot for electromagnetic transmissions) and the resulting phase delay is 5 chips. This represents the maximum phase difference then existing between the PN codes of the various stations, including the relevant first and second stations between which the data packet is to be transmitted. When the data packet is thereafter transmitted by the second station using the current phase value of its PN code, the phase of the received data packet is shifted relative to the PN code of the first station by no more than 10 chips (an additional phase delay having a maximum value of 5 chips being introduced during transmission of the data packet). Accordingly, the search at the first station to determine a phase value synchronizing the PN code of the station with the data packet requires only a restricted range of phase values to be considered. For example, upper and lower bounds for the restricted set of phase values might be set by adding positive and negative offsets of 10 chips to the current phase of the PN code of the first station. Accordingly, a range of no more than 20 chips might be searched, rather than a relatively exhaustive range corresponding to the number of chips in the PN code (typically in excess of 100 chips). Relative drifting of station PN codes owing to frequency differences between PN code generators will normally not be a significant factor in such matters, assuming periodic transmission of synchronizing packets. However, as discussed more fully below, the method of the invention can be readily adapted to accommodate loss of relative synchronization owing to such drift.

The method is applicable inter alia to polled local area networks. The term "polled local area network" as used in the specification should be understood as a network in which a base station transmits polling packets (packets uniquely addressed to specific polled stations) and in which each polled station transmits packets only in response to polling packets addressed to the station. A polling packet generated by the base station can serve as the synchronizing packet for purposes of the invention. It will be apparent to those skilled in the art that in a polled local area network each polling packet is decoded by all polled stations, but is discarded by a station if not addressed to the station. Although a polling packet may ultimately be discarded, the process of decoding the packet nevertheless produces or maintains a measure of synchronization among the PN codes of the various network stations.

In a polled local area network, the method may comprise a relatively simple method for determining phase values at the base station which synchronize the PN code of the base station with a response packet. The method involves encoding a polling packet with the PN code of the base station phase-shifted according to a predetermined phase value. A narrow-range search may be performed at the base station for an expected response packet, and to that end, a restricted search set may be determined by setting the lower bound of the search set to the predetermined phase value plus a first predetermined offset and setting the upper bound to the predetermined phase value plus a second predetermined offset. In such a system configuration, the response packet is effectively phased delayed relative to the predetermined phase value by an amount corresponding to the round-trip travel time of the associated polling and response packets. Under the exemplary system conditions discussed above (namely, the specified maximum station separation and chip rates), this delay would be a maximum of 10 chips. Accordingly, the lower bound might be set to the current phase value of the base station PN code (the predetermined phase value used to encode the polling packet plus a zero offset) and the upper bound might be set to the current phase value of the PN code plus an offset of 10 chips. This method intrinsically permits the response packets to be formed with shorter synchronization headers. Since the base station must decode one-half of all network packets in such a local area network, a considerable increase in data transfer rates can be achieved.

In another aspect, the invention provides a method of determining phase values that synchronize a PN code generated at a station of a local area network with temporally spaced-apart network packets transmitted through the air and received by the network station. The method comprises performing a wide range search for a phase value synchronizing the PN code of the network station with a first of the packets. The wide-range search comprises repeatedly selecting different phase values for the PN code of the network station from a relatively exhaustive set of phase values, producing phase-shifted PN codes corresponding to the PN code of the network station shifted in phase according to each of the selected phase values, combining the phase-shifted PN codes with the first packet to produce signals corresponding to the selected phase values and detecting from the signals a phase value which synchronizes the PN code of the network station with the first received packet. The method comprises performing a narrow-range search for phase values synchronizing the PN code of the network station with each of the packets succeeding the first packet. The narrow-range search comprises selecting different phase values for the PN code of the network station from a restricted set of phase values with predetermined upper and lower bounds corresponding to a phase value that synchronized the PN code with an immediately proceeding one of the packets, producing phase shifted PN codes corresponding to the PN code shifted in phase according to each of the phase values selected from the restricted set, combining the phase-shifted PN codes with a current one of the packet to produce signals each corresponding to a different phase value selected from the restricted set, and detecting from the signals a phase value of the restricted set which synchronizes the PN code of the network station with a current data packet. In a more general approach, this method involves switching from the wide-range search mode to the narrow-range search mode in response to detection of a phase value synchronizing the PN code of the station with a received data packet and switching from the narrow-range mode to the wide-range search mode whenever a predetermined period of time has expired from detection of a synchronizing phase value. The first packet serves effectively as a synchronization packet and might be sacrificed to produce a predetermined measure of synchronization between the source station and the receiving station (together with other network stations which must typically hunt at all times for transmitted packets). In the more general adaption of the method, the switching between search modes accommodates the likelihood that synchronization of source and receiving station PN codes will at some time be lost.

To permit greater reliability in the detection of synchronizing phase angles, the synchronizing packets may contain an indicator identifying the packet as a synchronizing packet. This synchronizing packet indicator ("SPI") serves essentially to distinguish the packet (or a portion which has been successfully decoded) from spurious noise. The SPI is preferably located in a trailing portion of the packet. This is important if very short synchronization headers are used, as a station may not synchronize to a received packet before a large portion of the packet has effectively been lost. In such circumstances, the SPI permits confirmation that a phase angle did in fact synchronize the PN code of the station to a synchronizing packet. In a polled local area network, an SPI may be inserted into each polling packet to assist the polled stations in identifying a true synchronizing phase value. If the base station is adapted to perform only narrow-range searches in the manner described above, there is no need to insert an SPI into the response packets of the polled stations.

A measure of parallel searching for phase values may be incorporated into either or both the broad-range and narrow-range searches. The selecting of different