Lecture 2.5. Broadband Communication Systems

1. Lecture 2.5.Broadband Communication Systems

2. Multiple users share the same frequency channel sequentially • Time slot sequence repeats over and over TIME TIME Each user assigned a different frequency - like ordinary radio User 3 1 2 3 User 2 User 1 FREQUENCY FREQUENCY FDMA TDMA • Channel is “spread” over wide frequency band • Many users share the same frequency band at the same time • Each user is assigned a unique “code” to identify and separatethem CODE TIME User 3 User 2 User 1 FREQUENCY CDMAalso known as “Spread Spectrum” Multiple Access MethodsMultiple users share the available spectrum

3. Spread spectrumSpread-spectrum techniques are methods by which electromagnetic energy generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth. These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference and jamming, to prevent detection, and to limit the power flux density on satellitedownlinks.

4. Frequency Spectrum (MHz) 88 2400 103 FM Band Power Power 2400 2500 Frequency Frequency Spread Spectrum Transmission Standard Radio Transmission

5. COMPLETE WAVEBAND ALLOCATED Time Time Spread Spectrum Technologies DS vs. FH • Direct Sequence • Each symbol is transmitted over multiple frequencies at the same time • Very efficient (no overhead) • Higher speed than FH at comparable distances • System capacity (multiple channels) higher than FH • Frequency Hopping • Sequential use of multiple frequencies • Hop sequence and rate will vary • “End hop waste time”

6. Digital Signal (Bits) Multiplier Source andChannelCoding RFModulator X FrequencySpectrum f Code Bits (Chips) f “Spread” FrequencySpectrum CodeGenerator Spread Spectrum Technologies Direct Sequence transmitter • Spreading: Information signal (I.e. a “symbol”) is multiplied by a unique, high rate digital code which stretches (spreads) its bandwidth before transmission. • Code bits are called “Chips”. • Sequence is called “Barker Code”

7. Symbol time ts “1” “0” “symbol” X = “Barker” sequence Result of multiplication Chip time tc 22 Mhz 2 Mhz Spread Spectrum Technologies What happens during “spreading” • Due to the multiplication of a symbol with Barker code, the “rate-of-change” increases with a factor 11 • This means that cycle rate increases from 1 MHz to 11 MHz • In terms of spectrum this means that after RF modulation the signal is spread from 2 MHz bandwidth to 22 MHz bandwidth

8. Digital Signal (Bits) Multiplied RFDemodulator ChannelandSourceDecoding X De-SpreadSignal f “Spread” FrequencySpectrum f Code Bits (Chips) CodeGenerator Spread Spectrum Technologies Direct Sequence receiver • At the receiver, the spread signal is multiplied again by a synchronized replica of the same code, and is “de-spread” and recovered • The outcome of the process is the original “symbol”

9. Direct Sequence Spread Spectrum Signal : 11 chip code Data Symbol time Spread Spectrum Technologies De-spreading • When the incoming signal is de-spread, it results in either a positive (+) or a negative (-) “spike” • These “spikes” arrive at intervals equal to the symbol time • A positive spike represents a “1” symbol, a negative spike represents a “0” symbol

10. Symbol time Spread Spectrum Technologies Direct Sequence receiver - effect of echoes • Echoes may arrive at the receiver, fluctuations can be noticed at positions other than at the symbol time boundaries • These fluctuations are ignored as the receiver will only interpret the spike at the synchronization points (separated from each other by the symbol time)

11. Spread-spectrum telecommunicationsThis is a technique in which a (telecommunication) signal is transmitted on a bandwidth considerably larger than the frequency content of the original information.Spread-spectrum telecommunications is a signal structuring technique that employs direct sequence, frequency hopping, or a hybrid of these, which can be used for multiple access and/or multiple functions. This technique decreases the potential interference to other receivers while achieving privacy. Spread spectrum generally makes use of a noise-like signal structure to spread the normally narrowband information signal over a relatively wideband (radio) band of frequencies.

12. The receiver correlates the received signals to retrieve the original information signal. Originally there were two motivations: either to resist enemy efforts to jam the communications (anti-jam, or AJ), or to hide the fact that communication was even taking place, sometimes called low probability of intercept (LPI).

13. Frequency-hopping spread spectrum (FHSS), direct-sequence spread spectrum (DSSS), time-hopping spread spectrum (THSS), chirp spread spectrum (CSS), and combinations of these techniques are forms of spread spectrum. Each of these techniques employs pseudorandom number sequences — created using pseudorandom number generators — to determine and control the spreading pattern of the signal across the alloted bandwidth. Ultra-wideband (UWB) is another modulation technique that accomplishes the same purpose, based on transmitting short duration pulses. Wireless Ethernet standard IEEE 802.11 uses either FHSS or DSSS in its radio interface.

14. Process gainIn a spread spectrum system, the process gain (or 'processing gain') is the ratio of the spread (or RF) bandwidth to the unspread (or baseband) bandwidth. It is usually expressed in decibels (dB).For example, if a 1 kHz signal is spread to 100 kHz, the process gain expressed as a numerical ratio would be 100,000/1,000 = 100. Or in decibels, 10log10(100) = 20 dB.

15. Note that process gain does not reduce the effects of wideband thermal noise. It can be shown that a direct sequence spread spectrum (DSSS) system has exactly the same bit error behavior as a non-spread spectrum system with the same modulation format. Thus, on an additive white Gaussian noise (AWGN) channel without interference, a spread system requires the same transmitter power as an unspread system, all other things being equal.Unlike a conventional communication system, however, a DSSS system does have a certain resistance against narrowband interference, as the interference is not subject to the process gain of the DSSS signal and hence the signal-to-interference ratio is improved.

17. Notes- Techniques known since 1940s and used in military communication system since 1950s- "Spread" radio signal over a wide frequency range several magnitudes higher than minimum requirement. The core principle of spread spectrum is the use of noise-like carrier waves, and, as the name implies, bandwidths much wider than that required for simple point-to-point communication at the same data rate.- Two main techniques: Direct sequence (DS)Frequency hopping (FH)

18. - Resistance to jamming (interference). DS is better at resisting continuous-time narrowband jamming, while FH is better at resisting pulse jamming. In DS systems, narrowband jamming affects detection performance about as much as if the amount of jamming power is spread over the whole signal bandwidth, when it will often not be much stronger than background noise. By contrast, in narrowband systems where the signal bandwidth is low, the received signal quality will be severely lowered if the jamming power happens to be concentrated on the signal bandwidth.

19. - Resistance to eavesdropping. The spreading code (in DS systems) or the frequency-hopping pattern (in FH systems) is often unknown by anyone for whom the signal is unintended, in which case it "encrypts" the signal and reduces the chance of an adversary's making sense of it.

20. - What's more, for a given noise power spectral density (PSD), spread-spectrum systems require the same amount of energy per bit before spreading as narrowband systems and therefore the same amount of power if the bitrate before spreading is the same, but since the signal power is spread over a large bandwidth, the signal PSD is much lower, often significantly lower than the noise PSD, therefore the adversary may be unable to determine if the signal exists at all.

21. - Resistance to fading. The high bandwidth occupied by spread-spectrum signals offer some frequency diversity, i.e. it is unlikely that the signal would encounter severe multipath fading over its whole bandwidth. - Multiple access capability. Multiple users can transmit simultaneously on the same frequency (range) as long as they use different spreading codes. See CDMA.

22. Frequency-hopping spread spectrumFrequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. It is utilized as a multiple access method in the frequency-hopping code division multiple access (FH-CDMA) scheme.

23. A spread-spectrum transmission offers three main advantages over a fixed-frequency transmission:- Spread-spectrum signals are highly resistant to narrowbandinterference. - Spread-spectrum signals are difficult to intercept. An FHSS signal simply appears as an increase in the background noise to a narrowband receiver. An eavesdropper (соглядатай) would only be able to intercept the transmission if they knew the pseudorandom sequence.- Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference.

24. Basic algorithmTypically, the initiation of an FHSS communication is as follows1. The initiating party sends a request via a predefined frequency or control channel.2. The receiving party sends a number, known as a seed.3. The initiating party uses the number as a variable in a predefined algorithm, which calculates the sequence of frequencies that must be used. 4. The initiating party sends a synchronization signal via the first frequency in the calculated sequence, thus acknowledging to the receiving party it has correctly calculated the sequence.5. The communication begins, and both the receiving and the sending party change their frequencies along the calculated order, starting at the same point in time.

25. Direct-sequence spread spectrumIn telecommunications, direct-sequence spread spectrum (DSSS) is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 'spread spectrum' comes from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency.

26. Features1. It phase-modulates a sine wavepseudorandomly with a continuous string of pseudonoise (PN) code symbols called "chips", each of which has a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much faster chips(каждому символу передаваемого сообщения ставится в соответствие некоторая достаточно длинная псевдослучайная последовательность) .2. It uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal.

27. Transmission method- Direct-sequence spread-spectrum transmissions multiply the data being transmitted by a "noise" signal. This noise signal is a pseudorandom sequence of 1 and −1 values, at a frequency much higher than that of the original signal, thereby spreading the energy of the original signal into a much wider band.- The resulting signal resembles (напоминает)white noise. However, this noise-like signal can be used to exactly reconstruct the original data at the receiving end, by multiplying it by the same pseudorandom sequence.- This process, known as "de-spreading", mathematicallyis a correlation of the transmitted PN sequence with the received PN sequence.

28. For de-spreading to work correctly, the transmit and receive sequences must be synchronized. This requires the receiver to synchronize its sequence with the transmitter's sequence via some sort of timing search process.

29. The resulting effect of enhancing signal to noise ratio on the channel is called process gain. This effect can be made larger by employing a longer PN sequence and more chips per bit, but physical devices used to generate the PN sequence impose practical limits on attainable processing gain.

30. If an undesired transmitter transmits on the same channel but with a different PN sequence (or no sequence at all), the de-spreading process results in no processing gain for that signal. This effect is the basis for the code division multiple access (CDMA) property of DSSS, which allows multiple transmitters to share the same channel within the limits of the cross-correlation properties of their PN sequences.

31. A plot of the transmitted waveform has a roughly bell-shaped envelope centered on the carrier frequency(just like a normal AM transmission, except that the added noise causes the distribution to be much wider than that of an AM transmission).In contrast, frequency-hopping spread spectrum pseudo-randomly re-tunes the carrier, instead of adding pseudo-random noise to the data, which results in a uniform frequency distribution.

32. Benefits- Resistance to intended or unintended jamming.- Sharing of a single channel among multiple users- Reduced signal/background-noise level hampers interception (stealth).- Determination of relative timing between transmitter and receiver.

33. Uses- The United States GPS and European Galileosatellite navigation systems- DS-CDMA (Direct-Sequence Code Division Multiple Access) is a multiple access scheme based on DSSS, by spreading the signals from/to different users with different codes. It is the most widely used type of CDMA.- Cordless phones operating in the 900 MHz, 2.4 GHz and 5.8 GHz bands- 2.4 GHz Wi-Fi

34. Time-hoppingTime-Hopping in communications, a type of spread spectrum technology in which the carrier is turned on and off by the pseudorandom code sequence.

36. Overview- Chirp Spread Spectrum uses its bandwidth to broadcast a signal, making it robust to channel noise. - Because the chirps utilize a broad band of the spectrum, Chirp Spread Spectrum is also resistant to multi-path fading even when operating at very low power. - However, it does not add any pseudo-random elements to the signal to help distinguish it from noise on the channel, instead relying on the linear nature of the chirp pulse.

37. База сигнала - это произведение ширины полосы спектра сигнала на длительность сигнала. Чем короче сигнал, тем шире его спектр.База сигнала это произведение эффективного значения длительности сигнала и эффективного значения ширины его спектра. В простых случаях за эффективную ширину спектра можно принять ширину главного лепестка спектра. Длительность сигнала и ширина его спектра подчиняются соотношению неопределенности, гласящему, что база сигнала не может быть меньше единицы. Ограничений на максимальное значение базы сигнала не существует. То есть короткий сигнал с узким спектром существовать не может, а бесконечный сигнал с широким спектром — может (так называемый широкополосный сигнал, сигнал с большой базой).

38. The basic terms and definitionsTendencies in construction of perspective digital systems for a mobile radio communication provide use of broadband signals. In this lecture broadband signals properties and features of their application in broadband communication systems are considered.Broadband signals – are signals, for which base is equaled to product of band spectrum width and duration , satisfies to a condition:B=FT>>1.

39. For simple (elementary) signals in which B≈1.In digital communication systems effective spectrum width spectrum is R=1/T. Therefore at B>>1 we have F >>R. In other words, broadband signals spectrum width F always is much more than spectrum width of the message which is transferred.

40. The value of T=1/Rinfluences efficiency of communication. For preset value R a condition B=FT>>1 can be fulfilled by expansion of spectrum width.