United States Patent [19]
`O'Sullivan et al.
`
`lllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll
`US005487069A
`5,487,069
`[11] Patent Number:
`[ 45] Date of Patent:
`Jan.23, 1996
`
`[54] WIRELESS LAN
`
`26-line 35.
`
`[75]
`
`Inventors: John D. O'Sullivan, Ermington;
`Graham R. Daniels, Willoughby;
`Terence M. P. Percival, Lane Cove;
`Diethelm I. Ostry, Petersham; John F.
`Deane, Eastwood, all of Australia
`
`[73] Assignee: Commonwealth Scientific and
`Industrial Research Organisation,
`Australia
`
`[21] Appl. No.: 157,375
`
`[22]
`
`Filed:
`
`Nov. 23, 1993
`
`[30]
`
`Foreign Application Priority Data
`
`Nov. 27, 1992
`
`[AU] Australia ................................. PL6069
`
`Int. Cl.6
`.........•.............•.•.•••••..............•••••...• H04B 7/01
`[51]
`[52] U.S. Cl .......................... 370/94.3; 375/284; 375/348;
`455/52.3; 455/65
`[58] Field of Search .................................. 375/34, 39, 51,
`375/57, 58, 99, 101, 254, 261, 279, 284,
`285, 346, 348; 370/95.3; 455/56.1, 54.1,
`63, 65, 52.3
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`3,605,019
`4,630,314
`4,679,227
`4,888,767
`5,095,535
`5,191,576
`5,283,780
`
`911971 Cutter et a!. .............................. 375/58
`12/1986 Smith ........................................ 375/58
`7/1987 Hartogs ..................................... 375/58
`12/1989 Furuya et a!. ............................ 375/58
`3/1992 Freeburg ................................... 455/55
`3/1993 Pommier et a!. ......................... 370/50
`211994 Schuchman eta! ...................... 455/65
`
`OTHER PUBLICATIONS
`
`Supercomrn!ICC'92 vol. 2, Jun. 1992, Chicago US pp.
`1025-1031 D. Buchholz et al. 'Wireless In-Building Net(cid:173)
`work Architecture and Protocols' p. 1029, left col., line
`
`IEEE Transactions on Communications, vol. 39, No. 5, May
`1991, New York US pp. 783-793 E. F. Casas et al. 'OFDM
`for Data Communication over Mobile Radio FM Chan(cid:173)
`nels-Part I: Analysis and Experimental Results' p. 784, left
`col., line 1-right col., line 2; FlG. 1 p. 790, right col., line
`18-line 22.
`
`42nd VTS Conference vol. 2, May 1992, Denver US pp.
`819-822 T. Le-Ngoc 'A CSMA/CD Portable Data System
`Using Adaptive Reed-Solomon Coding' p. 820, left col.,
`line 2-line 9.
`
`IEEE Transactions on Communications, vol. 33, No. 7, Jul.
`1985, New York US pp. 665-675 L. J. Cimini Jr. 'Analysis
`and Simulation of a Digital Mobile Channel Using Orthogo(cid:173)
`nal Frequency Division Multiplexing' par. I-par. II. Par IV.
`
`Primary Examiner-Benedict V. Safourek
`Attorney, Agent, or Firm-William S. Frommer
`
`[57]
`
`ABSTRACT
`
`The present invention discloses a wireless LAN, a peer-to(cid:173)
`peer wireless LAN, a wireless transceiver and a method of
`transmitting data, all of which are capable of operating at
`frequencies in excess of 10 GHz and in multipath transmis(cid:173)
`sion environments. This is achieved by a combination of
`techniques which enable adequate performance in the pres(cid:173)
`ence of multi path transmission paths where the reciprocal of
`the information bit rate of the transmission is short relative
`to the time delay differences between significant ones of the
`multipath transmission paths. In the LANs the mobile trans(cid:173)
`ceivers are each connected to, and powered by, a corre(cid:173)
`sponding portable electronic device with computational
`ability.
`
`72 Claims, 8 Drawing Sheets
`
`BITCLOCKS SYMBOL aOCKS
`
`FROM CONTROL and TIHING UNIT 38
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`Cisco Systems, Inc., Exhibit 1112
`Page 1
`
`

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`U.S. Patent
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`Jan. 23, 1996
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`5,487,069
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`Cisco Systems, Inc., Exhibit 1112
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`U.S. Patent
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`Jan. 23, 1996
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`U.S. Patent
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`Jan.23, 1996
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`
`

`
`U.S. Patent
`
`Jan. 23, 1996
`
`Sheet 8 of 8
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`5,487,069
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`Cisco Systems, Inc., Exhibit 1112
`Page 9
`
`

`
`5,487,069
`
`1
`WIRELESS LAN
`
`BACKGROUND OF THE INVENTION
`
`2
`This engineering designer concludes that the inadequate
`performance, and the large size, expense and power con(cid:173)
`sumption of the hardware needed to adaptively equalize
`even a 10 Mbit/s data signal are such that the problems of
`5 multi path propagation cannot thereby be overcome in Wire(cid:173)
`less In-Building Network (WIN) systems. Similarly, spread
`spectrum techniques which might also be used to combat
`multipath problems consume too much bandwidth (300
`MHz for 10 Mbits/s) to be effective. A data rate of 100
`10 Mbit/s utilizing this technology would therefore consume 3
`GHz of bandwidth.
`Instead, the solution adopted by Motorola and Mitzlaff is
`a directional antenna system with 6 beams for each antenna
`resulting in 36 possible transmission paths to be periodically
`checked by the system processor in order to locate the "best
`quality" path and "switch" the antennae accordingly. This
`procedure adds substantial bulk and cost to the system. This
`procedure is essentially the conversion of a multi path trans(cid:173)
`mission problem into a single path transmission environ-
`20 ment by the use of directional antennae.
`
`OBJECTS AND SUMMARY OF THE
`INVENTION
`
`The present invention relates to local area networks
`(LANs) which enable devices with computational ability to
`communicate with each other and, in particular, to a wireless
`LAN in which the devices communicate by means of radio
`transmissions.
`In recent years the personal computer has become an
`increasingly important tool in business and commerce and
`many workers now spend a good portion of their working
`day operating such computers. Similarly, business organi(cid:173)
`sations are increasingly structuring their businesses to not
`only enable, but to oblige, their workers to access informa- 15
`tion by means of a personal computer or equivalent terminal,
`which is connected to a local area network which extends
`around or through the office environment.
`Hitherto such local area networks have been provided
`either by electrical conductor or optical fibre and this
`requires the office premises to be extensively cabled. This
`cabling must be adjusted if, for example, partitions within an
`office are to be adjusted. In addition, the cabling required for
`a classroom or tutorial arrangement where a large number of
`personal computers are intended to be operated within a 25
`small areas, can be quite substantial.
`Furthermore, an increasing trend in recent times has been
`the sale of mobile or portable devices with computational
`ability. These include both laptop/notebook and handheld
`computers. Whilst the primary impetus for the purchase of
`such a computer is the ability to use its computational power
`outside of the normal office environment, once a portable
`computer has been purchased, the desire arises to use the
`portability within the office premises so as to allow the user
`of the portable computer to take the computer with him and 35
`use it in the closely adjacent offices of colleagues, for
`example, and yet still be able to access the LAN of the
`business organisation, which may be spread over several
`adjacent buildings in "campus" style.
`While this is possible by means of plug-in connectors
`which enable the portable computer of one operator to be
`plugged into the office LAN at any particular location, it is
`generally inconvenient since the LAN may not provide for
`two or more points of connection within a single office, the
`portable computer loses its portability, and so on.
`Accordingly, the need arises for a LAN to which such
`portable devices can be connected by means of a wireless or
`radio link.
`Such wireless LANs are known, however, hitherto they
`have been substantially restricted to low data transmission
`rates. In order to achieve widespread commercial accept(cid:173)
`ability, it is necessary to have a relatively high transmission
`rate and therefore transmit on a relatively high frequency, of
`the order of 1 GHz or higher. As will be explained hereafter,
`radio transmission at such high frequencies encounters a
`collection of unique problems.
`One wireless LAN which is commercially available is that
`sold by Motorola under the trade name ALTAIR. This
`system operates at approximately 18 GHz, however, the
`maximum data transmission rate is limited to approximately
`3-6 Mbit/s. A useful review of this system and the problems
`of wireless reception at these frequencies and in "office"
`environments is contained in "Radio Propagation and Anti(cid:173)
`multipath Techniques in the WIN Environment", James E. 65
`Mitzlaff IEEE Network Magazine November 1991 pp.
`21-26.
`
`40
`
`45
`
`The object of the present invention is to provide a wireless
`LAN in a confined multipath transmission environment
`having a high bit rate even through the reciprocal of the data
`or information bit rate (the data "period") is short relative to
`the time delay differences between significant transmission
`30 paths.
`According to one aspect of the invention there is disclosed
`a transmitter for operation in a confined multi path transmis(cid:173)
`sion environment, said transmitter comprising antenna
`means coupled to transmission signal processing means in
`turn coupled to an input data channel, said transmitter being
`operable to transmit data at radio frequencies in excess of 10
`GHz, and said transmission signal processing means com(cid:173)
`prising modulation means for modulating input data of said
`input data channel into a plurality of sub-channels com(cid:173)
`prised of a sequence of data symbols such that the period of
`a sub-channel symbol is longer than a predetermined period
`representative of the time delay of significant ones of
`non-direct transmission paths.
`According to another aspect, there is disclosed a trans-
`mitter for operation in a confined multipath transmission
`environment, said transmitter comprising antenna means
`coupled to transmission signal processing means in tum
`coupled to an input data channel, said transmitter being
`50 operable to transmit data at radio frequencies, said trans(cid:173)
`mission signal processing means comprising modulation
`means for modulating input data of said input data channel
`into a plurality of sub-channels comprised of a sequence of
`data symbols such that the period of a sub-channel symbol
`is longer than a predetermined period representative of the
`time delay of significant ones of non-direct transmission
`paths, means to apply data reliability enhancement to said
`data passed to said modulation means and means, interposed
`between said data reliability enhancement means and said
`60 modulation means, for interleaving blocks of said data.
`A transmitter can further be incorporated into a trans(cid:173)
`ceiver for operation in a confined multipath transmission
`environment. The transceiver also comprises reception sig(cid:173)
`nal processing means coupled to the antenna means and an
`output data channel to receive data at radio frequencies. A
`transceiver can be incorporated in a peer-to-peer wireless
`LAN, in that a plurality of mobile such transceivers for data
`
`55
`
`Cisco Systems, Inc., Exhibit 1112
`Page 10
`
`

`
`5,487,069
`
`10
`
`20
`
`30
`
`3
`transceiving operation by radio transmissions between ones
`thereof in a confined multipath environment. Furthermore,
`transceivers can be included in a wireless LAN, in that a
`plurality of such mobile transceivers have data transceiving
`operation by radio transmissions to one of a plurality of hub 5
`transceivers, the hub transceivers being coupled together to
`constitute a plurality of data sources and destinations.
`According to another aspect, the invention discloses a
`method for transmitting data in a confined multipath trans(cid:173)
`mission environment at radio frequencies in excess of 1 0
`GHz, said data being provided by an input data channel
`coupled to transmission signal processing means in tum
`coupled to antenna means, said method comprising the steps
`of:
`modulating said data, by modulation means of said trans- 15
`mission signal processing means, into a plurality of
`sub-channels comprised of a sequence of data symbols
`such that the period of a sub-channel symbol is longer
`than a predetermined period representative of the time
`delay of significant one of non-direct transmission
`paths; and
`transmitting, by said antenna means, said sub-channel
`symbols at said radio frequencies in excess of 10 GHz.
`According to a yet further aspect of the invention, there is 25
`disclosed a method for transmitting data in a confined
`multipath transmission environment of radio frequencies,
`said data being provided by an input data channel coupled to
`transmission signal processing means in tum coupled to
`antenna means, said method comprising the steps of:
`applying data reliability enhancement to said data;
`interleaving blocks of said enhanced data;
`modulating said data, by modulation means of said trans(cid:173)
`mission signal processing means, into a plurality of
`sub-channels comprised of a sequence of data symbols 35
`such that the period of a sub-channel symbol is longer
`than a predetermined period representative of signifi(cid:173)
`cant ones of non-direct transmission paths; and
`transmitting, by said antenna means, said sub-channel
`symbols.
`Preferably, transmission is enhanced by the use of one or
`more of the following techniques, namely interactive chan(cid:173)
`nel sounding, forward error correction with redundancy
`sufficient for non-interactive correction, modulation with
`redundancy sufficient for interactive error correction by
`re-transmission of at least selected data, and the choice of
`allocation of data between sub-channels.
`The radio transmission is also preferably divided into
`small packets of data each of which is transmitted over a
`time period in which the transmission characteristics over 50
`the predetermined range are relatively constant.
`The encoding of the data is preferably carried out on an
`ensemble of carriers each costituting a sub-channel and
`having a different frequency with the modulation of each
`individual carrier preferably being multi-level modulation of 55
`carrier amplitude and/or phase (mQAM). The modulation
`family mQAM includes amplitude shift keying (ASK),
`multi-level ASK (mASK), permutation modulation, binary
`phase shift keying (BPSK), multi-level phase shift keying
`(mPSK), amplitude phase keying (APK), multi-level APK 60
`(mAPK) and the like.
`
`40
`
`45
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a schematic plan view of an office illustrating
`multipath transmissions of radio frequencies of at least 10
`GHz caused by reflections;
`
`65
`
`4
`FIG. 2 is a graph of received power as a function of time,
`for an impulse transmission, illustrating the received signals
`of reduced magnitude which are delayed owing to the
`possibility of multiple path transmission;
`FIG. 3 is a graph of the received amplitude of steady state
`signals as a function of the transmitted frequency, this
`characteristic itself being time dependent;
`FIG. 4 is a schematic diagram illustrating a local area
`network including a plurality of hubs each of which is able
`to communicate with mobile transceiver(s) within a corre(cid:173)
`sponding cell;
`FIG. 5 is a schematic block diagram of the circuit arrange(cid:173)
`ments within each hub and mobile transceiver;
`FIG. 6 is a more detailed block diagram illustrating part
`of the mobile transceiver of FIG. 5.
`FIG. 7 is a more detailed block diagram of the framing,
`FEC and modulator section 32 of the transmit path of the
`mobile transceiver of FIG. 6;
`FIG. 8 is more detailed block diagram of the framing,
`FEC and demodulator section 32 of the mobile transceiver
`of FIG. 6; and
`FIG. 9 is a more detailed block diagram of the mm-wave
`transmitter 36 and receiver 35 of the mobile transceiver of
`FIG. 6.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`Embodiments of the present invention will now be
`described with reference to the drawings.
`In schematic form, FIG. 1 illustrates a room 1 in a typical
`office environment which includes items of furniture 2 and
`a transmitter 3 and receiver 4. For radio transmissions at a
`frequency in excess of 10 GHz, a multipath mode of
`transmission from the transmitter 3 to the receiver 4 occurs.
`Reflections from the walls (and floor and ceiling) of the
`room 1, items of furniture 2, and the like, within the room
`1 cause the multiple path transmissions.
`As illustrated in FIG. 2, the effect of the multiple path
`transmissions is that the receiver 4 receives an undelayed
`signal 5 which has travelled directly from the transmitter 3
`to the receiver 4, and a number of delayed signals 6 which
`are received at a time after receipt of the undelayed signal 5.
`The magnitude of the delayed signals 6 is usually somewhat
`attenuated. Under some conditions, the magnitude of the
`undelayed signal 5 can be attenuated also, sometimes by
`more than some delayed signals 6.
`As a consequence of the delayed signals 6, it is necessary
`for the length of time during which a single symbol is
`transmitted (the symbol period) to be substantially longer
`than the delay time in order that the received echoes of a first
`symbol not mask the receipt of a subsequent symbol. This
`requirement has provided a severe upper limit to the rate at
`which data can be transmitted in such an environment.
`Furthermore, as illustrated in FIG. 3, the office environ(cid:173)
`ment is by no means a good one for radio transmission. FIG.
`3 illustrates a typical channel characteristic over a short time
`period illustrating the magnitude of the received signal as a
`function of frequency in the 1 GHz band between 60 and 51
`GHz. It will be seen that the received amplitude is by no
`means constant and, in particular, at various frequencies
`fading occurs. Furthermore, as indicated by dotted lines in
`FIG. 3, the frequency at which fading occurs varies as a
`function of time because of movements within the room.
`
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`5,487,069
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`15
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`25
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`5
`Such a communication channel is called a time varying
`frequency selective fading channel.
`Similar, but different, communications channels are
`known in both telephone and long distance radio commu(cid:173)
`nications and various strategems, generally known as equali(cid:173)
`sation, are used to overcome the problems such channels
`present. However, in these fields since such fading is due to
`changes in temperature, or atmospheric conditions, once
`such telephone or long distance radio communication chan(cid:173)
`nels are established, the fading characteristic changes rela(cid:173)
`tively slowly. Also in telephone applications advantage of
`the fact that channel degradation is generally low near the
`centre of the channel, can be taken when arranging the
`equalisation. This is not the case in an office or indoor
`environment.
`Rather, in the above described office environment, the
`change in the transmission characteristic indicated by dotted
`lines in FIG. 3 can, for example, be caused by the simple act
`of someone opening a briefcase positioned on a desk. The
`raised lid of the briefcase results in a change in the charac(cid:173)
`teristic. Similar extremely short term changes can be caused
`by the receiver 4 itself moving, or other objects moving such
`as doors opening, people moving, and the like. Clearly the
`transmitter 3 can also move. The foregoing establishes a
`very hostile environment within which the desired radio
`transmissions are to take place. In particular, there is no
`preferred channel or even a guaranteed channel within the 1
`GHz band.
`It would be possible to overcome the abovementioned
`difficulties by the use of highly directional antennae so as to
`eliminate all paths of transmission but the direct path.
`However, attempting to mechanically align such an antenna
`which was in turn affixed to a portable computer is com(cid:173)
`mercially unattractive.
`FIG. 4 illustrates in schematic form the general layout of
`a wireless LAN in accordance with a preferred embodiment
`of the present invention. A plurality of hubs 8 and mobile
`transceivers 9 are provided. The hubs 8 are interconnected
`by means of a backbone 10 which can take the form of either
`electrical conductors or optical fibre cable. As indicated by
`a dotted line in FIG. 4 the backbone 10 can constitute a loop.
`If desired, the backbone 10 can be connected to other
`computers 7 and, if desired, via a gateway 11 to the public
`switched telephone network 12. In a typical arrangement,
`each office (or each office in each building of a campus)
`would be provided with a single hub 8 which would com(cid:173)
`municate with the, or each of the, mobile transceivers 9 in
`that room. Either the backbone 10 can extend over the entire
`area to be covered, or the area can be covered by the use of 50
`multiple gateways and multiple backbones. The effective
`range of the transceiver within the hub 8 is arranged to
`essentially cover only that room. The limited transmission
`range for the hub 8 creates a corresponding cell 13 as
`indicated by broken lines in FIG. 4. For a large room such 55
`as a lecture room in an educational institution, the length of
`the room can require that the room be provided with two
`hubs 8 in which case two partially overlapping cells 13
`would be present within the one room.
`As seen in FIG. 5, for the hub transceiver 8, a number of
`component blocks are provided. These take the form of a
`network interface 20, a buffer memory 21, a framing,
`forward error correction (FEC) and modulating section 22,
`a framing, forward error correction and demodulation sec(cid:173)
`tion 23, an IF (intermediate frequency) system section 24, a
`mm-wave receiver 25, a mm-wave transmitter 26, and an
`antenna 27 which is sufficiently broad in its radiation pattern
`
`6
`to illuminate the entire cell. The antenna 27 can achieve this
`result statically or dynamically (with electronic or mechani(cid:173)
`cal beam steering). All these items are connected to, and are
`operable by, a control and timing section 28. In addition, all
`5 are powered by an AC mains operable power supply 29.
`Equivalent portions of the mobile transceiver 9 are indi(cid:173)
`cated by a designator having a magnitude higher by 10 in
`FIG. 5 and also in FIGS. 6-9. The mobile transceiver 9 has
`a battery powered power supply 39. This is possible because
`10 of the use of low power gallium arsenide devices in the
`receiver 35 and transmitter 36, in particular.
`It will be noted that the antenna 37 is preferably a
`steerable antenna which is electronically steerable by the
`control and timing section 38 so as to at least partially direct
`the transmissions to and from the mobile transceivers 9
`towards the corresponding hub 8. A suitable antenna for this
`purpose is that disclosed in Applicant's Australian Patent
`Application No.
`66100/94
`entitled
`"A PLANAR
`ANTENNA", the contents of which are hereby incorporated
`by cross-reference. This antenna improves the signal to
`20 noise ratio on the wireless link and attenuates delayed
`signals thereby improving multipath performance.
`A more detailed block diagram of a portion of the trans(cid:173)
`ceiver 9 is illustrated in FIGS. 6--9. In FIG. 6 the general
`arrangement of the transceiver 9 (excepting the terminal
`interface 30 and buffer memory 31) is illustrated. An inter(cid:173)
`mediate stage of detail is given for the receiver 35 and
`transmitter 36, the receive intermediate frequency system 34
`and receive demodulator 33 and the transmit intermediate
`frequency system 34 and transmit modulator 32. Full details
`30 of the modulation are given in FIG. 7 and of the demodu(cid:173)
`lation in FIG. 8.
`In FIG. 7, the transmit path framing, FEC and modulating
`section 32 of FIGS. 5 and 6, is illustrated in detail. From the
`buffer memory 31 of FIG. 5 a binary data stream is applied
`to a CRC (cyclic redundancy check) Generate and Append
`block 40. The output of this block 40 or that of an End of
`Packet Pattern Generator 41 is selectively input to a rate Y2
`TCM (trellis coded modulation) Encoder 42. The output of
`encoder 42 is in turn input to a Di-bit Interleaver 43, the
`40 output of which is in turn input to a QPSK Encoder 44 which
`carries out differential encoding on a frame-by-frame basis.
`The output of QPSK Encoder 44 and a synchronising header
`generator 45 are combined in frame assembly and zero pad
`insertion block 46 so that the frames are assembled and four
`45 zero pads inserted so that six carriers are generated to each
`side of, but not coincident with, the centre frequency.
`The assembled frames are then passed through an Inverse
`Fast Fourier Transform device 47 which uses a 16 point
`complex IFFT. The resultant signal is passed through Frame
`Serializer and Cyclic Extender block 48 to correctly
`sequence with 4 point cyclic extension the serial frames. The
`result is then passed via digital to analogue converters 49,50
`to the intermediate frequency stage 34 of FIGS. 5 and 6.
`In the receive path in the Framing, FEC and Demodulat(cid:173)
`ing section 33 of FIGS. 5 and 6, essentially the reverse
`procedures are carried out as illustrated in detail in FIG. 8.
`The received signal from the intermediate frequency stage
`34 is passed through the analogue to digital converters 60,61
`and thence to the cyclic extractor and frame assembler 62.
`60 The resultant signal is passed through the Fast Fourier
`Transform device 63 to provide the essentially decoded
`signal. This signal is then simultaneously passed to a frame
`dis-assembler and zero pad remover 64 and to a synchro(cid:173)
`nising calculator and detector 65 which provides start of
`65 message, end of message and symbol timing signals. These
`are passed to the control and timing unit 38 of FIGS. 5 and
`6.
`
`35
`
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`7
`The output of the frame dis-assembler and zero pad
`remover 64 is passed to a demodulator/detector 66 which
`carries out the necessary soft decision frame-by-frame dif(cid:173)
`ferential demodulation and detection. The resulting output is
`passed to de-interleaver 67 and then to TCM decoder which 5
`is again a soft decision decoder. The decoder output is
`passed both to the buffer memory 31 of FIG. 5 and to CRC
`Accumulator and Checker 69. This latter device produces an
`error signal for the control and timing unit 38 of FIGS. 5 and
`6 if the demodulating/decoding has not correctly recovered
`the transmission data.
`Turning now to FIG. 9, from the antenna 37 a schemati(cid:173)
`cally indicated bi-directional amplifier 71 leads via a filter
`72 to an image rejection mixer 73. The preferred form of
`bi-directional amplifier 71 is that disclosed in Applicant's
`co-pending International Patent Application No. PCT/
`A U94/00704 entitled "A Bi-Directional Amplifier", the con(cid:173)
`tents of which are hereby incorporated by cross-reference.
`Alternatively the bi-directional amplifier 71 can be realised
`by use of a separate transmit amplifier and a separate receive
`amplifier as illustrated connected between the antenna 37
`and filter 72 by appropriate switches under the control of the
`control and timing unit 38 of FIGS. 5 and 6.
`The image rejection mixer 73 receives a 58 GHz signal
`from a local oscillator (LO) unit 74. In the preferred form the
`first local oscillator (LO) is at a frequency of 58 GHz,
`resulting in an intermediate frequency band of 2-3 GHz. In
`the preferred embodiment illustrated in FIG. 9, this signal is
`obtained by doubling the output signal of a 29 GHz oscil(cid:173)
`lator. It is also preferable to perform some form of frequency
`stabilization on this oscillator, either by using an external 30
`frequency discriminator as illustrated in FIG. 9, a stable
`internal resonator or some form of frequency/phase locked(cid:173)
`loop.
`The image rejection mixer 73 is connected to both the
`receive IF system 34 and the transmit IF system 34 and can
`be shared between them by the use of an appropriate switch
`again under the control of the control and timing unit 38 of
`FIGS. 5 and 6. The use of the filter 72 provides additional
`rejection of the image frequency.
`From FIGS. 6-9, It will be seen that the preferred form of
`modulation includes not only encoding but also fast fourier
`transforming, and its inverse. The transceiver 35,36 is pref(cid:173)
`erably realised by means of one or more monolithic Inte(cid:173)
`grated circuits. Furthermore, in order to reduce power con- 45
`sumption in the mobile transceiver 9, the control and timing
`section 38 can power down each mobile transceiver 9 except
`when it is transmitting or receiving. This is determined by a
`polling scheme initiated by the hub transceivers 8. For
`example, the hub 8 can communicate with each mobile 50
`transceiver 9 in tum inquiring if any data is required to be
`transmitted or access to other parts of the LAN is required.
`This polling of the various stations can comprise one o