Wideband Modulation and Broadband Modem Techniques PDF

Wideband Modulation and Broadband Modem Techniques Emmanuel Trinidad MSc ECE Overview ❑ Wideband Modulation ❑ Spread Spectrum ❑ Frequency Hopping ❑ Direct-Sequence Spread Spectrum ❑ Orthogonal Frequency Division Multiplexing ❑ Wireless Standards ❑ Broadband Modem Techniques...

Wideband Modulation and Broadband Modem Techniques Emmanuel Trinidad MSc ECE Overview ❑ Wideband Modulation ❑ Spread Spectrum ❑ Frequency Hopping ❑ Direct-Sequence Spread Spectrum ❑ Orthogonal Frequency Division Multiplexing ❑ Wireless Standards ❑ Broadband Modem Techniques ❑ xDSL Modems ❑ Cable Modems Digital Communication System Wideband Modulation Most modulation methods are designed to be spectrally efficient, where the goal is to: ❑ Transmit as many bits per hertz as possible ❑ Minimize the use of spectrum space ❑ Transmit the highest speed possible in the given bandwidth Wideband modulation are designed to occupy more bandwidth than the information bandwidth. Two most widely used wideband modulation methods are ❑Spread Spectrum ❑Orthogonal Frequency Division Multiplexing (OFDM) Spread Spectrum ❑ Spread spectrum (SS) is a modulation and multiplexing technique that distributes a signal and its sidebands over a very wide bandwidth ❑The continued growth of all types of radio communication, the resulting crowding, and the finite bounds of usable spectrum space have made everyone in the world of data communication sensitive to how much bandwidth a given signal occupies. ❑ The initial application of spread-spectrum (SS) techniques was in the development of military guidance and communication systems. Spread Spectrum ❑After World War II, 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 use of spread spectrum in civilian applications. Currently, 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. ❑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). Spread Spectrum There are two basic types of spread spectrum: ❑ Frequency-Hopping (FH/SS) – 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. ❑ Direct-Sequence (DS/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. ❑In addition, hybrid formats of the two schemes are also used. Other forms such as time-hopping and chirp modulation. Frequency-Hopping Spread Spectrum Frequency-Hopping Spread Spectrum ❑ The serial binary data is applied to a conventional two-tone FSK or higher MFSK ❑ The output of the FSK is applied to a mixer ❑ The mixer is driven by the frequency synthesizer ❑ The mixer output is the sum or difference between the FSK wave and the frequency synthesizer ❑ The synthesizer is driven by a pseudorandom code generator – either a special digital circuit or the output of a microprocessor. Pseudorandom Noise (PSN) ❑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 pseudorandom noise (PSN). The binary sequence is actually predictable, since it does repeat after many bit changes (hence “pseudo’’). Pseudorandom Noise Code Rate ❑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. ❑ The time that the synthesizer remains on a single frequency is called the dwell time. ❑ FCC regulations specify that there be a minimum of 75 hopping frequencies and that the dwell time not exceed 400 𝜇s PSN Code Rate Random Frequency-Hop Sequence ❑ The SS signal does not remain on any one frequency for a long time ❑ The SS signal jumps around randomly ❑ Does not interfere with a traditional signal on any of the hopping frequencies ❑ It appears to be more like background noise Direct-Sequence Spread Spectrum Direct-Sequence Spread Spectrum ❑ The serial binary data is applied to an XOR gate along with a serial pseudorandom code ❑ One bit time for the pseudorandom code is called a chip, and the rate of the code is called the chipping rate. Direct-Sequence Spread Spectrum Comparison between narrowband and spread spectrum signals Processing Gain ❑ 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 𝐵𝑊 𝐺= 𝑓𝑏 where 𝐵𝑊 is the channel bandwidth and 𝑓𝑏 is the data rate. For example 1.25 MHz 𝐺= = 96.15 13 kbps Code-Division Multiple Access (CDMA) ❑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. MATLAB generation of PRBS or PN Benefits of Spread Spectrum ❑ Security ❑ Resistance to jamming and interference ❑ Band sharing ❑ Resistance to fading and multipath propagation ❑ Precise timing Orthogonal Frequency Division Multiplexing (OFDM) ❑ Modulation – a mapping of the information on changes in the carrier phase, frequency or amplitude or combination. ❑ Multiplexing – method of sharing a bandwidth with other independent data channels. ❑ OFDM is a combination of modulation and multiplexing. Fading Channel Fading Channel Orthogonal Frequency Division Multiplexing (OFDM) ❑ Also known as multicarrier modulation (MCM) ❑ First proposed in the 1950s ❑ OFDM was not widely implemented until the late 1990s because of its complexity and cost ❑ Today, fast DSP chips make OFDM practical ❑ Transmits data by simultaneously modulating multiple carriers spaced throughout the channel bandwidth. ❑ 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 Orthogonal Frequency Division Multiplexing (OFDM) ❑ 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. Subcarrier spectrum in OFDM ❑Nulls occur at those points equal to the symbol rate. ❑ Typically, BPSK, QPSK, or some form of QAM is used as the modulation method. Processing Scheme for OFDM in DSP Fast Fourier Transform ❑ A significant breakthrough was the realization that the conversion from source signal to modulated signal could be performed by the Discrete Fourier Transform (DFT), and furthermore that the demodulation could be performed by its counterpart, the Inverse Discrete Fourier Transform (IDFT). ❑ Additionally, a much faster way to implement the DFT was suggested, using the Fast Fourier Transform (FFT), which had recently been discovered (Cooley and Tukey, 1965). These two ideas form the basis of OFDM as it is employed today. Orthogonal Frequency Division Multiplexing (OFDM) ❑ OFDM is widely used in wireless local-area networks (LANs) IEEE 802.11a ❑ Long Term Evolution (LTE) ❑In wired communication, a version of OFDM is known as discrete multitone (DMT) used in ADSL modems. ❑When digital data to be transmitted is accompanied by forward error correction (FEC) scheme, the method is called coded OFDM or COFDM. Drawbacks of OFDM ❑ There are two main drawbacks to OFDM: (1) Frequency offset and Doppler shift between the transmitter and receiver cause intercarrier interference (ICI) in the DFT frame, and (2) there is a relatively large peak-to-average power ratio (PAPR) associated with OFDM. 1st Generation Cellular Systems 2nd Generation Cellular Systems 3rd Generation Cellular Systems xDSL Modems ❑ 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 (ITU) 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 rates up to 640 kbps using the existing telephone lines. (Asymmetric means unequal upstream and downstream rates.) ❑The modulation scheme used with ADSL modems is called discrete multitone (DMT), another name for OFDM. xDSL Modems xDSL Modems ❑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 ❑ VVDSL, G.fast  VVDSL Migration Before After xDSL Curbside Nodes 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 onto the cable that also carries the TV signals. ❑ Cable TV systems use a hybrid fiber-coaxial (HFC) network Cable Modems ❑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. ❑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). Activity ❑ Research the specifications (protocols) of five electronic devices that you are utilizing for communications (mobile phone, modems, computer, tablets, cable TV). ❑ Tabulate and describe the techniques related to digital communication used by your devices.