Digital Modulation and Multiplexing
Cables and wireless channels transmit analog signals such as voltage, light intensity or loudness in continuous mode. The conversion process between the bits and the signals that represent them is called digital modulation.
Direct conversion bits in the signal, called baseband transmission, in which the signal occupies frequencies from zero to a maximum, which depend on the signal transmission rate. This is common for cables.
Regulates the amplitude, phase, or frequency of the carrier signal to transmit bits, which is called bandwidth transmission, in which the signal occupies a frequency band around the frequency of the carrier signal. This is typical of wireless and optical channels, for which the signals must be in a given frequency band.
Channels are often used by several signals. After all, it is much more practical to use a single cable to transmit multiple signals than to install a cable for each signal. This type of exchange is called multiplexing.
This can be achieved in many different ways. Modulation and multiplexing methods are widely used for cable, fiber optics, terrestrial radio channels, and satellite channels.
The most direct form of digital modulation is to use a positive voltage for presentation 1 and a negative voltage to represent 0. For an optical fiber, the presence of light can represent 1 and the absence of light – 0. This scheme is called NRZ (No Return to Zero). A strange name for historical reasons, and it simply means that the signal follows the data. After sending the signal, NRZ is distributed by cable. At the other end, the receiver converts it into bits by sampling the signal at regular intervals.
This signal will not look the same as the signal sent. It will be weakened and distorted by the channel and the sound of the receiver. To decode the bits, the receiver assigns the signal samples to the nearest characters. For NRZ, a positive voltage will be taken to indicate that 1 has been sent and a negative voltage to indicate that 0 has been sent.
1.1 Bandwidth Efficiency
When using NRZ, the signal can alternate between positive and negative levels up to every 2 bits (when switching between 1 and 0). This means that the bandwidth is at least B / 2 Hz when the transmission rate is B bit / s. This relationship comes from the Nyquist level. This is a fundamental limit. Therefore, we can’t use NRZ faster without using more bandwidth. Bandwidth is usually a limited resource, even for cable channels. High-frequency signals are becoming weaker, making them less useful, and high-frequency signals also require faster electronics.
1.2 Clock Recovery
For all schemes that encode bits into characters, the receiver needs to know when the character ends and the next character starts decoding the bits correctly. In NRZ, where the symbols are simply voltage levels, a long line of 0 or 1 does not change the signal. After a while, it is difficult to distinguish the bits, because 15 zeros are very similar to 16 zeros if you do not have very accurate clocks.
One strategy is to send a separate clock to the receiver. The other clock line is not important for computer buses or short cables with many parallel lines, but it is a waste for most network connections because if we had another line to send a signal, we could use it to send data. An intellectual trick is to mix the clock signal with the data signal when linked to the XOR so that an extra line is not needed. The clock makes the clock transition at each bit, so it operates at double speed. When it is in the XOR with a level of 0, it goes from a low level to a high level, which is only a clock. This transition is a logical 0. When you are in XOR with level 1, it changes completely and makes the transition from top to bottom. This transition is logical and is called Manchester encoding and has been used for classical Ethernet.
1.3 Balanced Signals
Signals with both positive and negative voltages, even for short periods, are called symmetric signals. On average, they are zero, which means that they do not have any DC electrical components. The absence of a DC component is an advantage because some channels, such as coaxial cables or transformer lines, greatly weaken the DC component due to its physical properties. Besides, a method of connecting a receiver to a channel, called capacitive coupling, transmits only a portion of the variable signal.
The balancing helps provide transitions for clock recovery because there is a combination of positive and negative voltages. It also provides a simple way to calibrate receivers because the average value of a signal can be measured and used as the decision threshold for symbol decoding. With unbalanced signals, the average may deviate from the actual decision level, for example, due to the 1s density, which will result in the decoding of a larger number of erroneous characters.
Digital modulation is achieved by transmitting in a passband transmission by regulating or modulating a carrier signal in a bandwidth (bandwidth called passband). Here can modulate the amplitude, frequency or phase of the carrier signal. Each of these methods has a corresponding name. In ASK (Amplitude Shift-Keying), two different amplitudes are used to represent 0 and 1. More than two levels can be used to represent more characters. Likewise, two or more different tones are used with FSK (Frequency Shift Keying). In the simplest form of PSK (Phase Shift Keying), the carrier wave is systematically shifted by 0 or 180 degrees at each symbol period. Since there are two phases, this is called BPSK (Binary Phase Shift Keying). “Binary” here refers to two characters and not to the fact that the characters represent 2 bits.
The best way to use channel bandwidth more effectively is to use four offsets, for example, 45, 135, 225 or 315 degrees, to transfer 2 bits of information per symbol. This version is called QPSK (phase change quadrature coding).
Frequency Division Multiplexing
FDM (Frequency Division Multiplexing) takes advantage of bandwidth transmission for channel sharing. It divides the spectrum into frequency bands, and each user has exclusive ownership of the band to send his signal. AM broadcast illustrates FDM. The distributed spectrum is approximately 1 MHz, approximately 500 to 1500 kHz. Different frequencies are assigned to different logical channels (stations), each of which operates in a certain part of the spectrum, and the distance between the channels is large enough to avoid interference.
By sending digital data, it is possible to divide the spectrum without using guard bands.
3.1 Orthogonal Frequency Division Multiplexing
In OFDM (Orthogonal Frequency Division Multiplexing), the channel bandwidth is divided into several subcarriers that send data independently (for example, using the QAM method). Subcarriers are very close together in the frequency domain. Thus, the signals of each sub-carrier propagate to the neighbors.
The frequency response of each subcarrier is calculated to be zero at the center of the adjacent subcarriers. As a result, the subcarriers can be sampled at their center frequencies without disturbing their neighbors. For this to work, time is required to repeat a portion of the symbol signals over time to obtain the desired frequency response. However, this overhead is much less than necessary for many guard bands.
Time Division Multiplexing
An alternative to FDM is TDM (Time Division Multiplexing). Here, users play in turn (as a loopback), each of them periodically receiving maximum throughput for a short time. Example of three streams multiplexed with TDM. The bits of each input stream is taken over a fixed time slot and sent to the aggregated stream. This sequence is performed with the total speed of the individual sequences. For this to work, the threads must be synchronized in time. Small analog guard time intervals can be added to the frequency guard band to accommodate small variations in time.
TDM is widely used in telephone and cellular networks. To avoid confusion, let’s explain that it is very different from the STDM (Statistical Time Division Multiplexing) option. A “statistical” prefix has been added, indicating that individual flows do not contribute to the multiplexed stream according to a fixed schedule, but under their demand statistics. STDM is the exchange of packages under a different name.
Code Division Multiplexing
There is a third type of multiplexing, which works completely differently than FDM and TDM. CDM (Code Division Multiplexing) is a form of spread spectrum communication in which a narrow-band signal propagates over a wider frequency band. This can make it more interference tolerant, in addition to the fact that multiple signals from different users can share the same frequency band. Since code division multiplexing is mainly used for this purpose, it is usually called Code Division Multiple Access (CDMA).
CDMA allows each station to transmit continuously over the entire frequency spectrum. Multiple simultaneous transmissions are separated using the coding theory. In CDMA, each bit is divided into m short intervals, called chips. In general, 64 or 128 chips per bit, but in the example here, we will use 8 chips/bits for simplicity. A unique m-bit code, called chip sequence, is assigned to each station. For pedagogical purposes, it is convenient to use a bipolar record to write these codes as -1 and +1 sequences. The chip sequences show in parentheses.
To transmit a bit, 1 station sends its sequence of chips. To transmit bit 0, it sends the negative of its chip sequence. Other models are not allowed. Therefore, for, m = 8, if a chip sequence is assigned to station A (-1 -1 -1 +1 +1 +1 +1 +1 +1), it can send 1 bit by transmitting a sequence of 1 – 1 -1 +1 -1 -1). The signals are sent with these voltage levels, it is sufficient in terms of the sequences.
In given above the figure:
(a) Chip sequences for four stations.
(b) Signals the sequences represent.
(c) Six examples of transmissions.
(d) Recovery of station C’s signal.