展频通讯干扰和原理PhysicalLayerOverview

1、展频通讯干扰和原理Physical Layer Overview2022/8/112展频通讯（Spread Spectrum） 原始目的是用于军事上的保密通讯，并可抵制敌方的刻意干扰 的物理层所采用的即为展频通讯技术中的跳频展频（Frequency Hopping Spread Spectrum，简称FHSS或FH）技术、与的物理层所采用直接序列展频（Direct Sequence Spread Spectrum，简称DSSS或DS）技术 2022/8/113展频通讯: 延展通讯时所用到的频带宽度，使得所占用的带宽远大于要传送原始数据所需的最小速率 展频通讯系统模型 2022/8/114展频通

2、讯的优点 抑制故意的干扰 窃听的困难度较高 可用于定位系统 可提供多人同时共享无线频道 2022/8/115抑制故意的干扰 2022/8/116窃听的困难度较高W大P小2022/8/117可用于定位系统在定位系统中，通常以脉冲在空间中传递所发生的延迟作为距离长短计算的依据 而在延迟大小的量测中，其不确定的部分则与单一脉冲所占用的带宽成反比 2022/8/118可提供多人同时共享无线频道展频通讯中所使用延展信号为一特定之代码串行，只要每个频道的用户所使用的代码串行彼此独立（也就是线性代数理论上所谓的正交（Orthogonal）可以形成多人公用频道的状况 2022/8/119直接序列展频 直接序列

3、展频的原理为将原始数据速率为的数据串行转换成一数据速率为伪随机代码串行， 以所得之伪随机代码串行进行调变才传递去。 2022/8/11102022/8/11112022/8/1112BPSK直接序列展频系统之信号PSD Power Spectral Density 2022/8/1113伪随机码（也就是片序列）的选取原则 伪噪声序列（Pseudo-Noise（PN）Sequence） 若N为序列的周期，则 2022/8/1114二进制的周期性序列是否具有类似噪声的特性 ?平衡（balance）： 在一个周期的序列中，二进制1（+1）的个数与二进制0（-1）的个数最多只相差一个。 同号连续（Ru

4、n）： 长度为一的同号连续的个数要占该序列所有同号连续总数的一半，长度为二的同号连续要占总数的1/4，长度为四的同号连续要占总数的1/8， 相关性运算（correlation）： 一个序列若要具有类噪声的特性，其比较后的结果最好是相同数值的总位数与相异数值的总位数最多只差1。 2022/8/11151. 平衡属性来说，+1的个数为8个，-1的个数为7个 2.3.Example:2022/8/11162022/8/11172022/8/1118如果因第二路径效应而产生之信号的时间延迟大于，也就是内含的伪随机码信号被位移超过一个位，则在进行解调变时，其对于正常信号的干扰就会变得很小。 2022/8

5、/1119Any girl can be glamorous. All you have to do is stand still and look stupid. - Hedy Lamarr2022/8/11202022/8/1121The ISM PHYs: FH, DS, and HR/DS802.11 FH PHYThe electronics used to support frequency-hopping modulation are relatively cheap and do not have high power requirements.The main advanta

6、ge to using frequency-hopping networks was that a greater number of networks could coexist and the aggregate throughput of all the networks in a given area was high.2022/8/1122Frequency-Hopping TransmissionTiming the hops accurately is the key to success; both the transmitter and receiver must be sy

7、nchronized so the receiver is always listening on the transmitters frequency.2,8,4,72022/8/1123Each frequency is used for a small amount of time, called dwell time. (max: 390 time units) 1 TU= 1024 micro-sec. Usually, beacon interval=100TU=0.1 sec. 19TU is suggested by 802.11. The hopping process ca

8、n take no longer than 224 micro-sec.2022/8/1124Hopping sequences that do not overlap are called orthogonal.2,8,4,76,3,7,22022/8/1125802.11 FH detailsChannels are defined by their center frequencies, which begin at 2.400 GHz for channel 0. 99% of the radio energy is confined to the channel.Successive

9、 channels are derived by adding 1-MHz steps:2022/8/11262022/8/1127802.11 hop sequencesAs an example, hopping sequence 1 for North America and most of Europe begins with the sequence 3,26,65,11,46,19,74,50,22,2022/8/11282022/8/1129Joining an 802.11 frequency-hopping networkThe FH Parameter Set elemen

10、t includes the hop pattern number and a hop index. (e.g, (1,2)By receiving a Beacon frame, a station knows everything it needs to synchronize its hopping pattern.Example: next channel 65, timestamp/dwell time when to switch to channel.2022/8/1130ISM emission rules and maximum throughput (US FCCs rul

11、es)There must be at least 75 hopping channels in the band, which is 83.5-MHz wide.Hopping channels can be no wider than 1MHz.1 MHz 1 Mbps ( 2-level encoding)Devices must use all available channels equally. In a 30-sec period, no more than 0.4 sec may be spent using any one channel.2022/8/1131Effect

12、of interferenceWith approximately 80 usable channels in the U.S. and Europe, interference on one channel reduces the raw bit rate of the medium by approximately 1.25%2022/8/1132Gaussian Frequency Shift Keying (GFSK)2-level GFSKNoise changes the amplitude of signal; (advantage of using frequency for

13、modulation)2022/8/1133The rate at which data is sent through the system is called the symbol rate.Although the carrier frequency is roughly 2.4GHz, the symbol rate is only 1 or 2 million symbols per second.Frequency changes with GFSK are not sharp changes. ( sharp high cost)2022/8/11344-level GFSKTh

14、e four symbols (00,01,10,and 11) each correspond to a discrete frequency, and therefore 4GFSK transmits twice as much data at the same symbol rate.Distinguishing between two levels is fairly easy. (four is harder.) Each doubling of the bit rate requires that twice as many levels be present, and the

15、RF components distinguish between ever smaller frequency changes. 2022/8/11352022/8/11362 Mbps.2022/8/11372022/8/1138FH PHY Convergence Procedure (PLCP)Framing and whiteningThe PLCP for the FH PHY adds a five-fields header to the frame it receives from the MAC.2022/8/1139The PLCP framing:PreambleSyn

16、c 10)Start Frame Delimiter (SFD) (0000 1100 1011 1101)HeaderPSDU Length Word (PLW) (The length of MAC frame)PLCP Signaling (PSF) (bit 0 is set to 0)Header Error Check (HEC)2022/8/11402022/8/1141WhitenArbitrary data may contain long strings of consecutive 0s or 1s, which makes the data much less rand

17、om.To make the transmitted data more like random white noise, the FH PHYs apply a whitening algorithm to the MAC frame.This algorithm scrambles the data before radio transmission. Receivers invert the process to recover the data.2022/8/11422022/8/11432022/8/1144Frequency-Hopping PMD SublayerThe gene

18、ral design of the transceiver used in 802.11 frequency-hopping networks2022/8/1145PMD for 1.0-Mbps FH PHYIn keeping with the common regulatory restriction of a 1-MHz bandwidth, 1 million symbols are transmitted per second.2GFSK is used as the modulation scheme, so each symbol can be used to encode a

19、 single bit.2022/8/1146PMD for 2.0-Mbps FH PHYIn the PLCP header, the PSF field indicates the speed at which the frame body is transmitted.At the higher data rate, the frame body is transmitted using a different encoding method than the physical-layer header.Regulatory requirements restrict all PMDs

20、 to a symbol rate of 1 MHz, so 4GFSK must be used for the frame body.Two bits per symbol yields a rate of 2.0 Mbps at 1 million symbols per second. 2022/8/1147Characteristics of the FH PHY2022/8/1148802.11 DS PHYDirect-Sequence Transmission2022/8/1149ChippingA chip is a binary digit used by the spre

21、ading process.Chipping stream (pseudorandom noise codes (PN codes) must run at a much higher rate than underlying data.2022/8/1150Barker code LengthCode2+ -, + +3+ + -4+ - + +, + - - -5+ + + - +7+ + + - - + -11+ + + - - - + - - + -13+ + + + + - - + + - + -+2022/8/1151一个巴克码的数字序列循环位移，其结果仍为巴克码 一个巴克码的所有

22、数字之正负符号颠倒(也就是+1变1，1变+1)，其结果也仍是巴克码 一个巴克码的相反顺序也仍是一个巴克码 2022/8/1152802.11 DS details802.11 adopted an 11-bit barker word for PN code.2022/8/1153Operating channelsThe DS PHY has 14 channels in the 2.4-GHz band, each 5 MHz wide. Channel 1 is placed at 2.412 GHz.2022/8/1154Channel energy spread2022/8/1155

23、Max theoretical throughputISM band 80 MHzSpreading factor 11Max bit rate slightly more than 7 MbpsInterference response2022/8/1156Differential Phase Shift Keying (DPSK)2022/8/11572022/8/11582022/8/1159DS PLCPFramingThe FH PHY uses a data whitener to randomize the data before transmission, but the da

24、ta whitener applies only to the MAC frame.The DS PHY uses a scrambler and applies to PLCP header + preamble + MAC frame.2022/8/1160SynSFD 0000 0101 1100 1111Signal0000 1010 1 Mbps0001 0100 2 MbpsService (reserved)Length micro secondCRC2022/8/1161DS MDPTransmission at 1.0 Mbps, 2.0 MbpsCS/CCA for the

25、 PHYMode 1: When the energy the energy detection (ED) threshold busyMode 2: An actual DSSS signal busyMode 32022/8/1162DS PHY Parameters2022/8/1163802.11b:HR/DSSS PHYDS PHYs11 M chips per secChip stream 11-bit Barker words 1 M wpsEach word encodes 1 or 2 bits 1 or 2 M bpsHigh rate Phase-shift encoding ?Complementary code keying (CCK) divides the chip stream into a series of 8-bit code symbols, so the underlyi