Frequency Hopped Spread Spectrum

Transcription

1 FH- 5. Frequency Hopped pread pectrum ntroduction n the next ew lessons we will be examining spread spectrum communications. This idea was originally developed or military communication systems. However, there are aspects o the hostile military communication environment that naturally occur in riendly communication systems. For example, we have seen that intererence rom other cells can cause the / to be low enough to disrupt our channel. n a military environment the intererence might be intentional whereas it is typically unintentional in a civilian environment. onetheless, regardless o the intent, intererence causes problems. ot surprisingly, thereore, techniques developed or hostile environments ind applicability in civilian systems. There are two main types o spread spectrum modulation: requency-hopped, and directsequence. n this lecture we ocus on requency hopping. The Jamming roblem A problem that arises when trying to communicate in a hostile environment is that o jamming in which intererence is created speciically to drive the / down enough to make communication impossible. Let s say you are trying to listen to Friend on a channel centered on requency and having bandwidth B. You need enough / or your link to unction, so a spectrum analyzer would show something like that illustrated in Fig. 5.. Here is Friend s signal. Figure 5.: n normal operation a radio signal occupies some channel bandwidth and has a spectral intensity that diers rom that o the noise by /. Anyone else could use a spectrum analyzer to see the same thing, including a hostile Foe. Having determined that you are transmitting and at what requency, Foe could then deliberately transmit on the same channel. This is illustrated in Fig. 5.. Here is Foe s signal, which is intererence with respect to Friend s signal. this creates a / that is low enough then the channel is jammed and you cannot receive Friend s signal. An obvious solution at this point would be to switch channels. You and Friend could have an agreed upon sequence o channels with the understanding that when one channel gets jammed you both switch to the next in the sequence. O course the sequence should be secret and appear essentially random or Foe can EE43: RF Engineering or Telecommunications cott Hudson, Washington tate University 05/30/7

2 FH- 5. switch the intererence right along with you. On the new channel you can communicate with Friend until Foe looks at his spectrum analyzer and igures out which channel you ve switched to. Then Foe could start jamming the new channel. This process is illustrated in Fig Figure 5.:Jamming involves deliberately transmitting over the same bandwidth your link is using. t 0 t t t 3 Figure 5.3:Top rame: ormal communication over channel at carrier requency. econd rame: Channel is jammed. Third rame: ignal hops to a new channel. Fourth rame: jammer determines new channel and changes jamming requency. EE43: RF Engineering or Telecommunications cott Hudson, Washington tate University 05/30/7

3 FH- 5.3 But why not stay ahead o Foe s jamming by agreeing beore hand to switch channels every Th seconds. This should be a short enough time that Foe won t be able to ind the new channel and begin jamming it beore you ve switched to yet another channel. This requency hopping strategy is an eective counter measure or jamming. ote that while at any instant you are perorming narrow-band communication with a bandwidth B, over time you spread your signal out over a spectrum MB where M is the number o channels or requency slots you switch between. This is reerred to as spread spectrum communication. t might seem that a down side o this approach is that you use up M channels or a single radio link and those channels would not be available or other users. However, this is not the case. Other users can requency hop also provided only that they use a dierent hopping sequence. This is illustrated in Fig UER UER t 0 t t t 3 Figure 5.4:Dierent users can requency hop in the same spectrum provided they use a dierent hopping sequence. How many users can share the spectrum? Obviously no more than M since there are only M slots available at any time. n principle, though, M users could requency hop simultaneous as long as they coordinated their hopping so that no two o them every tried to use the same slot. EE43: RF Engineering or Telecommunications cott Hudson, Washington tate University 05/30/7

4 FH- 5.4 pead-pectrum or tealth Communication Consider UER in Fig While a particular requency slot is being used, the average spectral intensity is / B Watts/Hz where is the power in Watts and B the bandwidth in Hz. This is illustrated at the let in Fig /B B MB /MB Figure 5.5: Let, short-term spectrum while a single requency slot is in use; right, time-average spectrum or spread-spectrum communication. Because the same power is spread over a greater spectral width, the spectral intensity is reduced. However a user makes use o any given slot only a raction / M o the time. o on average the spectral intensity at all requencies is only / MB. Another way o looking at this is that we have spread the power out over a bandwidth MB so the intensity is only / MB. This is illustrated at the right o Fig n both cases we have that the spectral intensity times the bandwidth is equal to the power. M is very large then the spectral intensity will be much smaller that what we would have in a narrow-band system. n act / MB might be so small that it is much less than the noise loor o Foe s spectrum analyzer. so then Foe will not be able to see that Friend is transmitting. We have thus obtained a stealth channel. This is o obvious value in military scenarios. ow you might think that / would suer because we are using a larger bandwidth and the total noise power would increase accordingly (since 0( BW ) ). However, at any given time we are only receiving over the relatively narrow bandwidth B, so the noise power is never more than 0 B. o, / / 0B which is independent o M. We do not look at the noise over the entire spectral width o MB at the same time so the noise power is not M0B. We get the stealth eect without sacriicing /! Example 5. Let s say we need / 0dB and B 0kHz or a good link, but we want the spectral intensity o the spread-spectrum signal to be 0 db below the noise loor. We can achieve this by using a spreading actor o M 000. This reduces the spectral intensity by 30 db which will be 0 db relative to the noise loor. We will use a total spectral width o 0 MHz. Commercial Application o pread pectrum t might seem that jamming and stealth would not be o much interest in a commercial environment where (hopeully) people are not trying to use radios or hostile purposes. However, i two users just happen to try and use the same channel then the eect is the same as i they were jamming one another. By the same token, even i don t care whether or not people know EE43: RF Engineering or Telecommunications cott Hudson, Washington tate University 05/30/7

5 FH- 5.5 m transmitting, i use spread spectrum to lower my spectral intensity below the noise loor o other users then will most probably not create intererence or them. These eects are important, particularly in the types o unsupervised communications that typiy the use o unlicensed spectrum. n the U.., the Federal Communication Commission (FCC) regulates the use o spectrum. Much o the available spectrum is auctioned to commercial users, such as radio and TV stations, and cellular companies. These commercial users pay the government or a license giving them exclusive right to use a particular portion o the spectrum in a particular geographical region. However, some regions o the spectrum are unlicensed, meaning they are available or use by anyone, provide one ollows some basic rules set down by the FCC. Two important unlicensed bands are the so-called M (ndustry cientiic Medical) bands at MHz and,400.0-,483.5 MHz. The later, in particular, is used extensively or wireless LAs. BER Example 5. Bluetooth is a wireless interace or a wide variety o short-range ( picocell ) applications including computer LAs and wireless peripherals. t uses FK in the.4 GHz unlicensed ( M ) band. There are 79 RF channels each with MHz bandwidth. Frequency slots are typically occupied or 65 s. Maximum transmitter power is 00 mw (0 dbm). Because the carrier requency is constantly changing, it is diicult to establish phase coherence in a FH- system. Consequently, incoherent FK modulation is typically employed. Recall that or incoherent FK the probability o a bit error is Eb 0 e e (5.) a single FH- link were operating, then it would ideally achieve this same BER. However, i other links are also operating in the same spectrum, there is a possibility that two or more links may hop to the same carrier requency at the same time. ay there are M slots available and K links operating. The probability that a given link will hop to a given slot is just / M. The probability that it will not hop to that slot is / M. Thereore the probability that the other K K links will not hop to the same slot that you hop to is / M. o, the probability o your requency slot being hit by at least one other link is The probability o not getting hit is K p h (5.) M p. you get hit then the / will be driven very low h and the probability o a bit error will go way up. n the worst case e Thereore, on average, EE43: RF Engineering or Telecommunications cott Hudson, Washington tate University 05/30/7

6 FH- 5.6 ( p e e h Eb 0 ) e Eb 0 M p K h M K (5.3) where the irst term accounts or errors due to noise and the second term or errors due to E b 0 hits. Assume that / is very strong so that e 0, i.e., there are no noise-induced errors. Then errors arise solely due to hits and e is the second term o (5.3). This is plotted in Fig. 5.6 or M ( K) Figure 5.6: robability o a requency slot being hit vs. number o radio links operating or M 79 slots. n practice you would probably try to send several bits during each hop. For example, in the Bluetooth standard, the bit period is s and hops last or the 65 s duration o a packet. o the curve in Fig. Gives the probability that a packet will be destroyed by a hit. Destroyed packets need to be resent, and this represents the overhead resulting rom the unsupervised sharing o spectrum between K users. As a very interesting historical aside, requency hopping was invented and patented (U.. atent number,9,387) by the legendary movie actress Hedy Lamarr and avant-garde musical composer George Antheil in August 94 under the title ecret Communication ystem. The goal was to provide an unjammable radio link or guiding torpedoes during World War. Their system used 88 requencies (corresponding to the 88 keys on a piano) and perorated papers rolls (such as those used at the time in player pianos ) to coordinate the hops. K EE43: RF Engineering or Telecommunications cott Hudson, Washington tate University 05/30/7

7 FH- 5.7 Reerences. Bray, J. and C. F.turman, Bluetooth.: Connect Without Cables, rentice Hall, 00, B Rappaport, T.., Wireless Communications: rinciples and ractice, rentice Hall 00, B EE43: RF Engineering or Telecommunications cott Hudson, Washington tate University 05/30/7