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Rypinski LACE, Inc.    UD #xP7\:P#IEEE 802.11 ă 0802 LAN Access Method for Wireless Physical Medium    x yxdddy #X~xP7kXP#  R  DATE: ` ` "March 8, 1993  a yx dddjy  Rb  AUTHOR: ` ` "Chandos A. Rypinski, ` ` " +Chief Technical Officer ` ` "LACE, Inc.`&(#mTel: +01 415 389 6659 ` ` "655 Redwood Highway #340`(#mFax: +01 415 389 6746 ` ` "Mill Valley, CA 94954 USA`(#jEm: rypinski@netcom.com   yxHdddy  P  TITLE: ` ` " RADIO PHY LAYER FOR USE WITH MEDIUM INDEPENDENT MAC (#`   yxddd>y (#  V ԇX` hp x (#%'0*,.8135@8:7 chips Chipping rate:`856 Mchips/sec Bits per symbol:`dD2 Radio bandwidth:`S?66 MHz Chip modulation:` 7POLAR AM (PSK) Quadrature phase transmission is not used, because of the probability of degradation in the medium of the isolation between phases and for other reasons. It is probable that this matter will be decided differently after there is more system experience. This change in the modulation',((nn might be part of the evolution to transfer rates much higher than 16 Mbits/sec. DDRTwo bits of information are derived from each 7chip symbol transmitted requiring four patterns for data in addition to those used for synchronization. DDRThe ratio of obtainable data rate to occupied spectrum is approximately:  SMbits/sec (data rate) = 0.25 x MHz (width) DDRUsing quadrature phase, the rate would be doubled and the resistance to time dispersion diminished. DDRTwo data modes are provided: NORMAL and FALLBACK. The fallback mode is half rate relative to normal using only the synch symbol.  S  RF Modulation DDRThe radio frequency signal is generated by linearly multiplying the NRZ baseband waveform by the operating carrier frequency where the polar form of NRZ is +1 and 1. The carrier frequency is suppressed in this operation. As will be later described, the NRZ is shaped to provide a minimum bandwidth modulating signal. DDRppVThe result is described as amplitude modulation since the operation must be linear. The representation of +1 and 1 in AM is the same amplitude for both values but of opposite phase. The appearance of the signal is identical to that of binary phase shift keying excepting for a lesser occupied bandwidth.8',((nn( ((nn8Ԍ S ԙ Baseband Waveform for Chipping  The baseband NRZST signal is described in  S  Attachment B . It is NRZ with smoothed transitions. There are two cases where NRZST is used: 1),As the chipping waveform on the RF carrier at the chipping rate. 2),As the bit stream transfer modulation between the MAC and the PHY when this interface is exposed and uses a length of unshielded twisted pair transmission. The following discussion relates to the radio frequency use: 1) above. For comparison, note that the first zero for randomdata squarewave baseband NRZ is at the bit rate frequency. 1st zero frequency:`K675% of bit rate 95% of power below:`K660% of bit rate Sidelobes above 1st zero:`=<33 dBc Above the first zero, the maximum level of the side lobes was 36 dB or more down in experimental models with FIR window size of 5 bits. The degree of reduction depends upon the window size in the FIR generating filter and in the precision of the sample and assembly execution. The radio frequency spectrum will be the spectrum of the modulating signal both sides of the carrier frequency when the radio system is linear. Nonlinearities will create additional radiated sidebands in the transmitter and increased intersymbol interference in the receiver.  S`  Transmitted Reference Carrier  Radio frequency reference carrier is transmitted as part of the signal so that the demodulating carrier at the receiver may be more quickly recovered than if it were synthesized. The radio front end may select this carrier or lock to it to recover the baseband signal. Technical parameters of the reference carrier are: Phase relative to data:`;quadrature Allocated power level:`933% of total Modulation:`[Anone&,((nnԌDDRThese proportions allow the reference carrier to be recovered in the receiver by selection with a centerfrequency passband filter which is much narrower than the passband of the chipping rate.  S8  MACPHY INTERFACE DDRThere are two MACPHY exposed interfaces: 1),DDRStation interface = fourpair balanced logic level for distances up to 1 meter. (#D 2),DDRAccesspoint interface = twopair balanced NRZST data and multiplexed control for telephone pairs up to 200 meters at 16 Mbits/sec.(#D DDRThe PHY is powered from the logic side in all cases through the common interface. The signals and information crossing this interface include the following: a)DDRtransmit data` "(#~pair b)DDRtransmit clock` "(#~pair c)DDRtransmit pending and ON` (#{clk mux d)DDRnot clear to send` (#{clk mux e)DDRtransmit power set`s (#zdata mux f)DDRreceive data` "(#~pair g)DDRreceive data clock` "(#~pair h)DDRreceive data valid` (#{clk mux i)DDRreceive signal level`6 (#y data mux j),DDRpower(#D` (#{phantom k)DDRsleep mode control`s (#zdata mux l)DDRremote loopback from radio side`s (#zdata mux m)DDRremote loopback from logic side`s (#zdata mux n)DDRfallback mode`s (#zdata mux DDRDetail on the definition, operation and multiplexing of these functions is shown as part  S of the description of PHY Services Sublayer  S in Attachment A . DDRSleep mode messages are decoded in the MAC and returned to the PHY as control commands in stations. DDRVia this interface, the vocabulary of processable management messages is a small set including the above functions and such others as may be needed for maintenance and fault isolation. Management communication is multiplexed on the data and clock lines.8H$,((nn'OL!((nn8  S ԈAxn?'ggstphy329.dpf <} '?W N #&txP74k&P#юBlock DiagramPHY Layer Detail Function for Radio Wireless Station.x   Ӈ THE PHY LAYER FUNCTIONS The PHY is made up of the following elements: 1),Antenna and radio frequency updown converter to/from analog baseband. This is the only frequency dependent component in the PHY.  2),Baseband signal processor analog to/from logic level. This component is specific to the modulation and the data transfer rate. 3),PHY services including all logic functions necessary to make different PHYs interchangeable at the MAC interface. PHY services may include baseband signal conditioning for transmission over a length of twopair cable.  Sp#  Antenna And Radio Frequency  SH$ UpDown Converter The possible operating frequencies for this front end are from 1.8 to 6.0 GHz, and the signal bandwidths from 20 to 80 MHz.&,((nnԌDDRIt is necessary to avoid or minimize cable length between the antenna and the radio frontend because of high losses and more complex matching and reflection problems. It may also be convenient to have separate antennas for transmit and receive rather than more complex duplexers and TR switching. DDRAbout 25% of the radio receiver gain may be in this unit. Much more is not permissible because of automatic gain control and dynamic range considerations. DDRThe receiver output is signal and noise at baseband with the bandwidth required to support the chipping rate in the medium.  S!  Signal Processor DDRThe transmit portion of the signal processor converts the logic level data and clock to chip patterned symbols, and then to baseband NRZST to modulate the transmit upconverter. DDRThe receive part of the signal processor accepts the analog chipping pulses and detects synch and data symbol patterns. The output8',((nn'((nn8 noise bandwidth of the correlator function is the singlesideband width of the data stream which for the chipping rate selected is 1/14th of the input noise bandwidth. After correlation, the information is converted to a logic level serial bit stream. The receive part of the signal processor deals with the variations in signal level and with recovery of chip and symbol clock.  Sp  Signal Level Adaptation It is important that a large part of the automatic gain control function depend upon sensing the desired signal after correlation. The observed level then is usable for a signal level indication responding only to liketype signals rather than randomtype interferers. A gain control signal might be returned to the radio down converter. Other techniques, in addition to feedback gain control, could also be used to diminish the effects of variable signal level.  S  Clock Acquisition This must be accomplished very quickly after the appearance of a valid signal. For this purpose it is assumed that signal preamble is 16 bits of synch and a 7bit start delimiter at binary level. It is expected that recovered bit clock will be valid within 5 bits of usable preamble A system assumption must be that all stations have accurate clocks with near identical rates. The acquisition adjustment is then one of phase and not frequency. The necessary accuracy is that there is only a fraction of a chip runout within one burst transmission.  S`  Quantizer and Combiner The accuracy and reliability of the receive function is significantly improved by a space diversity function. Two receivers and two antennas independently reach decisions on the value of a received symbol. The two results are linearly combined with each component weighted by the a function of the observed signal level. Combining at baseband enables summing that is independent of RF phase, but does depend on approximate chip alignment.&,((nnԌDDRUsing a comparator, the result can be quantized to logic level as 2bits parallel.  S  Fallback Mode DDRThe receive part of the signal processor is also capable of a separately but not simultaneously detecting a half rate signal using only the synch pattern. This output is provided alternately to the PHY Services sublayer, along with the indication that this is the current mode. DDRThe transmit part can generate either type signal on command. DDRThe transmit signal processor sends at a rate determined by the chip clock. Halving the rate by using only two instead of four symbol values has no effect on this processor.  SX  PHY Services Sublayer DDRThis sublayer and its function are described  S in much greater detail in Attachment A . DDRThe function of the PHY Services Sublayer is to provide the convergence necessary to make all PHYs look alike to the MAC. In addition, all logic functions which are necessary for the PHY operation are positioned here. The interface between the signal processor and the PHY services sublayer is the demarcation between analog modem and logical circuit function. DDRThe functions the on signal processor side may be specific to the particular PHY, but the functions on the MAC interface side must be common to all PHYs. DDRThis sublayer has a very limited capacity to process the data stream passing through it. Specifically, it can recognize the start and end delimiters and it can encode and decode one octet in the frame space used by the predetermined preamble before the start delimiter or following the end delimiter. It is this capability that is used for transfer of management information and control particularly including loopback, sleepmode, power control and received signal level 8H$,((nn'FC((nn8  S  SCRAMBLERS AND FEC Scrambling and/or forward error correction (FEC) should be in the PHY Services Sublayer if for the radio PHY, but would be different or absent for other mediums. Though these functions appear in the PHY  S block diagram, it is concluded that neither scrambling or FEC is needed for the radio path, but that the scrambler would be the simplest solution for the long telephone pair  Sp linking a remote PHY to a central hub . The primary reason for using scramblers is to mitigate dc effects from singular bit patterns when the medium has zero transmission at dc and possibly insufficient low frequency response. Another antidote is a dc restorer circuit which works quite well when the bit mix is near random and there is too not much noise in the signal. Scramblers have been required for modulations of highly compressed bandwidth to enable clock recovery from the guaranteed presence of transitions. This is not relevant to the present case. The benefit of FEC comes in raising the quality of a good channel to excellent at a price in redundancy. The problem of wireless LAN PHY is raising a rotten channel to good.   Sx  Air Interface  Because the 7bit symbols used are near square codes (near equal 1's and 0's) there is no low frequency problem regardless of the bit pattern, and therefore no scrambler is needed. On the basis of the following reasoning, it is also concluded that FEC would not provide sufficient benefits to offset its cost in delay and power drain. Furthe, the means of spreading and detection is a strong form of forward error correction. When only 6 patterns out of 128 are used, there is a strong error detection code when the framing function is also strong. The result is quite different looking for one of 6 patterns that is either recognized logically or with linear summing. A logical pattern form of error correction at best would be very limited using a short symbol.' ,((nnԌDDRThe summing with analog combining means that each element is weighted according to its amplitude. A chip which is of the opposite sign from what it should be but of low amplitude will not by itself materially degrade the result. A majority of chips would have to be wrong and have an aggregate power content exceeding those that were right. It is believed that the modulation and chipping described is a power forward error correcting means for this context. DDRIt might be useful to add a conventional FEC  SH wrapper which could surely raise 10é4 BER to 10ê S 7 at a cost of 30 to 50% added overhead and some further tranfer delay. The first point to consider is that BER is not meaningful in this context. The criteria is the proportion of errorfree messages out of those transmitted DDRErrors are more likely to be clustered as a result of some temporary impairment of the transmission path. Simple BER assumes randomly distributed errors. If a message is flawed, it should be retransmitted possibly using a different alternative in the medium. If a message is lost because someone walks past the antenna, that is better fixed with retry than FEC. DDRFEC does not appear sufficiently beneficial in this context.  S  Pair Interface  DDRThe pair interface is transporting at the bit rate rather than the chip rate. The bit is polar modulated with binary values +1 and 1. A sustained state of one binary value will produce dc or low frequency components. DDRThe copper pair does indeed transmit dc unless someone puts a transformer (repeat coil) in the path or shunts it with an inductor to get a centertap for the phantom. DDRIf power is transmitted at dc, then scrambled data is desirable. If power is transmitted at ac, then there is a possibility of retaining the usefulness of the dc path. 8p# ,((nn( w rH((nn8  R   Ј(#(#X` hDp x (#%'0*,.8135@8:PHY SERVICES SUBLAYER ă  S ԇ#&n P7 U&P# BACKGROUND It is believed imperative that there be a PHY Services Sublayer in order to isolate the design of the 802.11 MAC from excessive transmission medium dependence. The most acute matters are management of channel selection, transmitter power control, threshold settings and use of observed received signal level. There are lesser matters related to fault isolation. In this area the number of variants is so large that the probability of an acceptable conclusion is very small. The presently assumed radio transmission medium is minimal in these areas, yet there is considerable function for this sublayer. Any implementation will require the design of this sublayer in order to separate the design of the analog and digital parts of the PHY. The need for this sublayer is now demonstrated by pointing out the specific functions which it must include.  Su  THE MACPHY INTERFACE The presumed system plan requires physical separation between the MAC and the PHY creating a further intermediate transmission medium (like the AUI drop cable in 802.3). While this interface could be left to proprietary solutions, this direction could seriously degrade the effectiveness of the Standard. There is an assumption that this interface must be defined so that the radio part of the system can be separately supplied from the remainder of the function. Some the details addressed are present to indicate what must be defined for a multivendor environment. The services sublayer adapts to this interface. The station interface is seen as 4pairs where 2Pair are transmitting and 2pair are receiving. The accesspoint interface is seen as 2pair used alternately for send and receive. One of the 2pair is used for data and the other for clock. The multiplexing of data, clock, power and management on a small number of conductors is an important further function of the PHY' ,((nn Services Sublayer. Good choices are necessary to minimize the cost and power drain of this function.  S  PHY SERVICES SUBLAYER DDRThe main function of this sublayer is to provide convergence to make all PHYs look alike to the MAC. In addition, all logic functions, which are necessary for the PHY operation are in this sublayer. Competent definition of this sublayer will simplify the definition of the MAC.  S  Management Communication  DDRAll management communication must reach the PHY via the MAC. This requires a path and a vocabulary to be defined which is as rapid as the normal data transfers. A bidirectional mode is now defined obtained by replacing the 16 bits of preamble before the message start delimiter with 16 bits formatted: 8bits normal preamble and 8bits of management message. The start delimiter that follows is recognized so that only the preceding 8bits are interpreted. DDRThe preamble and delimiter provide enough message capacity for power control, signal level reporting, loopback and parameter setting. DDRPresently, it appears sufficient for management information to appear as a prefix, but if needed, the same information could be suffixed after an end delimiter for transmission. DDRIn addition some management functions are multiplexed on the clock and power paths as later detailed.  SE  Formatting and Framing  DDRThe received signal comes out of the signal processor as 2bits parallel. The transmitted signal is transferred to the signal processor with spreading code already applied. These and other functions are performed in the PHY Services Sublayer.  S'  BitSymbolChip Formatting 8' ,((nn( , ,((nn8ԌThe output of the receive signal processor is chip rate clock and symbol value. The services sublayer must convert this to bit rate clock and a serial bit stream. The services sublayer receives a serial bit stream and clock from the MAC which must be converted into bit or synch symbol values. This forms the serial bitstream at the chip rate which is presented to the transmit signal processor.  Sp  Transmit and Receive Frame Format The preamble preceding the start delimiter is formed in the sublayer for transmitting and stripped and replaced for receiving. This message space is then use to pass management information between MAC and PHY.  SX  Clock and Multiplexed Management  The services sublayer converts received chip and symbol clock to bit clock. The reverse occurs for transmitted messages. The MAC can be defined to start a transmit cycle with transmitted clock in parallel with the data. When the sublayer receives clock, it assumes that the transmitter will be turned ON soon after. The first clock pulse received is assumed valid and used to clock the data shift register of length two octets. Presence and absence of the clock signal is the transmit onoff function.  S(  Data and Multiplexed Management  Data from the MAC is clocked into the two octet shift register creating a delay before transmission sufficient to evaluate the effect of any management message. The eight bits preceding the start delimiter may contain management information.  S  Clock Line Signal Clock is transmitted as a sine wave at half of the data rate (e.g.; 8 MHz for 16 Mbits/sec). Clock will be present from the PHY only when valid data is present. The zero crossing of the sine wave will be aligned with the preferred sampling instant of the data. If the detection bandwidth for this sine wave is 4 to 12 MHz, it may be assumed that it will' ,((nn take 125 nanoseconds (at 16 Mbits/sec) before the amplitude will be close to the final value, an interval equal to one cycle or two bits. DDRThe presence of clock on the line is the first indication that data will be received, and its absence is a backup end delimiter indication. DDRIn no case, does the PHY recognize or process data between the delimiters  S  Physical Medium Delimiting DDRPHY layer delimiting occurs on the basis of chip patterns on the radio side and bit patterns on the MAC side. They are entirely separate. DDRRecognition of the Start delimiting symbol in the PHY layer is a useful function. The use of that information might be: 1),DDRTo inform following logic to wakeup and process.(#D 2),DDRTo mark the beginning of the transferred data message.(#D DDREnd delimiting in the MAC is primarily dependent on a MSG TYPE or a LENgth field in a header. This must be backed up by a default end delimiter that is loss of signal indication from the PHY implemented as a transition out of the data/clock valid state and indication. DDRCertain types of abnormal relationships in the correlator can be interpreted as loss of valid signal for this purpose. This function cannot be performed reliably by an envelope carrier detector in the radio. DDRThe 7bit MAC start and end delimiter are detectable by the PHY services function, and are also used to separate preceding and following management messages from transferred frames.  S`  Power Control and Signal Level  S8 Detection DDRThe speed with which signal level and transmit power level need to be moved is similar to that of the data. It multiplexed on the data pair transmitted in the management field. DDRReceived Signal Level is appended after the end delimiter at the end of each received transmission. The value is entered by the PHY services sublayer. DDRTransmitter Power Reduction Command is a field in a management message received by the8' ,((nn( ] ].((nn8 PHY from the MAC as a prefix on the message to be transmitted. Transmit power is always full unless a specific management message is received indicating a reduction.  S8  Loopback and Fault Isolation  Polling is the basic evidence that all parts of the system are working. The station MAC should send upward an "infrastructure present" indication if invitation or polling messages are heard. A station that recognizes the presence of infrastructure knows that a great deal of the station is operating including all of the receiving system. A station that cannot signon has a fault that cannot be located without further testing. Since the station, normally, cannot send and receive at the same time, the transmitter cannot be tested by radio frequency loopback. Whether designin of this capability is reasonable in cost remains to be determined. As an alternative, it is possible to loopback from the PHY services sublayer which would show that the cable between MAC and PHY was good. It is possible to have an RF sniffer that reports transmitter power output above a threshold. Any of these result in additional function which the PHY services sublayer must support. In a station, local loopback is initiated by a management message from the MAC via the management preamble. The services sublayer is cable of storing 64 octets received either from the transmit logic or from a receiver capable of local monitoring, and the sublayer can repeat back these octets in the same form as any other received message. In a station, remote loopback is commanded by a received message which is interpreted in the MAC. The reply is formed in the MAC and returned as any other transmitted message. In an accesspoint, there is no remote loopback. There is local loopback at the PHY Services Sublayer which uses the same 64 octet storage as defined for local loopback at the station. Local loopback passing through the radio is well done using a test station with a& ,((nn store and forward at the sublayer. Alternatively, the radio may be rigged for duplex monitoring.  S  Interface Signal Conditioning  DDRSome sort of conditioning is necessary on the data and clock signals for copper wire transmission of 12" from the PC-MCIA card to the radio, and from an accesspoint to an intelligent hub within the infrastructure. DDRIn the station interface, the cable will be short introducing little delay distortion at the bit rate, however balanced transmission line must be used to limit radiation and pickup. In addition, logic level rise and fall times must be limited by filtering, and then the pulse distortion from the filtering should be at least mildly compensated. The level of these signals may be considerably lower than that of conventional line drivers to minimize battery drain and radiation. DDRThe accesspoint interface must be more elaborate because of the reduced number of pairs. DDRData at the bit rate will be transmitted in NRZST pulse shaping with compensation for up to 150 meters of pair.  S  Interface Power Supply DDRPrimary power is sent from the MAC to the PHY over the "phantom" of the two pairs. Each of the 2pair is used in parallel or longitudinal mode for one side of the power. This results in ohmic loss that is onehalf of what it would be for one pair alone. DDRThe station interface would have a total of 4wires for power so that multiple voltages could be transferred including those which are absent in sleep mode. DDRThe accesspoint interface would transfer power at 24 Vac or Vdc to be determined. The accesspoint PHY services layer would have to convert the received level to operating supply voltages. 8p# ,((nn' _((nn8  P  A ӈ  # X~xP7kXP# CATTACHMENT B  P ,NRZST NONRETURN TO ZERO SMOOTH TRANSITION MODULATION ă X` hDp x (#%'0*,.8135@8: