The antenna is gettin' respect. More wireless designers now realize that the antenna holds the key to higher data rates, longer range, increased number of users per system, and greater reliability.
Most antennas continue to be a passive collection of conductors optimized for the application. On the rise, though, are intelligent antennas using spatial diversity, multiple-input/multiple-output (MIMO) techniques, and adaptive arrays for cell-phone basestations, Wi-Fi access points, and the new broadband wireless system called WiMAX.
Many designs can still incorporate the antenna on the pc board as just another pattern—but optimized for the radio. Typical of these designs are the loop, inverted-F, patch, meander line, and slot (see "Printed-Circuit-Board Antennas" at www.elecdesign.com, ED Online 9986).
For example, Fujitsu has developed an antenna for ultra-wideband (UWB) applications (Fig. 1). UWB uses the 3.1- to 10.6-GHz frequency range. Any one UWB signal will have at least a 500-MHz bandwidth, and often it's more than 1 GHz. Such wideband signals are a challenge for designers because antennas are resonant and have a relatively narrow bandwidth (
To solve this problem, UWB designers have been conducting experiments with a wide variety of pc-board designs. For instance, Fujitsu's recent antenna is a modified monopole with a special pattern that gives it wide bandwidth and a voltage standing-wave ratio (VSWR) of less than 2.5 to 1 over the entire UWB band. Other UWB-chip suppliers investing in antenna development include Alereon and Freescale Semiconductor.
In many applications, commercial component antennas have replaced pc-board antennas. Commercial component antennas mount on the pc board like any other part. Dielectric antennas, a popular type of component antenna, employ conductors formed on a dielectric such as ceramic with a very high dielectric constant. The conductors use some variation of the dipole, inverted-F, patch, meander line, or other configuration. Others use a helical design. The ceramic dielectric usually narrows the bandwidth down to 5% to 8%.
While such antennas work well, some are inefficient and usually introduce attenuation. But this loss often is traded off for ease of use and minimal detuning. Just placing your hand near most antennas or moving the antenna near other objects can detune it, ruining performance. Dielectric antennas are more immune to such effects, making them more popular. Suppliers include Centurion and Toko.
In a variation of the dielectric antenna, the dielectric resonator antenna, the dielectric itself is resonant at the frequency of operation and becomes the actual source of radiation. The antenna uses no ground plane, and it has a much wider bandwidth.
Antenova's High Dielectric Antenna (HDA) employs two inverted-F antennas back-to-back to provide a balanced feed point (Fig. 2). It has very wide bandwidth and good efficiency. The copper part of the antenna radiates the lower frequencies, while the dielectric radiates the upper frequencies within the bandwidth. A parasitic element in the design greatly enhances performance.
The company's Radionova modules use the antenna for CDMA, GSM, and UMTS 3G cell phones. These modules may incorporate the radio transceiver itself to produce a more compact assembly that fully integrates both radio and antenna. The approach is customizable for any cell-phone technology.
Component antennas can be found in cell phones, Wi-Fi laptops, Bluetooth-enabled devices, and GPS receivers. This is probably the fastest and easiest way for designers who aren't antenna wizards to solve antenna problems. Designers just need to follow the manufacturer's precautions about mounting, then make sure their receiver has sufficient gain/sensitivity and the transmitter has enough power gain to offset any losses encountered.
DIVERSITY AND MIMO
An ancient antenna technology—diversity—has become rather prominent in recent years. Designers use this method to overcome multipath problems in a wireless application. As the radio wave leaves the transmitting antenna, it should go directly line-of-sight (LOS) to the receiving antenna. But because most antennas aren't that directional, much of the signal travels off in other directions. Thus, it's reflected by multiple objects before appearing at the receiving antenna along with the direct signal.
Because the direct and reflected signals arrive at different times, the signals add and subtract. This produces fading and signal cancellation, as well as addition that can actually boost signal strength. The problem is further exacerbated if one or both of the communicating radios is moving. Diversity provides a way to mitigate the multipath problem by using two or more antennas for receiving.
For example, the spatial diversity system in Figure 3a uses two antennas spaced at least one wavelength apart, or preferably more. The idea here is simply that the farther apart the antennas, the likelier they will receive different versions of the signal from all direct and reflected sources.
The signals are amplified in low-noise amplifiers (LNAs) and then combined linearly to produce a stronger signal and reduced fading effects. The signals may actually be combined after the LNA as shown or in almost any other part of the receiver, such as after the IF, after demodulation, or even in a DSP after being digitized somewhere along the way.
Another type of spatial diversity system uses switched antennas, where the signal strength is monitored in the receiver, then a switch that tests each antenna is activated (Fig. 3b). The antenna with the best signal is connected to the receiver. Cell-phone basestations and most 802.11/Wi-Fi access points and routers employ the two-antenna switch-diversity arrangement. The newer radio chips include the diversity circuitry. It's possible to use three, four, and even more antennas to further boost the signal-to-noise ratio.
Though not yet widely deployed, the developing MIMO is drawing attention. Using multiple antennas on the transmit as well as the receive end of the link creates some amazing benefits. A minimum MIMO arrangement utilizes two transmitters, sending data simultaneously on the same channel and four antennas at the receiving end in a spatial diversity arrangement (Fig. 4).
Orthogonal frequency-division multiplexing (OFDM) is the most common modulation/access arrangement. The receiving antennas pick up all direct and reflected signals. The signals are combined through some algorithm, and the result is a far more robust output.
MIMO systems actually use the multipath to gather as much signal as possible to create a superior output. The outcome is longer range or lower power for a given bit error rate (BER).
A key benefit of MIMO is that it can transmit two data streams simultaneously in the same channel. This lets designers send higher-speed data in multiple lower-speed streams. It also greatly improves spectral efficiency, because more bits/hertz are transmitted in a given bandwidth. MIMO is just now appearing on Wi-Fi access points and some wireless routers/gateways with a data rate of 108 Mbits/s—twice the 802.11a/g standard of 54 Mbits/s.
Wireless access point and gateway/router manufacturers have widely adopted the AGN100RF radio and AGN100BB baseband/MAC ICs from Airgo Networks. These devices use two transmit antennas and three receive antennas to produce 108 Mbits/s in a single 20-MHz channel.
The transmit diversity adds 3 to 4 dB of gain over standard switched-diversity-only systems. Additionally, the receive diversity gives 3 to 5 dB in gain. This extends the range and area coverage by 20% to 30% with Airgo on one end of the link, and by up to 300% with Airgo on both ends of the link. MIMO is a mandatory feature of both the TgN and WWiSE proposals for the forthcoming IEEE 802.11n wireless local-area network (WLAN) standard, which is expected to deliver a minimum of 250 Mbits/s over a given channel.
Though Motia's Javelin was designed to improve Wi-Fi products, this MIMO chip offers other capabilities. It uses multiple antennas and handles the signal processing by analog methods (Fig. 5). The Javelin also works with four external antennas. The signals in the 2.3- to 2.7-GHz range are amplified in LNAs and then downconverted to baseband, where circuits develop outputs for the control circuitry.
Then, the control circuitry weighs the inputs and develops signals that adjust the signal phase before they're summed. This results in a composite signal that's optimized by adding in all phase and amplitude corrected multipath signals. The downconverted sum signal also is used as feedback by the control circuits. The output goes to the input of a standard Wi-Fi transceiver chip.
In the transmit mode, the signal from the transceiver is power-divided and fed to four signal-processing chains that use the same control signals developed by the receiver. Assuming that the link hasn't changed from the receive-to-transmit transition, the four signals are corrected and amplified before passing to the antennas.
With this arrangement, the overall gain over a single antenna system is 13 dB with a Javelin at only one end of the link. A Javelin at both ends brings 18 dB. Typically, the range is at least doubled, and in some cases it's tripled depending on the local environment. Link reliability is improved, thereby increasing the data rate. The Javelin works with any of the popular 802.11b/g transceivers.
Other wireless chips also are taking advantage of antenna properties. For instance, Provigent's PVG310, though not a MIMO chip, uses an external array with vertically and horizontally polarized elements. (Polarization refers to the orientation of the electric field of the radio wave.)
The PVG310 can transmit and receive two separate datapaths on the same frequency, while the orthogonal polarization enables the receiver to separate them. It uses special processing in its cross-polarization interface cancellation (XPIC) to double the wireless channel data rate. The chip also can achieve a rate of up to 622 Mbits/s—the OC-12 or STM-4 optical data rate—providing a wireless option instead of optical links.
The ultimate antenna is the smart antenna system. Also known as adaptive antennas, these intelligent systems use various beam-forming and beam-pointing methods essentially developed for military phased arrays used in radars. Smart antennas employ multiple antenna elements along with signal processing to pinpoint received signals and even null out interfering signals.
By combining multiple antennas, the radiation or received patterns can be shaped from the normal omnidirectional or figure-eight shapes that essentially waste signal strength. Multiple antennas spaced correctly form more highly directive antenna transmission or reception patterns. It's possible to create very narrow beams. Directional antennas also can select only the desired signal and suppress any interfering signal. Even if they're on the same frequency, the highly directional antenna acts like a spatial bandpass filter.
Highly directional antennas also have gain. They narrow the antenna pattern beam width, so they concentrate the signal and produce a result that equals amplification. With a directional antenna, a 1-W transmitter can be made to develop an equivalent radiated power (ERP) of 10 W or more, but only in a very restricted direction. The same goes for reception, as highly directional antennas amplify weak signals. Smart antennas use this concept to improve signal reception by automatically selecting desired antenna patterns or forming new ones optimized to the signals received.
There are two basic types of smart antennas: the switched-beam array and the adaptive array. The switched-beam array uses multiple antenna elements (from four to over 100) to produce multiple beams pointing in different directions. Each antenna, or group of antennas forming a specific beam, is switched in and out while monitoring the received signal strength. The beam receiving the strongest signal is selected. The directional beam provides gain, and the directionality helps eliminate interfering signals that are more likely in a different location outside the selected beam (Fig. 6a).
Adaptive arrays, or tracking beam antennas, dynamically adjust their directional nature to conform to the received signals, whether they're direct, multipath, or interfering signals. Adaptive arrays zero in on the desired signals, optimizing the beam width and gain. They also can identify interfering signals and adjust their beam to null out the interferer (Fig. 6b). These systems use multiple antennas in arrays and a variety of signal-processing algorithms to optimize the reception. Processing takes place after digitization in a standard DSP.
Adaptive antennas actually track and fine-tune the antenna pattern to provide the best reception with minimum interference. They also take advantage of multipath, much the way MIMO systems do to improve reception. The link budget is constantly maximized.
Smart antennas provide far-reaching benefits. First, the signal gain offered by the directional nature of the antennas extends the system's range. Distances can be extended from 20% to 200%, depending on the system. This means signals are received from greater distances, and nearby sources consume less power. Second, interference rejection is dramatically improved. Interfering signals on the same frequency can be minimized or actually nulled out. This allows for increased system capacity via frequency reuse.
Cell-phone systems reuse the same frequency and keep users separated by carefully spacing cell sites. Most cell sites already use sectorized antennas divided into 120° beams to improve the frequency reuse capability. But dynamic beam forming further isolates multiple users, making it possible to add even more users on the same frequency.
With the shortage of spectrum space and its very high cost, adaptive arrays become particularly attractive. Adaptive antennas actually implement an entirely new access mode called spatial division multiple access (SDMA), which makes it possible to increase the efficiency of the existing spectrum.
The high gain of the smart-antenna systems also makes the link more robust, minimizing the effect of noise and improving reliability. Multipath problems are reduced, thereby improving the link quality. Consequently, a higher data rate can be attained.
Smart antennas, developed primarily for cell-phone systems, can expand the subscriber capacity of a given cell site while elevating performance. Case in point: ArrayComm's IntelliCell uses standard antenna arrays and its specialized software algorithms on standard Texas Instruments DSP chips in basestation radios. This produces a basestation that delivers three times the standard capacity of a conventional basestation and up to twice the coverage area.
Availability and quality of service are improved. The higher gains involved in transmit and receive operations minimize power, too. Such adaptive array systems (AASs) are installed on over 250,000 cell sites worldwide, with more on the way. This kind of technology can significantly reduce the total number of new basestations in future buildouts.
The phased-array systems from Vivato have used smart antennas to expand Wi-Fi technology use at the usual access points. Smart antennas based on a 128- or 64-slot array can extend the range of an access point by two to three miles. The antenna panels cover a 90° to 100° angle and can provide pinpoint accuracy in linking up with those accessing the site (Fig. 7). In addition, antenna gains of 20 to 50 dB are possible.
Such high gain means the link is solid, making it possible for the connection to sustain the high data rates handled by the standard. With this kind of performance, voice-over-IP phones can be implemented using wireless systems, because they can provide the speed and quality-of-service required. Vivato systems also let hotspot carriers use fewer access points to cover an even wider area than current omnidirectional systems.
The forthcoming WiMAX wireless broadband systems represent another potential application for smart antennas. Based on the IEEE 802.16 standard, these systems offer low-cost backhaul and an alternative to cable and DSL systems where cables aren't available.
Smart antennas should improve the range and capacity. They also should make the system more versatile than originally intended. Steve Glapa, director of marketing at ArrayComm, says that combining AAS with WiMAX systems will make them far more useful. Such a combination also has the potential to eventually compete with the entrenched wireless services.
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