Digital modulation
A microprocessor's (Intel P4-E) core
Nowadays information transmission is much more complex than before. Quite only radio stations use simple AM or FM modulations, but telephones, TV stations... digitize data before casting them. In this part you shall see how information is digitized, and in the next one you shall see how digitized information is cast. The process discribed here is called PCM (acronym standing for Pulse Code Modulation). It consist in two steps : sampling and quantification. PCM is used for example in telephony, television or in music recording.
1°/ Sampling Sampling consists in taking values of the signal at regular rate : for example 8 kHz (it means 8,000 values per second) for a telephone or 44 kHz for an audio CD. These values are called samples. The signal obtained is called a discrete signal. On the picture below, the analogue signal is the contineous curve and the discrete one is reprensented by the dots. Obvioulsy, the higher the sampling rate is, the smaller the loss of information is.
2°/ Quantification Once the signal is sampled, it has to be quantified. Quantification consists in associating each point to a step. Each step is coded thanks to n bits (a bit is either 0 or 1). The number of steps is 2n. For example, a 3 bits quantification allows 23 = 8 steps. Actually it consists in only « rounding » the values :
The incoming signal is the one at left, the quantified one is at right There is of course a loss of information, however the more steps there are, the smaller the loss is. For telephony a 8-bit quantification is used (256 steps can then be used), for audio CD a 16-bit quantification is used, that allows 65,536 steps to be used. After being quantified and coded (that is to say each step has been associated with a sequence of bits) the signal is ready for transmission. For the graph above, the digital sequence that has to be cast is for example : 000 010 010 011 011 100 .............111 111 110 110 101 101 If the receptor knows the quantification and sampling rates, it will be able to approximate the original analogue signal that was broadcast. As you read above, the quality depends on the sampling and quantification rates. But these features depend on the bandwidth of the transmission channel (it means the amount of information that can be cast in one second, in these casesit is expressed in bits per second), or on the storage capacity of the media used (CD, DVD...), becausethe higher the sampling and quantification rates are, the more information has to be cast.
Broadcasting the information
In modern wireless communications, radio waves are used to cast most information. Their use is always the same: a high frequency wave (a radio wave that is called carrier wave) is modified, or modulated by the signal that has to be broadcast, called modulating signal (because of its low frequency, this signal cannot spread « alone » over long distances, that is why it has to be broadcast by a carrier wave). Then the modulated high frequency wave is emitted. When received, the signal is demodulated, so that the modulating wave can be reconstituted. Two examples are presented below : ASK (Amplitude Shift Keying), that is used for example in infrared remote controllers, and PSK (standing for PhaseShift Keying), which is used in most of modern wireless communication means.
1°/ ASK (Amplitude Shift Keying) A simple way to cast information is ASK (Amplitude Shift Keying), thanks to OOK (On Off Keying), which is called a binary modulation. When the digit to cast is 0, nothing is emitted, and when the digit to cast is 1, a sinusoidal signal is emitted This modulation is mainly used for optical transmissions, for example with infrared remote controllers. There is an important synchronization problem with OOK : the receptor is unable to make a difference between a “0” and the beginning or the end of a signal. That is why the clock (the signal that regulates the transmission) has to be cast on another channel (that is to say another “way” of transmission, for example another frequency).
3°/ PSK (Phase Shift Keying) a) Modulation Phase Shift Keying is the most used method to transmit digital information over radio waves. To well understand this process, a little mathematics is required. A sinusoidal wave is modelled by the formula below
U t =U m×cos2 × f ×t Um is the amplitude, in Volt f is the frequency, in Hertz ϕ is the phasein radians Um, f and ϕ are constants For example, with Um = 5 V, f = 0.5 Hz and ϕ = 0, the curve U(t) at right is obtained :
With the same values excepted the phase (ϕ = π), the curve W(t) at left is obtained :
As you can see, graphically, changing the phase corresponds to translating the curve : U(0) = 5 and W(0) = -5. The phase is expressed in radians or in degrees (radians have been used for the previous examples), that is mathematically coherent with the cosine function. The graph at right shows a signal with 2 phase positions (BPSK).
Generally, 4 or 8 phase positions are used. PSK using 4 phase positions is called Quadrature PSK, or QPSK (QPSK is used for example for WiFi : a wireless way of communicating for computers and other devices, or for digital telephony). With QPSK, 2 bits can be cast with 1 phase variation. Two repartitions over the trigonometric circle are available for QPSK : the one presented at left, whose form is π/4 + kπ/2, or the other one, whose form is kπ/4. For complex technical reasons, the first one is the most used.
Four different phase positions on the trigonometric circle
QPSK Modulated signal : the signal is disformed because of its propagation
b) Demodulation Contrary to simple Amplitude modulation, which uses quite a “mechanic” system to demodulate the signal, PSK uses nearly only mathematical operations. For example for QPSK, the modulated signal can be written :
U t = A×cos2 ft/ 4 k ×/ 2 A is the amplitude of the wave, f its frequency and k is the variable that would modify the phase (k is a natural integer). The aim of the demodulation is first to reconstitute a non-modulated signal which would have the same frequency as the carrier wave and one of the 4 phase positions of QPSK. Then, by comparison between this signal and the modulated one, the changes in phase will be detected and the digital information will be extracted. To reconstitute, this non modulated signal, the input one is elevated to the square a first time :
A cable modem, that uses QPSK
U 2 t = A2 / 2×cos4 ft/ 2 k Reminder :
cos x2=1/ 2 ×1cos2 x Then the signal is elevated to the square a second time :
U 4 t = A4 /8×cos 8 ft The expression does not depend on k anymore. To get the needed signal, the frequency of this one has just to be divided by 4, and the continuous component (what makes it always positive) has to be retrieved, thanks to a high pass filter (a dipole that enables only high frequency signals to pass through it). A DECT Phone, that uses OQPSK, To know which phase has the reconstituted signal (one of the fourth used in QPSK), a known series of bits is cast at the beginning of any transmission. that is a kind of QPSK
c) Different uses of PSK Norm
Description
Modulation
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Cable Modems
BPSK or QPSK
IEEE 802.11a/b/g (a.k.a. WiFi)
Computers wireless networks
QPSK
NADC, CDMA...
Mobile Phones
QPSK
DVB-S
Satellite TV
QPSK
GPS
Real-Time navigation
BPSK
Furthermore PSK is used in laser vibrometry, which is a process used for example in acoustic, to detect the characteristics of a room. PSK is also used to count molecules.
Conclusion Quoted from the World Health Organization (W.H.O) : Recent years have seen an unprecedented increasein the number and diversity of sources of electric and magnetic fields (EMF) used for individual, industrial and commercial purposes. Such sources include television, radio, computers, mobile cellular phones, microwave ovens, radars and equipment used in industry, medicine and commerce. All these technologies have made our life richer and easier. Modern society is inconceivable without computers, television and radio. Mobile phones have greatly enhanced the ability of individuals to communicate with each other and have facilitated the dispatch of emergency medical and police aid to persons in both urban and rural environments. Radars make air travelling much safer. At the same time, these technologies have brought with them concerns about possible health risks associated with their use. Such concerns have been raised about the safety of cellular mobile telephones, electric power lines and police speed-control "radar guns". Scientific reports have suggested that exposure to electromagnetic fields emitted from these devices could have adverse health effects, such as cancer, reduced fertility, memory loss, and adverse changesin the behaviour and development of children. However, the actual level of health risk is not known, although for certain types of EMF, at levels found in the community, it may be very low or non-existent. [...] The conflict between concerns about possible health effects from exposure to EMF and the development of electricity supply and telecommunications facilities have led to considerable economic consequences. For example, electrical utilities in many countries have had to divert high voltage transmission lines around populated areas and even halt their construction. The installation of base stations for mobile telephone systems has been delayed or has met opposition from the public becauseof concerns that the RF emissions from these base stations might cause cancer in children. In the United States, for example, 85% of the total number of base stations needed have yet to be constructed. What is known is that EMF can heat brain tissues (perhapsup to 3°C), but as the WHO said, all the risks and effects are not known for the moment. That is why people have to be careful and to moderate their uses of these fantastic but perhaps dangerous new technologies.
