Spread Spectrum | Wideband Modulation | Broadband Modem

Wideband Modulation

Most modulation methods are designed to be spectrally efficient, i.e., to transmit as many bits per hertz as possible. The goal is to minimize the use of spectrum space and to transmit the highest speed possible in the given bandwidth. However, there is another class of modulation methods that do just the opposite. These methods are designed to use more bandwidth. The transmitted signal occupies a bandwidth many times greater than the information bandwidth. Special benefits derive from such wideband modulation techniques. The two most widely used wideband modulation methods are spread-spectrum and orthogonal frequency-division multiplexing.

Spread Spectrum

Spread spectrum (SS) is a modulation and multiplexing technique that distributes a signal and its sidebands over a very wide bandwidth. Traditionally, the efficiency of a modulation or multiplexing technique is determined by how little bandwidth it uses. The continued growth of all types of radio communication, the resulting crowding, and the finite bounds of usable spectrum space has made everyone in the world of data communication sensitive to how much bandwidth a given signal occupies. Designers of communication systems and equipment typically do all in their power to minimize the amount of bandwidth a signal takes. How, then, can a scheme that spreads a signal over a very wide piece of the spectrum be of value? The answer to this question is the subject of this section.

After World War II, the spread spectrum was developed primarily by the military because it is a secure communication technique that is essentially immune to jamming. In the mid-1980s, the FCC authorized the use of spread spectrum in civilian applications. Currently, the unlicensed operation is permitted in the 902- to 928-MHz, 2.4- to 2.483-GHz, and 5.725- to 5.85-GHz ranges, with 1 W of power. Spread spectrum on these frequencies is being widely incorporated into a variety of commercial communication systems. One of the most important of these new applications is wireless data communication. Numerous LANs and portable personal computer modems use SS techniques, as does a class of cordless telephones in the 900-MHz, 2.4-, and 5.8- GHz ranges. The most widespread use of SS is in cellular telephones in the 800- to 900-MHz and 1800- to 1900-MHz ranges. It is referred to as code-division multiple access (CDMA).

There are two basic types of spread spectrum: frequency-hopping (FH) and direct sequence (DS). In frequency-hopping SS, the frequency of the carrier of the transmitter is changed according to a predetermined sequence, called pseudorandom, at a rate higher than that of the serial binary data modulating the carrier. In direct-sequence SS, the serial binary data is mixed with a higher-frequency pseudorandom binary code at a faster rate, and the result is used to phase-modulate a carrier.

Frequency-Hopping Spread Spectrum

Fig. 11-33 shows a block diagram of a frequency-hopping SS transmitter. The serial binary data to be transmitted is applied to a conventional two-tone FSK modulator, and the modulator output is applied to a mixer. Also driving the mixer is a frequency synthesizer. The output signal from the bandpass filter after the mixer is the difference between one of the two FSK sine waves and the frequency of the frequency synthesizer. As the figure shows, the synthesizer is driven by a pseudorandom code generator, which is either a special digital circuit or the output of a microprocessor.

The pseudorandom code is a serial pattern of binary 0s and 1s that changes in a random fashion. The randomness of the 1s and 0s makes the serial output of this circuit appear as digital noise. Sometimes the output of this generator is called pseudo-random noise (PSN). The binary sequence is actually predictable since it does repeat after many bit changes (hence “pseudo’’). The randomness is sufficient to minimize the possibility of someone accidentally duplicating the code, but the predictability allows the code to be duplicated at the receiver.

Spread Spectrum | Wideband Modulation | Broadband Modem
Spread Spectrum | Wideband Modulation | Broadband Modem

PSN sequences are usually generated by a shift register circuit similar to that shown in Fig. 11-34. In the figure, eight flip-flops in the shift register are clocked by an external clock oscillator. The input to the shift register is derived by X-ORing two or more of the flip-flop outputs. It is this connection that produces the pseudorandom sequence. The output is taken from the last flip-flop in the register. Changing the number of flip-flops in the register and/or which outputs are X-ORed and fed back changes the code sequence. Alternatively, a microprocessor can be programmed to generate pseudorandom sequences.

In a frequency-hopping SS system, the rate of synthesizer frequency change is higher than the data rate. This means that although the data bit and the FSK tone it produces remain constant for one data interval, the frequency synthesizer switches frequencies many times during this period. See Fig. 11-35, where the frequency synthesizer changes frequencies four times for each bit time of the serial binary data. The time that the synthesizer remains on a single frequency is called the dwell time. The frequency synthesizer puts out a random sine wave frequency to the mixer, and the mixer creates a new carrier frequency for each dwell interval. The resulting signal, whose frequency rapidly jumps around, effectively scatters pieces of the signal all over the band. Specifically, the carrier randomly switches between dozens or even hundreds of frequencies over a given bandwidth. The actual dwell time on any frequency varies with the application and data rate, but it can be as short as 10 ms. Currently, FCC regulations specify that there be a minimum of 75 hopping frequencies and that the dwell time not exceed 400 μs.

Fig. 11-36 shows a random frequency-hop sequence. The horizontal axis is divided into dwell time increments. The vertical axis is the transmitter output frequency, divided into step increments of the PLL frequency synthesizer. As shown, the signal is spread out over a very wide bandwidth. Thus a signal that occupies only a few kilohertz of the spectrum can be spread out over a range that is 10 to 10,000 times that wide. Because an SS signal does not remain on any one frequency for a long time but jumps around randomly, it does not interfere with a traditional signal on any of the hopping frequencies. An SS signal actually appears to be more like background noise to a conventional narrow-bandwidth receiver. A conventional receiver picking up such a signal will not even respond to a signal of tens of milliseconds duration. In addition, a conventional receiver cannot receive an SS signal because it does not have a wide enough bandwidth and it cannot follow or track its random-frequency changes. Therefore, the SS signal is as secure as if it were scrambled.

Spread Spectrum | Wideband Modulation | Broadband Modem
Spread Spectrum | Wideband Modulation | Broadband Modem

Two or more SS transmitters operating over the same bandwidth but with different pseudorandom codes hop to different frequencies at different times, and do not typically occupy a given frequency simultaneously. Thus SS is also a kind of multiplexing, as it permits two or more signals to use a given bandwidth concurrently without interference. In effect, SS permits more signals to be packed in a given band than any other type of modulation or multiplexing.

A frequency-hopping receiver is shown in Fig. 11-37. The very wideband signal picked up by the antenna is applied to a broadband RF amplifier and then to a conventional mixer. The mixer, like that in any superheterodyne receiver, is driven by a local oscillator. In this case, the local oscillator is a frequency synthesizer like the one used at the transmitter. The local oscillator at the receiving end must have the same pseudorandom code sequence as that generated by the transmitter so that it can receive the signal on the correct frequency. The signal is thus reconstructed as an IF signal that contains the original FSK data. The signal is then applied to an FSK demodulator, which reproduces the original binary data train.

One of the most important parts of the receiver is the circuit that is used to acquire and synchronize the transmitted signal with the internally generated pseudorandom code. The problem of getting the two codes into step with each other is solved by a preamble signal and code at the beginning of the transmission. Once synchronization has been established, the code sequences occur in step. This characteristic makes the SS technique extremely secure and reliable. Any receiver not having the correct code cannot receive the signal.

frequency hopping receiver
frequency hopping transmitter

Many stations can share a common band if, instead of assigning each station a single frequency to operate on, each is given a different pseudorandom code within the same band. This permits a transmitter to selectively transmit to a single receiver without other receivers in the band being able to pick up the signal.

Direct-Sequence Spread Spectrum

A block diagram of a direct-sequence SS (DSSS) transmitter is shown in Fig. 11-38. The serial binary data is applied to an X-OR gate along with a serial pseudorandom code that occurs faster than the binary data. Fig. 11-39 shows typical waveforms. One bit time for the pseudorandom code is called a chip, and the rate of the code is called the chipping rate. The chipping rate is faster than the data rate.

The signal developed at the output of the X-OR gate is then applied to a PSK modulator, typically a BPSK device. The carrier phase is switched between 08 and 180° by the 1s and 0s of the X-OR output. QPSK and other forms of PSK can also be used. The PSK modulator is generally some form of balanced modulator. The signal phase modulating the carrier, being much higher in frequency than the data signal, causes the modulator to produce multiple, widely spaced sidebands whose strength is such that the complete signal takes up a great deal of the spectrum. Thus the resulting signal is spread. Because of its randomness, the signal looks like wideband noise to a conventional narrowband receiver.

frequency hopping receiver

Fig. 11-40 shows a standard narrowband signal and a spread spectrum signal. Assume a binary information signal that is occurring at a rate of 13 kbps. If we use BPSK with its 1 bit/Hz effi ciency, we could transmit this signal in a bandwidth of about 13 kHz. Now, if we use DSSS with a chipping signal of 1.25 Mbps, the resulting signal will be spread over about 1.25 MHz of bandwidth if we use BPSK. The spread signal has the same power as the narrowband signal but far more sidebands, so the amplitudes of the carrier and sidebands are very low and just above the random noise level. To a narrowband receiver, the signal just looks like a part of the noise level.

The effect of spreading the signal is to provide a type of gain called processing gain to the signal. This gain helps to improve the overall signal-to-noise ratio. The higher the gain, the greater the ability of the system to fi ght interference. This processing gain G is

G = BW/fb

where BW is the channel bandwidth and fb is the data rate. For the example in Fig. 11-40, the process gain is

G = 1.25 MHz/13 kbps = 96.15

In terms of decibels, this is a power gain of 19.83 dB.

One type of direct-sequence receiver is shown in Fig. 11-41. The broadband SS signal is amplified, mixed with a local oscillator, and then translated down to a lower IF in mixer 1. For example, the SS signal at an original carrier of 902 MHz might be translated down to another IF of 70 MHz. The IF signal is then compared to another IF signal that is produced in mixer 3 using a PSN sequence that is similar to that transmitted. The output of mixer 3 should be identical to the output of mixer 1 but shifted in time. This comparison process, called correlation, takes place in mixer 2. If the two signals are identical, the correlation is 100 percent. If the two signals are not alike in any way, the correlation is 0. The correlation process in the mixer produces a signal that is averaged in the low-pass filter at the output of mixer 2. The output signal will be a high average value if the transmitted and received PSN codes are alike.

The signal out of mixer 2 is fed to a synchronization circuit, which must re-create the exact frequency and phase of the carrier so that demodulation can take place. The synchronization circuit varies the clock frequency so that the PSN code output frequency varies, seeking the same chip rate as the incoming signal. The clock drives a PSN code generator containing the exact code used at the transmitter. The PSN code in the receiver is the same as that of the received signal, but the two are out of sync with each other. Adjusting the clock by speeding it up or slowing it down eventually causes the two to come into synchronization.

The PSN code produced in the receiver is used to phase-modulate a carrier at the IF in mixer 3. Like all the other mixers, this one is usually of the double-balanced diode ring type. The output of mixer 2 is a BPSK signal similar to that being received. It is compared to the received signal in mixer 3, which acts as a correlator. The output of mixer 3 is then filtered to recover the original serial binary data. The received signal is said to be despread.

Direct-sequence SS is also called code-division multiple access (CDMA), or SS multiple access. The term multiple access applies to any technique that is used for multiplexing many signals on a single communication channel. CDMA is used in satellite systems so that many signals can use the same transponder. It is also widely used in cellular telephone systems, for it permits more users to occupy a given band than other methods.

Benefits of Spread Spectrum

Spread spectrum is being used in more and more applications in data communication as its benefits are discovered and as new components and equipment become available to implement it.

Security. SS prevents unauthorized listening. Unless a receiver has a very wide bandwidth and the exact pseudorandom code and type of modulation, it cannot intercept an SS signal.


Resistance to jamming and interference. Jamming signals are typically restricted to a single frequency, and jamming one frequency does not interfere with an SS signal. Similarly, unintentional interference from a signal occupying the same band is greatly minimized and in most cases virtually eliminated.

Band sharing. Many users can share a single band with little or no interference. (As more and more signals use a band, the background noise produced by the switching of many signals increases, but not enough to prevent highly reliable communication.)


Resistance to fading and multipath propagation. Frequency-selective fading occurs during signal propagation because signals of different frequencies arrive at a receiver at slightly different times due to reflections from other objects. SS virtually eliminates wide variations of signal strength due to reflections and other phenomena during propagation

Precise timing. The use of the pseudorandom code in SS provides a way to precisely determine the start and end of a transmission. Thus SS is a superior method for radar and other applications that rely on accurate knowledge of transmission time to determine distance.

Orthogonal Frequency Division Multiplexing (OFDM)

Another wideband modulation method growing in popularity is called OFDM. Also known as multicarrier modulation (MCM), this relatively new form of modulation was first proposed in the 1950s but was not seriously considered until the 1980s and early 1990s. OFDM was not widely implemented until the late 1990s because of its complexity and cost. Today, fast DSP chips make OFDM practical.

Although OFDM is known as a modulation method as opposed to a multiplexing method, the term frequency-division multiplexing is appropriate because the method transmits data by simultaneously modulating segments of the high-speed serial bit stream onto multiple carriers spaced throughout the channel bandwidth. The carriers are frequency-multiplexed in the channel. The data rate on each channel is very low, making the symbol time much longer than predicted transmission delays. This technique spreads the signals over a wide bandwidth, making them less sensitive to the noise, fading, reflections, and multipath transmission effects common in microwave communication. Because of the very wideband nature of OFDM, it is considered to be a hybrid of spread spectrum.

Fig. 11-42 shows the concept of an OFDM modem. The single serial data stream is divided into multiple slower but parallel data paths, each of which modulates a separate subcarrier. For example, a 10-Mbps data signal could be split into 1000 data signals of 10 kbps transmitted in parallel. A common format is to space the subcarriers equally across the channel by a frequency that is the reciprocal of the subcarrier symbol rate. In the situation described here, the spacing would be 10 kHz. This is what makes the carriers orthogonal. Orthogonal means that each carrier has an integer number of sine wave cycles in one-bit period.

A plot of the bandwidth of each modulated carrier is the familiar (sin x)/x curve discussed. (See Fig. 11-43.) Nulls occur at those points equal to the symbol rate. With this arrangement, all carriers lie at the null frequencies of the adjacent carriers.

This permits simplified demultiplexing. Typically, BPSK, QPSK, or some form of QAM is used as the modulation method. In using QPSK or QAM, multiple bits per symbol are transmitted to permit a higher overall data rate. The subcarriers are algebraically added, and the resulting composite transmitted. Again referring to Fig. 11-42, note that the demodulator or receiver uses filters to separate out the individual subcarriers, and demodulators to recover the individual bitstreams, which are then reassembled into the original serial data.

When tens, hundreds, or even thousands of subchannels are used, as is the case in modern systems, obviously traditional modulators, demodulators, and filter circuits are impractical because of size, complexity, and cost. However, all of these functions can be readily programmed into a fast DSP chip.

A simplified version of the process is shown in Fig. 11-44. At the transmitter or modulator, the serial data is modulated; then a serial-to-parallel conversion is performed. The inverse fast Fourier transform (IFFT) is then implemented. This process produces all the orthogonal subcarriers. The D/A converter converts the OFDM signal to analog form and transmits it over the communication medium.

At the receiver or demodulator, the OFDM signal is digitized by the A/D converter, and then an FFT is performed. Recall that an FFT essentially does a spectrum analysis of a time-domain signal. A sampled time-domain analog signal is translated to a frequency-domain plot of spectral content by the FFT. The receiver FFT DSP sorts out the subcarriers and demodulates the original data, which is then reassembled into the original high-speed data stream.

Like most other modems, the OFDM modem is a fast DSP or FPGA chip programmed with all the mathematical algorithms that produce the functions defined by the blocks in Figs. 11-42 and 11-44.

Today, OFDM is widely used in wireless local-area networks (LANs) and fourth-generation (4G) Long Term Evolution (LTE) cellular networks. A version of OFDM for wired communication systems, known as discrete multitone (DMT), used in ADSL modems is discussed later in this chapter. It is also the method chosen for transmitting high-quality audio in digital satellite radio broadcasting systems. More recently, it has been proposed as an alternative to the 8-VSB AM used in digital high-definition TV systems. OFDM is also used in the high-speed version of wireless LANs (802.11 or Wi-Fi) and in the broadband wireless system called WiMAX. It is also being considered for future cell phone systems. When the digital data to be transmitted is accompanied by some form of forward error correction (FEC) scheme (Trellis code, etc.), the method is called coded OFDM or COFDM. OFDM is also the modulation of choice for the fourth-generation LTE cellular system. All modern smartphones incorporate LTE.

Broadband Modem Techniques

A modem or modulator-demodulator is the circuitry used to translate a baseband signal, usually digital, to a higher transmission frequency that is better suited to the transmission medium. A good example is that digital data is not that compatible with the twisted-pair cable used in telephone systems. The bandwidth is too limited. However, modulating the data onto a carrier provides a way to transmit the data over a system originally designed for analog voice. Modems are used with all types of cables such as the telephone lines and the coaxial cable of cable TV. And modems can be of the radio variety where they are used to transmit data wirelessly. This section provides an overview of several types of popular modems.

xDSL Modems

Although the twisted-pair telephone line to the central office is normally said to have a maximum bandwidth of 4 kHz, the truth is that the bandwidth of this line varies with its length, and it can handle higher frequencies than expected. Because of the line characteristics, the higher frequencies are greatly attenuated. However, by transmitting the higher frequencies at higher voltage levels and using line compensation techniques, it is possible to achieve very high data rates. New modulation methods also permit previously unachievable line rates. The digital subscriber line (DSL) describes a set of standards set by the International Telecommunications Union that greatly extend the speed potential of the common twisted-pair telephone lines. In the term xDSL, the x designates one of several letters that define a specific DSL standard.

The most widely used form of DSL is called asymmetric digital subscriber line (ADSL). This system permits downstream data rates up to 8 Mbps and upstream rate up to 640 kbps using the existing telephone lines. (Asymmetric means unequal upstream and downstream rates.)

The connection between a telephone subscriber and the nearest telephone central office is a twisted-pair cable using size 24 or 26 copper wire. Its length is usually anywhere between 9000 and 18,000 ft (2.7 to 5.5 km). This cable acts as a low-pass filter. Its attenuation to very high frequencies is enormous. Digital signals are seriously delayed and distorted by such a line. For this reason, only the lower 0- to 4-kHz bandwidth is used for voice.

Spread Spectrum | Wideband Modulation | Broadband Modem

ADSL employs some special techniques so that more of the line bandwidth can be used to increase data rates. Even though a 1-MHz signal may have an attenuation of up to 90 dB on an 18,000-ft line, special amplifi ers and frequency compensation techniques make the line usable.

The modulation scheme used with ADSL modems is called discrete multitone (DMT), another name for OFDM, discussed earlier in this chapter. It divides the upper-frequency spectrum of the telephone line into channels, each 4.3125 kHz wide. See Fig. 11-45. Each channel, called a bin or subcarrier, is designed to transmit at speeds up to 15 kbps/Bd or 60 kbps.

Each channel contains a carrier that is simultaneously phase-amplitude-modulated (QAM) by some of the bits to be transmitted. The serial data stream is divided up so that each carrier transmits some of the bits. All bits are transmitted simultaneously. Also, all the carriers are frequency-multiplexed into the line bandwidth above the normal voice telephone channel, as Fig. 11-45 shows.

The upstream signal uses the 4.3125-kHz bins from 25.875 to 138.8 kHz, and the downstream signal uses bins in the 138-kHz to 1.1-MHz range. The number of bits per baud and the data rate per bin vary according to the noise on the line. The less noise there is in each bin, the higher the data rate. Very noisy bins will carry few or no bits, whereas quiet bins can accommodate the maximum 15 kbps/Bd or 60 kbps.

This system is very complex and is implemented with a digital signal processor. The DSP chip handles all modulation and demodulation functions by simulating them digitally.

Fig. 11-46 shows an ADSL modem. All DMT/OFDM modulation/demodulation is handled by the DSP chip. The digital output of the DSP is converted to analog by the D/A converter. The resulting signal is amplified, filtered, and sent to a line driver that applies a high-level signal to the line. The hybrid is a circuit or transformer that permits simultaneous transmit and receive operations on the telephone line. The transformer matches the circuit impedance to the line.

In the receive mode, the incoming DMT analog signal is amplified, filtered, and applied to a PGA for AGC. The signal is digitized by the A/D converter and applied to the DSP for recovery of the digital data.

Several different levels of ADSL are available. The data rate for each depends upon the length of the subscriber twisted-pair cable. The shorter the cable, the higher the data rate. The highest standard rate is 6.144 Mbps downstream and 576 kbps upstream at a line distance not to exceed 9000 ft. The minimum rate is 1.536 Mbps downstream and 384 kbps upstream at line distances up to 18,000 ft. This is the most common form of ADSL.

ADSL MODEL

ADSL is available in most cities. ADSL is the most widely used form of high-speed Internet access throughout the world. ADSL is in second place to cable TV modem broadband access.

Other forms of DSL have also been defined. Two of the most recent versions of ADSL are ADSL2 and ADSL2+. Fig. 11-45 shows the spectrum used by the ADSL2 versions. ADSL2 extends the upper download speed to the 8- to 12-Mbps range at a distance of about 8000 ft. ADSL2+ further boosts speeds to 24 Mbps at a distance of about 4000 ft. Some newer standards referred to as bonding standards make use of two twisted pairs in the telephone cable to carry parallel data streams that effectively double the data rate for a given distance.

VDSL, or very high-speed DSL, offers a data rate of up to 52 Mbps one way (download) or 26 Mbps fully symmetrical using QAM. Fig. 11-45 shows the spectrum used.

It uses 2048 subcarriers to achieve that speed. VDSL permits digital video to be transmitted and thus offers an alternative to cable TV systems. However, to get this speed, the twisted-pair length is limited to 1000 ft or less at 52 Mbps and less than 3500 ft at 26 Mbps. Internet service providers (ISPs) have also improved DSL service by adding neighborhood terminals called digital subscriber line access multiplexers (DSLAMs). The DSLAMs talk to the central office by way of high-speed fiber-optic cable. The DSLAMs greatly shorten the distance between the central office and the homes, making it possible to achieve the speeds of ADSL2 or VDSL2.

Cable Modems

Most cable TV systems are set up to handle high-speed digital data transmission. The digital data is used to modulate a high-frequency carrier that is frequency- multiplexed conto the cable that also carries the TV signals.

Cable TV systems use a hybrid fiber-coaxial (HFC) network. Refer to Fig. 11-47. It consists of the headend where TV signals are collected and packaged for delivery over the 6-MHz channels defined on the cables. The headend also connects to the Internet. The TV signals are delivered over fast fiber-optic cables to neighborhood optical nodes where optical-to-electrical and electrical-to-optical conversions are performed as well as amplification. Then the frequency-multiplexed signals, both TV and Internet access, present over coaxial cables to the homes on the network. The cable used is typically RG-6/U 75-ohm coaxial. Each optical node serves from about 500 to 2000 homes. Additional amplification is used along the way as needed.

Spread Spectrum | Wideband Modulation | Broadband Modem

Cable TV systems use a bandwidth of approximately 750 MHz to 1 GHz. This spectrum is divided into 6-MHz-wide channels for TV signals. The standard VHF and UHF channels normally assigned to wireless TV are used on the cable, along with some special cable frequencies. The TV signals are therefore frequency- division-multiplexed onto the cable. Some of the channels are used exclusively for Internet access.

Fig. 11-48 shows the spectrum of the cable. Television channels extend from 50 MHz (Channel 2) up to 550 MHz. In this 500 MHz of bandwidth, up to 83 channels of 6 MHz can be accommodated. In some systems, the number of channels is extended out to about 1 GHz.

The spectrum above the TV channels, from 550 to 850 MHz, is available for digital data transmission. Standard 6-MHz channels are used, giving approximately 50 channels or more. These channels are used for downstream data transmission (from the headend down to the user).

The spectrum from 5 to 50 MHz, as you can see from Fig. 11-48, is divided into seven 6-MHz channels that are used for upstream data transmission (from the user up to the server). This frequency range maybe 5 to 42 MHz in some systems or 5 to 65 MHz in other systems.

Cable modems use 64-QAM for downstream data. Using 64-QAM in a 6-MHz channel provides a data rate up to 31.2 Mbps. This method of modulation uses 64 different phase-amplitude combinations (symbols) to represent multiple bits. Because each channel is shared by multiple users, the 31.2-Mbps rate is not achieved. Typical rates are in the 500-kbps to 10-Mbps range for downloads. In some systems, 256-QAM is available to provide a maximum data rate of 41.6 Mbps in a 6-MHz channel. Higher subscriber download speeds can then be achieved. In older systems, only one TV signal was transmitted per channel. However, today with modern digital techniques, several digital TV signals can be transmitted per channel by using DSP compression techniques.

Spread Spectrum | Wideband Modulation | Broadband Modem
CABLE MODEM

Standard QPSK and up to 128-QAM are used in the upstream channels to achieve a data rate of up to about 27 Mbps. With multiple users, the upstream rate is less. Fig. 11-49 shows a typical cable modem. It is basically a VHF/UHF receiver connected to the cable for downloads and a modulator/transmitter for uploads. The signal from the cable passes through the diplexer, which is a filter circuit that permits simultaneous transmit and receive operations. The signal is amplified and mixed with a local oscillator signal from the frequency synthesizer to produce an IF signal. The frequency synthesizer selects the cable channel. The IF signal is demodulated to recover the data. Reed Solomon error detection circuitry (see Sec. 11-7) finds and corrects any bit errors. The digital data then goes to an Ethernet interface to the PC. Ethernet is a popular networking system to be discussed.

For transmission, the data from the computer is passed through the interface, where it is encoded for error detection. The data then modulates a carrier that is up-converted by the mixer to the selected upstream channel before being amplified and passed through the diplexer to the cable.

Cable modems provide significantly higher data rates than can be achieved over the standard telephone system. The primary limitation is the existence or availability of a cable TV system that offers such data transmission services.

Cable modem standards are set by an industry consortium called Cable Labs. The specification is referred to as the Data over Cable Service Interface Specification (DOCSIS). The latest version, DOCSIS 3.1, permits channel bonding of two or more channels to achieve higher data rates. Higher rates also derive from higher modulation levels of up to 4096-QAM. DOCSIS 3.1 also permits the use of OFDM with 25-kHz to 50-kHz subcarriers and up to 4096-QAM modulation. This allows the cable companies to deliver up to 10 Gbps downstream and up to 1 Gbps upstream. The cable companies will be able to compete with fiber-optic systems.

Watch Video

Multiplexer | Demultiplexer | FDM | TDM | PAM | Applications ( Spread Spectrum | Frequency-Hopping Spread Spectrum | Direct-Sequence Spread Spectrum | Orthogonal Frequency Division Multiplexing (OFDM) | Broadband Modem Techniques | Wideband Modulation | Broadband Modem )

Digital Codes | Hartley’s Law | ASCII | Asynchronous | Encoding Methods ( Spread Spectrum | Frequency-Hopping Spread Spectrum | Direct-Sequence Spread Spectrum |Orthogonal Frequency Division Multiplexing (OFDM) | Broadband Modem Techniques | Wideband Modulation | Broadband Modem )

FSK | PSK | DPSK | QPSK | QAM | Spectral Efficiency | Modem Concepts ( Spread Spectrum | Frequency-Hopping Spread Spectrum | Direct-Sequence Spread Spectrum | Orthogonal Frequency Division Multiplexing (OFDM) | Broadband Modem Techniques | Wideband Modulation | Broadband Modem )

PCM ( Pulse Code Modulation ) | T-Carrier Systems| Duplexing ( Spread Spectrum | Frequency-Hopping Spread Spectrum | Direct-Sequence Spread Spectrum | Orthogonal Frequency Division Multiplexing (OFDM) | Broadband Modem Techniques | Wideband Modulation | Broadband Modem )


Receiver and Transceiver | AM | FM | SW Radio | SDR | Wi-Fi

IF Amplifiers | RF Input Amplifiers | Squelch Circuits | Controlling Gain ( Spread Spectrum | Frequency-Hopping Spread Spectrum | Direct-Sequence Spread Spectrum | Orthogonal Frequency Division Multiplexing (OFDM) | Broadband Modem Techniques | Wideband Modulation | Broadband Modem )

Click Here To Learn More ( Spread Spectrum | Frequency-Hopping Spread Spectrum | Direct-Sequence Spread Spectrum | Orthogonal Frequency Division Multiplexing (OFDM) | Broadband Modem Techniques | Wideband Modulation | Broadband Modem )

Click Here to Learn ( Spread Spectrum | Frequency-Hopping Spread Spectrum | Direct-Sequence Spread Spectrum | Orthogonal Frequency Division Multiplexing (OFDM) | Broadband Modem Techniques | Wideband Modulation | Broadband Modem )

Reference : Electronic communication by Louis Frenzel

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