Broadband is a somewhat vague term that means many things to many people. Traditionally, the microwave industry has associated broadband systems with military applications, such as radar and electronic-warfare (EW) platforms, and typically covering a bandwidth of 2 to 18 GHz. But the rapid adoption of commercial mobile communications, and users' desires to be connected with voice, data, and video anywhere at any time, has given new meaning to the term "broadband" at RF and microwave frequencies, and is creating opportunities for suppliers of components and test equipment.
Commercial broadband communications systems have never counted on a single technology, with current broadband services delivered by a mixture of wired (metal and glass fiber) terrestrial and wireless terrestrial and satellite systems. The RF/microwave signals in use in these systems vary widely in frequency, from high-frequency (HF) signals still in use in two-way radios to even millimeter-wave links for backhaul connections between cell sites and switching stations. Government bodies usually establish the range of available frequencies for a given area. In the United States, for example, the Federal Communications Commission (FCC) handles spectrum licensing.
If the business potential for providing broadband electronic equipment is imagined in terms of the need to create a worldwide broadband network, then it becomes apparent that a worldwide broadband communications market has billions of users, requiring a diversity of both infrastructure and mobile electronic equipment. For example, schemes for cellular networks consist of cell sites and base stations, backhaul links, smaller cells, such as pico cells to fill coverage "holes," and the wide range of end-user mobile handsets. Considering that cellular technology has already evolved through three generations (3G), with a fourth-generation (4G) network under construction, it is probably no surprise at the number of cellular solutions for broadband service worldwide. And this is just one portion of a broadband network, with a variety of shorter-range technologies being employed, for example, for linking wireless devices throughout a home, or for enabling electronic devices carried by one person to communicate as part of a personal area network (PAN).
Although the term broadband is also associated with a generally wide bandwidth and high-speed data rate capacity, these values are also somewhat arbitrary. A broadband cellular standard, such as the Third Generation Partnership Project (3GPP) Long Term Evolution (LTE), uses channel bandwidths from 1.4 to 20.0 MHz, depending upon user requirements and operating conditions, while older broadband cellular technologies, such as wideband-codedivision- multiple-access (WCDMA), use a channel bandwidth of 5 MHz. Another wireless broadband technology, WiMAX, occupies channel bandwidths of 5 to 10 MHz. Compared to older wireless broadband technologies like WCDMA and HSPA, WiMAX and LTE are designed to take advantage of multiple antennas and multiple-input, multiple-output (MIMO) schemes to achieve higher data rates.
Bandwidth in wireless broadband networks is consumed by the modulation scheme. More information can be carried by a higher-order modulation scheme, at the cost of using more bandwidth than with a simpler scheme. A carrier frequency can be modulated in terms of its amplitude, phase, or frequency, with the simplest analog formats being the familiar amplitude modulation (AM) and frequency modulation (FM). Wireless broadband networks have come to rely on digital modulation schemes, in which typically a carrier's amplitude and phase are varied to represent data bits (0 and 1).
For example, in binary phase-shift keying (BPSK) modulation, the amplitude is constant and two phase states of the carrier are used to represent 0 and 1 bits. Quadrature phase-shiftkeying (QPSK) modulation increases the number of phase states to four to transmit and receive four digital symbols. In four-state quadrature amplitude modulation (4QAM), two amplitude states and two phase states are used to represent four data symbols (00, 01, 10, and 11) with a single carrier frequency. By increasing the number of amplitude and phase states, an even greater amount of information in digital form can be transmitted and received using a single carrier. For example, 64QAM uses eight amplitude states and eight phase states to represent 64 different digital symbols with a single carrier.
Many wireless broadband approaches employ orthogonal-frequency-divisionmultiplex (OFDM) to improve spectral efficiency. It is a modulation method by which the data to be transmitted is distributed over a large number of carriers spaced apart at known frequencies. In this way, modulation and demodulation circuits are tuned to work with those specific frequencies, avoiding problems with interference or spurious signals.
Unfortunately, providing wireless broadband coverage is not simply a matter of using a wireless broadband technology with a higher-order modulation scheme. Higher-order modulation schemes are more sensitive to channel noise and require excellent signal quality to differentiate received bits. In a noisy channel, the bit error rate (BER) will increase, often resulting in lost data. Simple modulation schemes are effective in noisy environments while higherorder schemes work in noise-free environments with good signal quality.
To optimize performance, broadband networks can take advantage of a number of communications techniques developed originally to provide security for military tactical radios, including frequency-hopping-spread-spectrum (FHSS) and direct-sequence-spreadspectrum (DSSS) techniques. In both approaches, the data are spread across a larger bandwidth than the data itself, to improve the chance of receiving transmitted data in the presence of noise. In FHSS, the center frequency jumps through a sequence of frequencies that are known to the transmitter and the receiver, at either a fast or a slow switching rate. In DSSS, transmitted data bits are multiplied by a sequence of bits randomly distributed in time and with a much higher data rate than the information to be transmitted. The multiplied signal occupies a much larger bandwidth than the original data and appears like noise in the communications channel. It is despread at the receiver using the same random code.
In long-range portions of a wireless broadband network, communications channels must maintain excellent frequency stability to avoid interference with devices operating within adjacent frequency allocations. This level of performance implies stable component performance, such as oscillators with low phase noise, amplifiers with good linearity to minimize the generation of spurious signals, and filters capable of rejecting unwanted signals outside of an allocated bandwidth while passing desired signals with minimal insertion loss within the allocated bandwidth.
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The requirements for the short-range portions of a wireless broadband network are somewhat different than the long-range segments. Wireless service providers have long been concerned with the quality of service in what has often been called "the last mile" of the wireless network, since it involves linking a longrange network with many shorter-range wireless devices working with different technologies and frequency allocations. Standards such as IEEE 802.11wireless local area networks (WLANs) have successfully connected computers and computer peripherals within an office or home to longer-range wireless networks. But as longer-range wireless networks have evolved to meet user demands for broadband services, shorter-range wireless technologies are following.
The evolution of IEEE 802.11, which operates within the unlicensed Industrial- Scientific-Medical (ISM) band in the United States, is typical of how shortrange wireless technologies are also taking on broader-bandwidth capabilities. Originally developed for use at 2.4 GHz over a 20-MHz bandwidth, and using FHSS and DSSS, versions of IEEE 802.11 now operate at 2.4 and 5.0 GHz, most still with a 20-MHz bandwidth and with several versions (802.11a, g, and n) using OFDM for high data rates over longer distances.
One of the short-range wireless technologies that is perhaps the most demanding on component and device performance is ultrawideband (UWB) technology. Almost a decade ago, the FCC set aside 3.1 to 10.6 GHz for unlicensed UWB use. In spite of the bandwidth required of its components, the technology is attractive for very shortrange (about 10 m) wireless applications because of its low power consumption. UWB signals, which appear as noise to more narrowband systems in that range, are limited by the FCC to very low transmit power levels: the FCC power spectral density emission limit for UWB emitters from 3.1 to 10.6 GHz is -41.3 dBm/MHz, which is actually the same level for unintentional emitters in that band established by the FCC Part 15 limit. Even at those low transmit power levels, UWB technology is enticing for use in PANs and other short-range wireless broadband applications because of its data rate capabilities to about 250 Mb/s.
Of course, for companies looking to supply components, devices, substrate materials, and other "building blocks" for emerging wireless broadband networks, it will be necessary to develop effective computer-aided-engineering (CAE) software models. And to conduct effective measurements during research and in production, it will be necessary to depart from traditional microwave test equipment, such as CW signal generators and spectrum analyzers, in favor of carrier test sets, signal simulators, and real-time spectrum analyzers capable of matching or exceeding the wide channel bandwidths of emerging wireless broadband networks.