Author: Analog Devices

Company: Analog Devices

Background Information about Wireless Communications --Detailed technical information about Wireless Communications Technology. Why Digital ? The move to digital is occurring for different reasons around the world. In the US, the initial reason was the extreme over-crowding of the existing analog cellular system (AMPS); areas in New York, Los Angeles, and Chicago were facing chronic congestion and there was little hope of more spectrum being awarded. Although a 'raw' unprocessed digital system typically requires more bandwidth than an analog one, there is much greater potential for using it efficiently, with coding, to increase the effective capacity. Within Europe, the drive was different; the need for a standard that could be widely used across the whole continent (now politically unified as the EU). Given the small size and dense populations of most European countries, any proposal would have to be (at least) pan-European in scope, and allow "roaming" between different operators and areas. Both these reasons could be sufficient, but digital networks also offer distinct advantages over their analog predecessors. For example, they are much more secure (the GSM system includes encryption as standard), and can be designed to allow for sophisticated services, such as direct data, voice mail, or store-and-forward fax. In addition, for countries which have never installed a conventional 'wired' phone network (or have only a rudimentary one) they offer the prospect of jumping straight to the most advanced technology with huge savings in the cost of infrastructure. China is currently installing a system based on fiber-optic backbone and the analog cellular standard, while in Bangkok there are now four times as many cellular phones as wired (conventional) ones. Similarly, in countries with severe climates, wireless systems can be more reliable than miles of exposed cabling - without the risk that 'entrepeneurial' types will steal the copper wiring for its scrap value! Summary of Technologies: FDMA, CDMA, TDMA Whenever a number of users need to share a piece of frequency spectrum, they must establish rules for allowing that sharing-'multiple access'. Frequency Division Multiple Access (FDMA) is the most widespread system, and the most comprehensible. Each user is allocated a unique frequency band, that they can use exclusively. Broadcast radios and TVs work like this-a given station 'owns' a given spot on the frequency dial. By using low power radios, analog cordless phones and walkietalkies use this system too; by having a number of possible channels and selecting a vacant one, so do AMPS phones. TDMA (Time Division Multiple Access), users share a frequency band; each user's speech is stored, compressed and transmitted as a quick packet, using controlled time slots to distinguish them-hence the phrase 'time division'. At the receiver, the packet is de-compressed. In GSM, eight users share a given channel; in the U.S. IS-54 protocol, it is three (on a narrower channel). Strictly this is TDMA/FDMA, (time division multiple access/ frequency division multiple access), because the packets are put on a given frequency channel. CDMA is a very different affair. Traditional systems transmit a single strong signal (perhaps intermittently), on a narrow band. CDMA (Code Division Multiple Access) works in reverse, sending a weak but very broadband signal. A unique code "spreads" the signal across a wide area of the spectrum (hence the alternative name-spread spectrum), and the receiver uses the same code to recover the signal from the noise. A very robust and secure channel can be established, even for an extremely low-power signal-theoretically, the signal can be weaker than the noise floor. More impressively, by using different codes, a number of different channels can simultaneously share the same spectrum, without interfering with each other. The drawback is the complexity and sophistication of the circuitry required; although used in some systems (e.g., cordless phones) it may be too complex for this generation of cellular systems. CDMA is still unproven for large-scale use in complex systems. In particular, there are serious questions about degradation with many users, power control (the 'near-far problem') and the hand-off at basestations. An analogy may help. Imagine trying to get a number of conversations at once: FDMA allows each meeting its own private room-obviously this is inefficient. TDMA has a strict chair, who allows each person to speak in turn. CDMA is a cocktail party-many conversations in progress at once, but you can focus on one of them (the image improves if you say each conversation is in a different-mutually incomprehensible-language There is a trade-off between efficiency and complexity. While FDMA has been in operation for many years and is cheap, it is wasteful of scarce spectrum. TDMA is more efficient, somewhat more complex and is now widely used in GSM. CDMA is potentially more efficient and more robust, but is even more complex. Summary of Technologies: RF, Mixed Signal, DSP There are several technologies required within a wireless system, and this section gives a brief over-view. Mixed signal Being a digital system in an analog world, the telephone needs to convert between analog and digital at several points. The first is the voice-band CODEC; converting speech to and from digital form. The second, more demanding, task, is the Baseband CODEC (sometimes called an RF Modem or RF Converter-misleadingly because it does not operate at RF) which connects the complex digital transmission stream to the radio stage. This has to generate fast signals on the transmit side and accurately capture the waveforms from the receive section of the radio. DSP A digital signal processor performs these tasks, as well as error-correction, equalization, echo cancelling, etc. Physical Layer Logic The mixed-signal stage provides digital data; the DSP massages that data, but the PLP or protocol-specific logic actually acts on that information, forming it into packets and operating on it in ways unique to that particular wireless standard. Controller This controls the handset itself-including the display, remembering quickdial numbers and other functions. Radio Stage The section that converts the fast signals to the carrier frequency (900 MHz, 1.9 GHz, etc.), and receives and down-converts incoming signals. Power Management Less glamourous, but essential. Battery life is crucial in a handset, and clever circuity is required to reduce power consumption. Devices are designed to be low power, and stages 'sleep' a lot-they are only switched on for the instant they are needed. GSM - Global System for Mobiles The most mature digital wireless standard is GSM, usually refered to as the 'European' digital standard, as it had origins there and is defined by the European Telecommunications Standards Institute (ETSI). However, it now has the support of 80 operators in over 40 countries (including Australia, Argentina, South Africa, Russia, China, Kuwait, Hungary, and Thailand) and deserves its 'Global' description. In addition, GSM forms the basis of Motorola's Iridium worldwide satellite system. The number of subscribers currently stands at about 2 million. Motorola, Inc.'s projections put the number of subscribers in Europe at around 20 million by the year 2000; many sources now feel this is conservative. BIS has forecast the number of new subscribers per year as rising rapidly, to reach a total in excess of 50 million by 2000. The GSM handset market will probably level out to be worth about $500 million world wide per annum from 1995 on, as the growth in unit sales is matched by declining ASPs. Many U.S. operators (including Motorola, Pacific Telesis, Ameritech International, Bell Atlantic, Shearson Lehman, U.S. West International, Bell-South, McCaw Cellular, MCI Communications and Northern Telecom) have close links with GSM; owning portions of licencees around the world, or having joint agreements. Although a number of these are evaluating GSM within the U.S., only one company operates a commercial service: Nextel Communications, Inc. is providing a full GSM-based service (including text messaging, data link, and voice mail) in Southern California, with expansion to other cities planned for next year. With such a large base of consumers over which to spread costs, prices for GSM handsets should fall drastically. Similarly, infrastructure will benefit from economies of scale and in most areas service providers face competition, so operating charges will be forced down. GSM is a TDMA standard, with 8 users per channel. The speech is taken in 20 ms windows, which are sampled (13-bit resolution at 8 ksps), processed and compressed. A channel is 200 kHz wide, and contains data from eight users. Each user has a time slot of 0.577 ms, during which a burst of 156 bits is transmitted at a modulation frequency of approximately 270 kHz. GSM is transmitted on a 900 MHz carrier. There is an alternative system operating at 1.8 GHz-DCS-1800, providing additional capacity, and often viewed is more of a PCS than a cellular system; smaller cells concentrated in urban areas, and smaller, lower-powered handsets. In a similar way, the U.S. has proposed PCS-1900; another GSM system operating on the different carrier of 1.9 GHz Two features add to the attractiveness of GSM: the SIM card and the 'roaming' agreements. The Subscriber Identification Module (SIM) is a credit card size card which is owned by a subscriber, who slides it into any GSM handset to transform it into 'their' phone. It will ring when their unique phone number is dialed; calls made will be billed to their account; all options and services connected; voicemail can be collected and so on. People with different SIMs can share one 'physical' handset, turning it into several 'virtual' handsets, one per SIM. One commentator describes this as "In effect, the GSM phone becomes as small and as light as your credit card. The SIM is all you need to carry." The obvious complement to this, is the 'roaming' agreement, by which network operators agree to recognize (and accept) subscribers from another network, as phones (or SIMs) move. Vodafone, the British operator recently announced that they had added roaming agreements with Hong Kong and South Africa to their list. So, a British subscriber could drive to France and Germany, fly to South Africa or Hong Kong-and use their GSM phone to make and receive calls (on their same UK number), with as much ease as American an businessman can use a phone in Boston, Miami, or Seattle. GSM has been designed to support both voice and non-voice (data, fax and imaging services). Future applications such as fax, store-and-forward, audiotex, and IVR (interactive voice response) are likely to follow soon. One other development in the future is Half-rate GSM, which seeks to double capacity, by more efficient speech compression. Although the algorithm has not been finalized, most sources predict it will be roughly four times more complex than the current compression standard (GSM Full Rate). As such, most chip-sets being developed now have been designed in anticipation of the processing load required. American Digital Cellular Standards There are two major standards being considered for use within the U.S., TDMA (IS-54) and CDMA (IS-95). Different service operators are supporting different standards, so it appears unlikely there will be seamless nationwide coverage or roaming in the near future. For this reason, all digital cellular phones support AMPS as a back-up standard-your TDMA phone can drop-back to the nationwide coverage if it enters an area of CDMA-only service. This obviously makes (expensive) digital phones less attractive-why pay extra for for features you cannot use for much of the time, as you will be restricted to the analog operation? IS-54 IS-54 is a TDMA (time division multiple access) standard, very similar to GSM in its principles, and using equally tested technology. There are slight differences (e.g., IS-54 uses a lower bit rate, narrower channel, and less interleaving), but these are not significant. It thus has the advantage of using a tested technology and can benefit from the development work already done. However, it is not clear that U.S. volume alone is sufficient to drive prices down to an acceptable level. IS-95 The IS-95 system was primarily developed by Qualcomm, which owns some key patents and charges manufacturers licence fees if they wish to use the standard. It is a CDMA standard, in which a code is used to spread a signal across a wide-band. This requires little power and is resistant to noise-allowing many conversations to share a spectrum. As a result, CDMA offers the attractive prospect of allowing a large number of users in a given channel, with very small (low power) handsets. However, there are questions over current feasibility-even if trials go well, volume production is unlikely before 1997. There are questions about how suitable it is for mass-production with current technology, and how performance will degrade in the presence of a lot of users (to what extent will they interfere?). Finally, the design of the base-station is a lot more complex. In particular, 'soft hand-off' is tricky-it is hard to know when and how to switch a caller to a new basestation. South Korea was an early adopter of CDMA, but appears to be having difficulties, and is now using a GSM system as an alternative network. Other Standards PCS "Toll quality voice, anywhere, any time." Personal Communication System (PCS) is not so much a technology or a particular standard; it is more the market for small, individual telephones. The descendents of domestic cordless, or the miniaturization of cellular systems, PCS is forecast to be an incredibly fast-growing market. Within the U.S., the 1.9 GHz band has been allocated for PCS systems; the allocated spectrum is 120 MHz wide and is licensed as two 30 MHz segments for the 51 major trading areas, and three 10 MHz segments for the 493 basic trading areas. Many people are proposing a variant of the GSM system ("PCS-1900" or "DCS-1900"), using the existing protocol at the new carrier frequency. But there are six other candidates: TDMA and CDMA (based on IS-54 and IS-95), broadband CDMA (supported by OKI and InterDigital), DCTU (Digital Cordless Telephone U.S.-a version of DECT), Omnipoint and PACS (Personal Access Communication System). DECT In Europe, the DECT standard has been developed, with a history very similar to the PCS charter, and the first systems are now reaching the market. There is discussion of using DECT as an in-office wireless phone, with sufficient data rate (a few hundred Kbps) to also be used as an adequate wireless LAN (e.g., to receive e-mail or to print from a laptop without the need for cabling). PDC Personal Digital Cellular (PDC) is the Japanese standard, previously known as JDC (Japanese Digital Cellular). A TDMA standard similar to the U.S. IS-54 protocol, it is not in use anywhere else in the world. Computers and WLANs The computer industry is responding to an increasing consumer demand for mobility and wireless connectivity. Wireless Local Area Networks (WLANs) operating in the 2.4 GHz and 5.8 GHz unlicenced ISM (Instrumentation, Scientific, and Medical) bands and using spread spectrum technology are presently under development. It is expected that data rates of 1 Mbps and 10 Mbps can be achieved at 2.4 GHz and 5.8 GHz, respectively. A WLAN standard operating at 2.4 GHz (IEEE 802.11) is being 'painfully' defined (with four different versions), while European countries are developing an alternative standard (HIPERLAN) for 10 Mbps transmission, using the 5.8 GHz band. An attractive alternative to a 'real' WLAN is to use the existing (voice) networks for data. In the U.S., the CDPD (Cellular Digital Packet Data) system uses the analog (AMPS) cellular system, sending data packets through empty channels, while GSM, DECT, and PHP can all support low data-rate communications. Rates range from 19 kbps for CDPD to a hundred kbps for DECT. This is well suited to less demanding applications-Personal Digital Assistants (PDAs) are increasingly using this approach for e-mail and fax connectivity. Satellite Communications A number of globe-spanning satellite telephone systems have been proposed and constructed. These provide communications coverage over very wide areas, including over the ocean as well as in remote land areas. Satellite systems fall into three broad classes: geosynchronous (GEO), "big" low earth orbit (LEO), and "little" LEO. Geosynchronous systems include Inmarsat and OmniTRACS. The former is mainly geared toward analog voice transmission (it was used by reporters to transmit from Baghdad during the Gulf War), and the first-generation Inmarsat-A system was designed for large (1-meter parabolic dish antenna) expensive terminals. Newer generations of Inmarsats are incorporating digital techniques for use with smaller, less expensive terminals. The Inmarsat system uses allocations in the 6 GHz band from the ground station to the satellite, 1.5 GHz for the satellite-to-terminal downlink, 1.6 GHz for the terminal-to-satellite uplink, and 1 GHz from the satellite to the ground station. Inmarsat is about to launch a world wide paging service (which uses ADI's ADSP-2166 processor-using clever coding and low data rates to remove the need for an antenna) and is considering a world-wide digital voice phone system-Inmarsat-P or Project 21. Little LEOs are relativelv small and inexpensive satellites that provide low-cost, low-data rate, two-way digital communications, and location positioning to small handheld terminals. The frequency allocations are in the VHF band below 400 MHz. The advantage of little LEOs are their small size and relatively low costs. Systems include Leosat, Orbcomm, Starnet, and Vitasat. For example, the Orbcomm system requires 34 satellites for reliable full-world coverage. The Motorola Iridium system is similar, and will use the GSM protocol as the basis for its communication system. Glossary AMPS Advanced Mobile Phone System - U.S. analog service CDMA Code Division Multiple Access CDPD Cellular Digital Packet Data - sending digital data over the existing AMPS system, by transmitting dense packets on vacant analog channels DCS-1800 Low power variant of GSM, with 1.8 GHz carrier, used in Europe (e.g., Mercury One-2-One) DCS-1900 Proposed use of GSM with 1.9 GHz carrier for PCS applications. DCTU Digital Cordless Telephone U.S. - a version of DECT proposed for the U.S. PCS market ETACS Enhanced TACS ETSI European Telecommunications Standards Institute (standards body responsible for GSM) FDMA Frequency Division Multiple Access - each conversation gets its own, unique, radio channel. This is the system used by AMPS. GSM Global System for Mobiles. Most established digital Cellular standard to date. Half-rate A variant on GSM; doubles capacity by more efficient coding HIPERLAN An ETSI proposed wireless WLAN, operating at 5.8 GHz and with data rates of up to 10 Mbps IEEE802.11 A WLAN standard (or set of standard), operating at 2.4 GHz and with data rates of up to 1 Mbps IS-41 The protocol for 'roaming' within the USA, describing how services should 'hand over' between operators IS-54 The TDMA standard for U.S. digital cellular IS-95 The CDMA (Qualcomm) standard for U.S. digital cellular JDC Japanese Digital Cellular - now renamed PDC LMR Land Mobile Radio - wireless for specialized applications - e.g., taxi or emergency services MMI Man/Machine Interface - how easy a phone is to use, how fun, how sexy. As phones conform to strict standards, the MMI becomes a key area of differentiation. NAMPS Narrowband AMPS PACS Personal Access Communication System - a candidate for the U.S. PCS PCS Personal Communications System - "Toll quality voice, anywhere, any time" PCS-1900 See DCS-1900 PDC Personal Digital Cellular - Japanese cellular standard PHS Personal Handyphone system - Japanese cordless standard SIM Subscriber Identification Module - a card used in GSM to 'personalise' a handset TACS Total Access Communication System - European analog cellular TETRA Trans European Trunked Radio Access - European digital cellular land mobile radio system TDMA Time Division Multiple Access - several users take it in turn to share one radio channel (e.g., GSM, IS54) TIA Telecommunications Industry Association - U.S. Standards making body UMPS Universal Mobile Phone Service