An oldie from Planet Analog.
RF & Wireless
New functionality squeezes wireless
By Sam Pritchett, RF Strategic Marketing Manager, Wireless Communications Business Unit, Texas Instruments Inc., Dallas
EE Times
February 7, 2000 (10:01 a.m. EST)
The meteoric growth of wireless communications over the last decade has shown just how critical the thoughtful and deliberate development of next-generation wireless handsets has become. As the number of wireless subscribers has exploded, so too has the number of new data-centric applications, in turn attracting more users. That has increased the pressure for the rapid development of sophisticated high-speed wireless communications standards.
As new standards roll, service providers are placed in the unenviable position of grandfathering their service offerings over an already installed infrastructure while migrating toward new standards that will best support next-generation applications. These service providers are certainly intent on migrating to next-generation applications because doing so will help them develop new revenue streams as voice-only service tariffs erode.
The current wireless communications environment is one where multiple standards have been implemented, evolutionary changes in existing standards introduced and entirely new standards developed. That situation will have far-reaching effects on how next-generation wireless handheld products will be designed.
When the cellular telecommunications market was just beginning in the late 1970s and early 1980s, the infrastructure and handsets were entirely analog. An embedded processor capable of the 40 million instructions per second a digital handset needs to sustain a voice conversation was just not available yet. The wireless communications standards in use at the time, such as Advanced Mobile Phone System, were developed to support voice conversations.
That began to change as second-generation (2G) digital wireless technology was introduced in the early 1990s in an attempt to increase wireless network capacity and to improve customer-oriented performance, such as talk time and standby time. The analog standards that were based exclusively on frequency-division multiple access for voice communications were joined by digital standards based on time-division multiple access or code-division multiple access (CDMA), each of which also employed frequency-division duplexing. The new digital standards were also capable of data communication speeds in the range of 9.6 kbits/second to 14.4 kbits/s.
The move to digital technology was a boon to the wireless market because of its effect on network capacity, handset talk/standby times and subscriber costs. With the use of digital compression and multiplexing techniques, the capacity of the wireless infrastructure was suddenly increased threefold. Once the cost of digital components for wireless communication reached a level where the economies of scale could be applied, the cost of wireless communication handsets dropped precipitously, bringing the world of wireless communications within the grasp of a much larger segment of consumers.
The floodgates are now wide open, and in recent years the demand for wireless communication services has skyrocketed. Dataquest Inc. recently reported that during 1997 and 1998, the digital cellular phone market grew by 80 percent each year. Similarly, worldwide annual sales of digital wireless telephones quadrupled between 1996 and 1999, from 48 million to 255 million units per year.
Once the door was opened to digital technologies, a Pandora's box of applications became possible. Fueled by the demand for higher data rates, the so-called 2.5 generation (2.5G) saw the emergence of multislot data transmission standards, such as High Speed Circuit Switched Data and General Packet Radio Service (GPRS).
In the case of GPRS, an enhanced modulation format known as the Enhanced Data-GSM Environment (Edge) has also been introduced; it increases the GSM data rate from 1 bit per symbol for Gaussian-filtered mean shift keying to 3 bits per symbol for eight-phase-shift keying. That enables data rates of up to 384 kbits/s in GPRS-enabled Edge networks. The advent of data-packetizing techniques such as GPRS enable the network provider to situate multiple subscribers on a single channel and time slot, thus making possible another increase in network capacity and a round of new applications.
Wireless applications like e-mail, Internet access, wireless data communications and voice-over-IP (VoIP) are suddenly not just pipe dreams but entirely realizable. Service providers are quickly seizing on the opportunities presented by 2.5G technology to offer new products. In addition, packetized data and, in particular, VoIP allow the service provider to reduce the cost of access to the installed (back-haul) wireline network, since the available bandwidth of each copper wire or fiber-optic channel is more fully utilized by residing multiple subscribers on a single channel. This will ultimately transform their revenue stream from the voice-channel-centric nature of plain old telephone service to the data-packet-centric nature of the information age.
A third generation (3G) of wireless communications will soon be introduced, adding a new-generation wireless standard created for the purpose of supporting a data-centric subscriber base, such as Universal Mobile Telecommunications System (UMTS), as well as cdma2000. The 3G systems will be overlaid upon the existing multimode and multiband 2G and 2.5G standards, such as CDMAOne/IS95 and GSM/Edge, thus increasing wireless handset complexity yet again. The 3G standards will support another leap forward in capacity as well as data rates up to 2 Mbits/s for stationary reception and transmission and 384 kbits/s when the terminal instrument is moving.
The capabilities provided by 3G standards and systems will bring about the convergence of what have been distinct functions into single personal communications devices. For example, the calendar function and address book information contained in today's personal digital assistants (PDAs) might be merged with digital audio players, e-mail, Internet and e-commerce devices. In addition to those integrated features, some of the new personal communications devices will include location service based upon GPS; speech recognition for inputting data and issuing commands; videoconferencing; and even office tools, such as spreadsheets and word processing. The wireless instruments will propel the wireless communications world well beyond the capabilities of instruments once depicted as fantasy by comic strips such as Dick Tracy.
Bluetooth and other new technologies will also bring about new applications, such as wireless synchronization between PCs and handheld personal communications instruments, impromptu wireless local-area networks, wireless telephone headsets and others. For many of those applications, UMTS and CDMAOne standards, based on wideband CDMA with its 2.2-GHz frequency and 5-MHz channel bandwidths, will be needed to support the high-rate data-centric subscriber. All of the myriad wireless signals emanating from 3G applications will converge on one wireless personal communications device.
This convergence of signals, each of which is centered at different points on the RF spectrum, will strain the way personal communications devices are designed. To accommodate legacy as well as new and emerging applications, designers will implement more than one radio architecture in a single personal communications device. The composite radio, a collection of multiple radios each with its own signaling scheme and transmit/receive frequencies, will be the result of a new-generation system definition and development.
For example, one radio might be used to receive and transmit both GSM and Edge signals, which are centered at 900 MHz, 1,800 MHz or 1,900 MHz. It might also have channels that are 200 kHz wide, with another radio dedicated to Bluetooth signals in the 2,400-MHz range with channel bandwidths of 1 MHz. Another radio might be dedicated to receiving the 1,575-MHz GPS signal range with a single channel of 2-MHz bandwidth, and still another radio to UMTS/cdma2000, with signals in the 2,200-MHz range and channel bandwidths of 5 MHz. In standalone implementations, each radio would have its own RF oscillators, filters and frequency translators specific to its unique signaling scheme and channel bandwidth.
Five radios into one
With all of the applications that are converging on personal communications devices, as many as five or six different radios could be merged into a single handheld instrument. As a result, designers of those instruments will have to come up with creative ways to synergistically weave these independent radios into a composite one with maximum commonality, co-integration compatibility, and higher and higher integration. New radio architectures will become a necessity in order to eliminate the plethora of now-external components such as channel filters and VCOs as wireless instrument board space becomes more and more at a premium. At the same time, the cost of these new-generation communications devices, in spite of their increased complexity, must not be much higher than today's 1G or 2G wireless handsets and PDAs.
The principal challenge is the possibility of interference among the multiple radios inside a single instrument. Because the multiple radios must be sensitive enough to detect signals as low as -110 dBm and even -130 to -150 dBm for GPS signals, it is highly probable that signals intended for one radio may end up interfering with the processing of signals used by others.
System designers address these potential problems by applying a methodology known as frequency design or frequency planning. Frequency design means that each radio is designed to be sensitive to those signals that it is intended to process, while ensuring that it is immune to spurious signals created within its radio's signal processing or by one of the other radios. Simultaneously, this frequency-design process must avoid creating spurious signals that will interfere with or affect the robustness of one of the other radios with which it is integrated. Although this task may appear very straightforward, it is actually extremely complex because it requires the simultaneous consideration of several deliberately created signals as well as a plethora of undesirable, but unavoidable, spurious signals.
Also emerging are new radio architectures that can mitigate the problems of space and interference as well as the cost of personal communications devices. Unfortunately, there is no one new radio architecture that is a panacea; each has its advantages and disadvantages. Implementing any one of these new architectures involves a trade-off.
The long-standing classical architecture employed is superheterodyne. Though the simplest to implement from a semiconductor and circuit-design perspective, it requires many external components-including bulky channel filters-and creates the most undesirable but unavoidable spurious signals. Two radio architectures that are gaining popularity are low IF and direct conversion. Although they require the fewest components and create fewer spurious signals, they place the most stringent demands from a semiconductor and circuit-design perspective.
Radio system architects are beginning to employ a variety of these architectures, as best suited for the particular radio, within the composite radio systems within these new communications devices. The strengths of each architecture are matched with the requirements of the signaling scheme the specific radio is intended to process. This allows the radio system architect to optimize for minimal interference caused by the interaction between the target signals, the deliberately created signals as well as the spurious ones. Simultaneously, the radio system architect is optimizing overall circuit complexity, battery-life and component count while minimizing the ever-present force: system cost. Needless to say, this task is not for the faint of heart.
In the years ahead, wireless device and system radio architects will be asked to squeeze 10 pounds of functionality into a one-pound bag. They will be asked to optimize performance, battery life, board space, time-to-market and cost to consumer at a time when the functionality is expanding by leaps and bounds.
The only way to achieve these objectives is to base the design process on solid, time-tested techniques that involve considerable up-front planning and deliberate decision-making long before prototypes are ever assembled. Once the savvy architect and design team have fully comprehended their objectives, they will have to avail themselves of every tool they can possibly muster to ensure their success in the ever-increasing time-to-market pressure of today's wireless instrument business.
[comment: Nokia and IDC are elevating to another level the essential methodology that IDCC used with TXN to produce the BCDMA SoC, which has since then been tested successfully in 5 fixed wireless trials in 5 countries. One clear difference is that Nokia and IDCC are using it in a hyper-volume production environment.]
Frequency-design techniques will be implemented and new radio architectures mixed and matched. The explosion in complexity will blur the line between RF-semiconductor suppliers and the wireless instrument OEMs they serve.
Suppliers with expertise in these areas can lend their support in a number of ways. Vertically integrated OEMs may offer simple and objective comments as an outside expert, or become involved as an active quasi-member of the end-equipment definition. OEMs more tailored for manufacturing can provide a range of complete RF system solutions.
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