Text A
Modulation Technology
Formula（2.1）introduced the basic function of a sine wave which already indicates the three basic modulation schemes(typically,the cosine function is used for explanation):
This function has three parameters: amplitude A，frequency f, and phase φt，which may be varied in accordance with data or another modulating signal ①For digital modulation,digital data (0 and 1) is translated into an analog signal (base-band signal). Digital modulation is required if digital data has to be transmitted over a medium that only allows for analog transmission 
. One example for wired networks is the old analog telephone system—to connect a computer to this system a modem is needed. The modem then performs the translation of digital data into analog signals and vice versa .Digital transmission is used,for example,in wired local area networks or within a computer. In wireless networks, however, digital transmission cannot be used. Here, the binary bit-stream has to be translated into an analog signal first .The three basic methods for this translation are amplitude shift keying(ASK),frequency shift keying(FSK),and phase shift keying (PSK).These are discussed in more detail in the following sections .
Apart from the translation of digital data into analog signals,wireless transmission requires an additional modulation,an analog modulation that shifts the center frequency of the base-band signal generated by the digital modulation up to the radio carrier.For example,digital modulation translates a 1 Mbps bit-stream into a base-band signal with a bandwidth of 1 MHz.There are several reasons why this base-band signal cannot be directly transmitted in a wireless system:
▶ Antennas: An antenna must be the order of magnitude of the wavelength of the signal in size to be effective. For the 1 MHz signal in the example this would result in an antenna some hundred meter high,which is obviously not very practical for handheld devices.With 1 GHz,antennas of a few centimeters in length can be used.
▶ Frequency division multiplexing:Using only base-band transmission,FDM could not be applied.Analog modulation shifts the base-band signals to different carrier frequencies.The higher the carrier frequency, the more bandwidth that is available for many base-band signals.
▶ Medium characteristics: Path-loss, penetration of obstacles, reflection, scattering, and diffraction depend heavily on the wavelength of the signal. Depending on the application, the right carrier frequency with the desired characteristics has to be chosen:long waves for submarines, short waves for handheld devices, very short waves for directed microwave transmission etc.
As for digital modulation, three different basic schemes are known for analog modulation:amplitude modulation(AM),frequency modulation(FM),and phase modulation(PM).Figure 2.1 shows a (simplified) block diagram of a radio transmitter for digital data. The first step is the digital modulation of data into the analog base-band signal according to one of the schemes presented in the following sections. The analog modulation then shifts the center frequency of the analog signal up to the radio carrier.This signal is then transmitted via the antenna.
Figure 2.1 Modulation in a transmitter
The receiver(Figure 2.2)receives the analog radio signal via its antenna and demodulates the signal into the analog base-band signal with the help of the known carrier.This would be all that is needed for an analog radio tuned in to a radio station.(The analog base-band signal would constitute the music.) For digital data, another step is needed. Bits or frames have to be detected, i.e., the receiver must synchronize with the sender.How synchronization is achieved,depends on the digital modulation scheme.After synchronization,the receiver has to decide if the signal represents a digital 1 or 0,reconstructing the original data.
Figure 2.2 Demodulation and data reconstruction in a receiver
The digital modulation schemes presented in the following sections differ in many issues,such as spectral efficiency(i.e.,how efficiently the modulation scheme utilizes the available frequency spectrum), power efficiency (i.e., how much power is needed to transfer bits – which is very important for portable devices that are battery dependent),and robustness to multi-path propagation, noise,and interference.
Amplitude shift keying
Figure 2.3 illustrates amplitude shift keying (ASK), the most simple digital modulation scheme.The two binary values,1 and 0,are represented by two different amplitudes.In the example, one of the amplitudes is 0 (representing the binary 0). This simple scheme only requires low bandwidth,but is very susceptible to interference.Effects like multi-path propagation,noise,or path loss heavily influence the amplitude. In a wireless environment, a constant amplitude cannot be guaranteed, so ASK is typically not used for wireless radio transmission. However, the wire transmission scheme with the highest performance,namely optical transmission,uses ASK.Here,a light pulse may represent a 1, while the absence of light represents a 0. The carrier frequency in optical systems is some hundred T Hz.ASK can also be applied to wireless infra red transmission, using a directed beam or diffuse light.
Figure 2.3 Amplitude shift keying(ASK)
Frequency shift keying
A modulation scheme often used for wireless transmission is frequency shift keying (FSK) (Figure 2.4).The simplest form of FSK,also called binary FSK(BFSK),assigns one frequency f1 to the binary 1 and another frequency f2 to the binary 0.A very simple way to implement FSK is to switch between two oscillators, one with the frequency f1 and the other with f2, depending on the input. To avoid sudden changes in phase, special frequency modulators with continuous phase modulation (CPM) can be used. Sudden changes in phase cause high frequencies, which is an undesired side-effect. A simple way to implement demodulation is by using two band pass filters, one for f1 the other for f2.A comparator can then compare the signal levels of the filter outputs to decide which of them is stronger.FSK needs a larger bandwidth compared to ASK but is much less susceptible to errors.
Figure 2.4 Frequency shift keying(FSK)
Phase shift keying
Finally,phase shift keying(PSK)uses shifts in the phase of a signal to represent data.Figure 2.5 shows a phase shift of 180° or π as the 0 follows the 1(the same happens as the 1 follows the 0). This simple scheme,shifting the phase by 180° each time the value of data changes,is also called binary PSK(BPSK).A simple implementation of a BPSK modulator could multiply a frequency f with+1 if the binary data is 1 and with-1 if the binary data is 0.To receive the signal correctly,the receiver must synchronize in frequency and phase with the transmitter. This can be done using a phase lock loop(PLL).Compared to FSK,PSK is more resistant to interference,but receiver and transmitter are also more complex.
Figure 2.5 Phase shift keying(PSK)
Advanced frequency shift keying
A famous FSK scheme used in many wireless systems is minimum shift keying(MSK).MSK is basically BFSK without abrupt phase changes,i.e.,it belongs to CPM schemes.Figure 2.6 shows an example for the implementation of MSK.In a first step,data bits are separated into even and odd bits, the duration of each bit being doubled. The scheme also uses two frequencies:f1, the lower frequency,and f2,the higher frequency,with f2=2f1.
According to the following scheme,the lower or higher frequency is chosen(either inverted or non-inverted)to generate the MSK signal:
▶ if the even and the odd bit are both 0,then the higher frequency f2 is inverted(i.e.,f2 is used with a phase shift of 180°);
▶ if the even bit is 1,the odd bit 0,then the lower frequency f1 is inverted.This is the case,e.g., in the fifth to seventh columns of Figure 2.6;
▶ if the even bit is 0 and the odd bit is 1,as in columns 1 to 3,f1 is taken without changing the phase;
▶ if both bits are 1,then the original f2 is taken.
A high frequency is always chosen if even and odd bits are equal.The signal is inverted if the odd bit equals 0.This scheme avoids all phase shifts in the resulting MSK signal.
Figure 2.6 Minimum shift keying(MSK)
Adding a so-called Gaussian low pass filter to the MSK scheme results in Gaussian MSK (GMSK),which is the digital modulation scheme for many European wireless standards.The filter reduces the large spectrum needed by MSK.
Advanced phase shift keying
The simple PSK scheme can be improved in many ways. The basic BPSK scheme only uses one possible phase shift of 180°.The left side of Figure 2.7 shows BPSK in the phase domain(which is typically the better representation compared to the time domain in Figure 2.5).The right side of Figure 2.7 shows quadrature PSK (QPSK), one of the most common PSK schemes (sometimes also called quaternary PSK). Here, higher bit rates can be achieved for the same bandwidth by coding two bits into one phase shift.Alternatively,one can reduce the bandwidth and still achieve the same bit rates as for BPSK.
Figure 2.7 BPSK and QPSK in the phase domain
QPSK(and other PSK schemes)can be realized in two variants.The phase shift can always be relative to a reference signal (with the same frequency).If this scheme is used,a phase shift of 0 means that the signal is in phase with the reference signal.A QPSK signal will then exhibit a phase shift of 45° for the data 11,135° for 10,225° for 00,and 315° for 01 — with all phase shifts being relative to the reference signal.The transmitter‘selects’parts of the signal as shown in Figure 2.28 and concatenates them.To reconstruct data,the receiver has to compare the incoming signal with the reference signal.One problem of this scheme involves producing a reference signal at the receiver. Transmitter and receiver have to be synchronized very often,e.g.,by using special synchronization patterns before user data arrives or via a pilot frequency as reference.
One way to avoid this problem is to use differential QPSK(DQPSK).Here the phase shift is not relative to a reference signal but to the phase of the previous two bits.In this case,the receiver does not need the reference signal but only compares two signals to reconstruct data.DQPSK is used in US wireless technologies IS-136 and PACS and in Japanese PHS.
One could now think of extending the scheme to more and more angles for shifting the phase. For instance, one can think of coding 3 bits per phase shift using 8 angles. Additionally, the PSK scheme could be combined with ASK as is done for example in quadrature amplitude modulation (QAM)for standard 9600 bps modems(left side of Figure 2.8).Here,three different amplitudes and 12 angles are combined coding 4 bits per phase/amplitude change. Problems occur for wireless communication in case of noise or ISI.The more‘points’used in the phase domain,the harder it is to separate them. DQPSK has been proven as one of the most efficient schemes under these considerations.
Figure 2.8 16 quadrature amplitude modulation and hierarchical 64 QAM
A more advanced scheme is a hierarchical modulation as used in the digital TV standard DVB-T. The right side of Figure 2.8 shows a 64 QAM that contains a QPSK modulation. A 64 QAM can code 6 bit per symbol.Here the two most significant bits are used for the QPSK signal embedded in the QAM signal.If the reception of the signal is good the entire QAM constellation can be resolved. Under poor reception conditions,e.g.,with moving receivers,only the QPSK portion can be resolved. A high priority data stream in DVB-T is coded with QPSK using the two most significant bits.The remaining 4 bits represent low priority data.For TV this could mean that the standard resolution data stream is coded with high priority,the high resolution information with low priority.If the signal is distorted,at least the standard TV resolution can be received.
Multi-carrier modulation
Special modulation schemes that stand somewhat apart from the others are multi-carrier modulation (MCM), orthogonal frequency division multiplexing (OFDM) or coded OFDM (COFDM)that are used in the context of the European digital radio system DAB and the WLAN standards IEEE 802.11a and HiperLAN2. The main attraction of MCM is its good ISI mitigation property.MCM splits the high bit rate stream into many lower bit rate streams(see Figure 2.9),each stream being sent using an independent carrier frequency. If, for example,n symbols/s have to be transmitted, each subcarrier transmits n/c symbols/s with c being the number of subcarriers. One symbol could,for example represent 2 bits as in QPSK.DAB,for example,uses between 192 and 1536 of these subcarriers.The physical layer of HiperLAN2 and IEEE 802.11a uses 48 subcarriers for data.
Figure 2.9 Parallel data transmission on several subcarriers with lower rate
Figure 2.10 shows the superposition of orthogonal frequencies.The maximum of one subcarrier frequency appears exactly at a frequency where all other subcarriers equal zero.
Figure 2.10 Superposition of orthogonal frequencies
Using this scheme,frequency selective fading only influences some subcarriers,and not the whole signal—an additional benefit of MCM. Typically, MCM transmits symbols with guard spaces between single symbols or groups of symbols. This helps the receiver to handle multi-path propagation. OFDM is a special method of implementing MCM using orthogonal carriers. Computationally, this is a very efficient algorithm based on fast Fourier transform (FFT) for modulation/demodulation. If additional error-control coding across the symbols in different subcarriers is applied,the system is referred to as COFDM.