60GHz毫米波技术白皮书——IEEE802.11ad.pdf

802.11ad-WLAN at 60 GHzA Technology IntroductionWhite PaperData rates in the range of several Gigabit/s are needed to transmit signals like uncompressed video sig-nals.Amendment 802.11ad to the WLAN standard defines the MAC and PHY layers for very high through-put(VHT)in the 60 GHz range.This white paper provides an introduction to the technology behind 802.11ad and highlights the test andmeasurement requirements.White Paper WLAN 802.11ad 1MA220_1eBernhard Schulz-1MA220_1eContents802.11ad-WLAN at 60 GHz2White Paper WLAN 802.11ad 1MA220_1eContents1Introduction.32Key Features.53The 11ad Physical Layer(PHY).64Measurement requirements.215Appendix.256About Rohde&Schwarz.28Introduction802.11ad-WLAN at 60 GHz3White Paper WLAN 802.11ad 1MA220_1e1 IntroductionDigital wireless communications will always demand more throughput than is available.Especially when several users must share the same physical resources,only a fractionof the nominal throughput remains.Multimedia data streams in particular require veryhigh throughput on a continuous basis.One example is uncoded 3D high definitionvideo streams(currently at 4k resolution(Ultra-HD:3840 x 2160 pixels),and the trendis toward higher frame rates of e.g.48 frames per second).To meet this need,the Wireless Gigabit Alliance(WiGig)has developed a specificationfor wireless transmission of data in the 60 GHz band at speeds in the multi-Gigabitrange.Why the 60 GHz band?In the range around 60 GHz,an unlicensed frequency band is available everywhere inthe world.This range permits higher channel bandwidths for greater throughput.Another advantage is the small wavelengths(approx.5 mm).These make it possible touse compact and competitive antennas or antenna arrays(e.g.for beamforming).At first glance,however,this range also has some apparent disadvantages:very high free field attenuation in this band.(attenuation after 1 m:68 dB,after 10m:91 dB)oxygen(O2)absorptionHowever,because the transmission typically takes place within a limited range ofunder 10 m(the typical living room),the high degree of attenuation can also be seenas an advantage.Interference from adjacent transmissions is very unlikely.The trans-mission is very difficult to intercept,making it even more secure.Finally,beamformingcan be used to focus the power to the receiver.WLAN 802.11The 802.11 WLAN standard has also been continuously updated to permit higherthroughput.These changes were developed as amendments,but have since beenassigned their own letter as enhancements to the standard using the MAC and PHYlayers.The relevant standard enhancements are 11a,b,g,n and these cover the twobands in the 2.4 GHz and the 5 GHz range.The 11n extension(also called HighThroughput(HT)uses up to four MIMO streams to achieve a data rate of up to 600Mbit/s.The 11p standard is an extension that permits robust data traffic between auto-mobiles(car to car).You can read more about the various standards in The WLAN Universe 3.For T&Mpossibilities using Rohde&Schwarz instruments,refer tohttp:www.rohde- Introduction802.11ad-WLAN at 60 GHz4White Paper WLAN 802.11ad 1MA220_1eThe latest developments are known as Very High Throughput(VHT)and are specifiedin two new amendments:11ac is intended for the frequency range under 6 GHz and uses conventionaltechnologies,such as those seen in 11n.It uses bandwidths of up to 160 MHz andeight MIMO streams to achieve data rates of more than 1 Gbit/s for the 80 MHzbandwidth(see http:www.rohde- covers the frequency range at 60 GHz.The WiGig standard mentioned abovewas fully integrated into 11ad.Chapter 2 provides an overview of the key features.Chapter 3 provides a detailed explanation of the three different PHYs being used andhighlights the differences.Chapter 4 describes the T&M requirements.Key Features802.11ad-WLAN at 60 GHz5White Paper WLAN 802.11ad 1MA220_1e2 Key Features802.11ad includes the following key features:Support for data rates of up to 7 Gbit/s,divided intoa mode with simple,robust modulation,but lower data rates(single carrier),an energy-saving mode for battery-operated devices(single carrier low power)and a high-performance mode with OFDM technology for very high throughputUse of the 60 GHz unlicensed bandprovides global availabilityavoids the overcrowded 2.4 GHz and 5 GHz bandsuses short wavelengths(5 mm at 60 GHz),making compact and affordableantennas or antenna arrays possibleBeamformingoptimizes power at the receiver.overcomes interference(changes in the channel conditions)during the trans-mission in realtimeis backward compatible with the WLAN universeSeamless use of 802.11a,b,g,nacross both bands 2.4 GHz and 5 GHz,plus 11ad in the 60 GHz range-tri-band devicesTypical applications for 11ad are:Wireless DisplayDistribution of HDTV content(e.g.in residential living rooms)Wireless PC connection to transmit huge files quicklyAutomatic sync applications(e.g.uploading images from a camera to a PC,kioskapplications)The 11ad Physical Layer(PHY)802.11ad-WLAN at 60 GHz6White Paper WLAN 802.11ad 1MA220_1e3 The 11ad Physical Layer(PHY)The 11ad physical layer was added as an amendment,chapter 21 of the 802.11-2012standard.It is called Directional Multi-Gigabit(DMG)PHY.This chapter provides adetailed description of the various implementations.3.1 ChannelsThe nominal channel bandwidth is 2.16 GHz.The useful ISM band around 60 GHz(57GHz to 66 GHz)is regulated differently in various regions of the world.Four channelsare defined for this band,but they are not universally available.Channel 2 is availablein all regions and is therefore used as the default channel.Table 3-1:Channel numbers in the 60 GHz bandChannels158.32 GHz260.48 GHz362.64 GHz464.80 GHzFig.3-1:Channels in the 60 GHz bandA transmitter operating in this band must match a spectrum mask;see chapterchap-ter 4,Measurement requirements,on page 21.ChannelsThe 11ad Physical Layer(PHY)802.11ad-WLAN at 60 GHz7White Paper WLAN 802.11ad 1MA220_1e3.2 Directional multi-Gigabit(DMG):Three different PHYs.In principle,three different modulation modes are available.They make it possible tofulfill differing requirements(such as high throughput or robustness).Not all threemodes need to be supported by every implementation:Table 3-2:Three different PHYs.PHYMCSAnmerkungControl PHY0 Single carrier PHY(SC PHY)1.1225.31(low power SC PHY)OFDM PHY13.24 All DMG PHYs use the same packet structure,but they differ in how the individualfields are defined as well as in the coding and modulation that is used.Fig.3-2:General structure of a packet in 11ad.A packet is made up of the following common parts:PreambleThe preamble consists of the short training field(STF)and the channel estimation(CE)field.It is required in every packet.It supports the receiver during automaticgain control(AGC),when recognizing the packet and in estimating the frequencyoffset,and it displays the type of PHY that is used(SC or OFDM).The receiver canalso use the known CE field to estimate the channel.HeaderThe header is different for every PHY and contains additional important informationfor the receiver,such as the modulation mode(MCS),the length of the data field ora checksum.DataThis part is used to transmit the actual data with different modulations(MCS).Thelength of the field varies(number of bytes/octets).TRNThis field is optional and can be appended to all packets.It includes beamforminginformation(see chapterchapter 3.3,Beamforming,on page 16)Individual fields in the packets(e.g.the preamble)are made up of Golay sequences(see chapter 5.1,Golay sequences,on page 25)Each sequence consists of bipolarsymbols(1).They are constructed mathematically in order to achieve specific auto-correlation characteristics.Each consists of a complementary pair(a and b).An addi-Directional multi-Gigabit(DMG):Three different PHYs.The 11ad Physical Layer(PHY)802.11ad-WLAN at 60 GHz8White Paper WLAN 802.11ad 1MA220_1etional index contains the length of the sequence.For example,Ga128 and Gb128 rep-resent a complementary sequence with a length of 128.In addition,four specific Gx128are then logically combined into Gu512 or Gv512The single carrier PHYs(SC,low power SC and control)nominally use a bandwidth of1760 MHz,while the OFDM PHY uses 1830.47 MHz.3.2.1Control PHYThis mode is used to exchange signaling and/or control messages in order to establishand monitor connections between the various devices.Support for this mode is there-fore mandatory for all devices.MCS0 was selected to provide very robust transmissionthat can withstand possible interference in the channel.BPSK modulation is used andthe preamble(STF)is longer than in the other packets to make the transmission morerobust.Fig.3-3:Control packet preamble:STF consists of 50 G128 sequences and CE transmits Gu+Gv.The STF is made up of a total of 50 G128 sequences.Gb128 is first repeated 48 times,followed by one Gb128 and one Ga128.As a result,the STF is 50*128=6400 Tc inlength.The CE field consists of one Gu512 and one Gv512,followed by a Gb128sequence.The same CE field is also used for SC PHY.As a result,the CE field is9*128=1152 Tc in length.Directional multi-Gigabit(DMG):Three different PHYs.The 11ad Physical Layer(PHY)802.11ad-WLAN at 60 GHz9White Paper WLAN 802.11ad 1MA220_1eFig.3-4:Control packet header(40 bits).The control header is a total of 40 bits in length.The most important fields areexplained here:Scrambler initialization:Provides the start point for the scrambler(see figure 3-5)Length(data):Specifies the length of the data field.For control,the range is 14octets to 1023 octetsPacket type:Specifies whether the beamforming training field is intended for thereceiver or the transmitter.It carries no information when Training Length=0Training length:Specifies whether a beamforming training field is used and if so,how long it isHCS:Provides a checksum per CRC for the headerThe data to be transmitted is scrambled,coded,modulated and spread.Fig.3-5:Control packet:Data processing.The scrambler is the same for all PHYs;only the initialization value is set differently(transmitted in the header)For the encoding,a low-density parity check(LDPC)coder is used.It always usesa codeword length of 672 bits,and the coding rate varies depending on the PHY.The 3/4 matrix with shortening is used here,which results in a code rate of 1/2 orless.Differential BPSK is used as a robust modulation.It is shifted by/2 to avoid zerocrossings in the I/Q diagram.As a result,the difference between the peak andaverage power remains low.Finally,the signal is spread with a Ga32 sequence.Directional multi-Gigabit(DMG):Three different PHYs.The 11ad Physical Layer(PHY)802.11ad-WLAN at 60 GHz10White Paper WLAN 802.11ad 1MA220_1e3.2.2Single CarrierIn SC mode,from 385 Mbit/s up to 4.620 Gbit/s are transmitted depending on theMCS.To support mobile devices that are sensitive to power consumption,an addi-tional(optional)low-power SC mode with an energy-saving encoder is defined.Fig.3-6:SC packet preamble:STF consists of 17 Ga128 sequences and CE transmits Gu+Gv.The basic packet structure is the same for both SC types.In the case of the low-powerPHY,the 16QAM modulation is not used during data transmission in order to saveenergy.The STF is made up of a total of 17 G128 sequences.Ga128 is first repeated16 times,followed by one Ga128.As a result,the STF is 17*128=2176 Tc in length.The CE field consists of one Gu512 and one Gv512,followed by a Gb128 sequence.As a result,the CE field is 9*128=1152 Tc in length.The same CE field is also used inthe control PHY.Fig.3-7:SC packet header(64 bits).The SC header is a total of 64 bits in length.The most important fields are explainedhere:Directional multi-Gigabit(DMG):Three different PHYs.The 11ad Physical Layer(PHY)802.11ad-WLAN at 60 GHz11White Paper WLAN 802.11ad 1MA220_1eScrambler initialization:Provides the start point for the scrambler(see figure 3-5)MCS:Displays the modulation and coding scheme used in the data fieldLength(data):Specifies the length of the data field.For control,the range is 1 octetto 262143 octetsPacket type:Specifies whether the beamforming training field is intended for thereceiver or the transmitter.It carries no information when Training Length=0Training length:Specifies whether a beamforming training field is used and if so,how long it isLast RSSI:Displays the power level of the last field receivedHCS:Provides a checksum per CRC for the headerThe additional steps for the data field differ for SC PHY and for low-power SC PHY.3.2.2.1SC PHYThis method is the simplest transmission option.A total of 12 MCS(1 to 12)aredefined,whereby support for MCS 1 to 4 is mandatory.The various MCS have differ-ent modulations and code rates,and therefore also different throughputs.MCS1(BPSK and code rate 1/2)can achieve 385 Mbit/s,MCS4(BPSK and code rate 3/4)upto 1.115 Gbit/s and MCS12(16QAM and code rate 3/4)up to 4.620 Gbit/s.A completetable of MCS1 to 12 for SC PHY is available at 1-Table 21-18.The data to be transmitted is scrambled,coded,modulated and blockwise transmitted.Fig.3-8:SC PHY packet:Data processing.The scrambler is the same for all PHYs;only the initialization value is set differently(transmitted in the header)A low-density parity check(LDPC)coder is used for the encoding.It always uses acodeword length of 672 bits,and the coding rate varies depending on the PHY.Inthis case,code rates 1/2,5/8,3/4 and 13/16 are used.Depending on the data rate,various types of modulation are used(BPSK,QPSK or16QAM).Each is shifted by/2 to avoid zero crossings in the I/Q diagram.As aresult,the difference between the peak and average power remains low.The data are transmitted blockwise at 448 symbols per block.Another 64 symbolsare inserted between the individual blocks as guard intervals(GI)in order to pro-vide a known reference signal to the receiver in the case of long data packets.Thecomplete block is therefore 512(448+64)symbols in length.The GI consists of aGa64 Golay sequence and is always modulated with/2-BPSK.Directional multi-Gigabit(DMG):Three different PHYs.The 11ad Physical Layer(PHY)802.11ad-WLAN at 60 GHz12White Paper WLAN 802.11ad 1MA220_1eFig.3-9:SC PHY data blocks:A data block is made up of 512 symbols(64 GI+448 data).3.2.2.2Low power SC PHYThis method is set up similarly to the SC PHY,but it is optimized to save energy inorder to support small,battery-operated devices as well.In contrast to all other PHYs,the LDPC is replaced by an energy-saving alternative,and unlike SC PHY,16QAMmodulation is not used.A total of seven MCS(25 to 31)are defined,and all areoptional.The various MCS have different modulations and code rates,and thereforealso different throughputs.MCS25(BPSK and code rate 13/28)permits rates of 626Mbit/s to be achieved,and for MCS31(QPSK and code rate 13/14)it is up to 2.503Gbit/s.A complete table of MCS25 to 31 for low-power SC PHY is available at 1-Table 21-22.The data to be transmitted is scrambled,coded,modulated and blockwise transmitted.Fig.3-10:Low-power SC PHY packet:Data processing.Directional multi-Gigabit(DMG)