Mobile Wireless Communications

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By Miro Stoichev


Everything is converging. The wired world and the wireless world are converging. The Internet and mobile wireless is converging. The distinction between the wireless, wireline and the Internet service providers is beginning to blur. And the glue certainly is “mobile wireless”.

Mobile wireless has exploded in popularity because of the fact that it simplifies and revolutionizes communication. The market for mobile wireless is increasing by leaps and bounds. The success of mobile communications lies in the ability to provide instant connectivity anytime and anywhere and the ability to provide high-speed data services to the mobile user. The quality and speeds available in the mobile environment must match the fixed networks if the convergence of the mobile wireless and fixed communication networks is to happen in the real sense. So, the challenges for the mobile networks lie in providing a very large footprint of mobile services (to make the movement from one network to another as transparent to the user as possible) and the availability of high speed reliable data services along with high quality voice. A range of successful mobile technologies exists today in various parts of the world and every technology must evolve to fulfill all these requirements. In the following sections I’ll talk about the mobile technologies existing today, how these technologies compare, how these technologies are shaping up and what we can expect to see in the near future.

The mobile wireless market is predominantly voice-oriented with low speed data services. The popularity of mobile voice services has been the deciding factor for the development of mobile networks so far. Data, mainly in the form of SMS has basically been an extra service. However, SMS is fast becoming very popular and in many European countries subscribers are spending more on SMS than voice. Both the voice and data markets continue to grow and the 2nd Generation networks are evolving to keep up and, in fact, are generating demands for newer services. Although digital technologies have improved the quality of service provided by the mobile networks, the voice quality is still not the same as the toll quality. New speech coding techniques like EFR and adaptive multi-rate are bridging this gap.

Technologies And Services Existing

Many second-generation mobile technologies exist today each having influence in specific parts of the world. GSM, TDMA (IS 136), and CDMA (IS 95) are the main technologies in the second-generation mobile market. GSM by far has been the most successful standard in terms of it’s coverage. All these systems have different features and capabilities. Although both GSM and TDMA based networks use time division multiplexing on the air interfaces, their channel sizes, structures and core networks are different. CDMA has an entirely different air interface.

In the following sections I will discuss the existing standards, the technologies, the situation today and also talk about some of the forecasts for these technologies. This should help you understand the situation today and also dispel some of the basic misconceptions about the viability of these technologies.

Global System for Mobile Communication (GSM)

GSM’s air interface is based on narrowband TDMA technology, where available frequency bands are divided into time slots, with each user having access to one time slot at regular intervals. Narrow band TDMA allows eight simultaneous communications on a single 200Khz carrier and is designed to support 16 half-rate channels. The fundamental unit of time in this TDMA scheme is called a burst period and it lasts 15/26 ms (or approx. 0.577 ms). Eight burst periods are grouped into a TDMA frame (120/26 ms, or approx. 4.615 ms), which forms the basic unit for the definition of logical channels. One physical channel is one burst period per TDMA frame. A GSM mobile can seamlessly roam nationally and internationally, which requires that registration, authentication, call routing and location updating functions exist and be standardized in GSM networks.

GSM offers a variety of data services. GSM users can send and receive data, at rates up to 9600 bps, to users on POTS (Plain Old Telephone Service), ISDN, Packet Switched Public Data Networks, and Circuit Switched Public Data Networks using a variety of access methods and protocols, such as X.25 or X.32. Other data services include Group 3 facsimile, as described in ITU-T recommendation T.30, which is supported by use of an appropriate fax adapter. A unique feature of GSM, not found in older analog systems, is the Short Message Service (SMS). SMS is a bi-directional service for short alphanumeric (up to 160 bytes) messages. Messages are transported in a store-and-forward fashion. For point-to-point SMS, a message can be sent to another subscriber to the service, and an acknowledgment of receipt is provided to the sender. SMS can also be used in a cell-broadcast mode, for sending messages such as traffic updates or news updates. Messages can also be stored in the SIM card for later retrieval.

The European version of GSM operates at the 900 MHz frequency (and now at the newer 1800 MHz frequency). Since the North American version of GSM operates at the 1900 MHz frequency, the phones are not interoperable, but the SIMs are. Dual-band 900 -1800 and 900 -1900 phones are already released and in production. Tri-band 900 -1800 -1900 GSM phone are expected to be manufactured in the next few years, which will allow interoperability between Europe and North America

A GSM network consists of mobile stations talking to the base transceiver station, on the Um interface. Many BTS are connected to a BSC via the Abis interface and the BSC connect to the MSC (The core switching network) via the A interface.

HLR and VLR provide customized subscriber services and allow seamless movement from one cell to another. The Authentication register and the equipment register provide security and authentication. An OMC and a cell broadcast center allow configuration of the network and provide the cell broadcast service in the GSM network (not shown in the diagram).The voice transmitted on the air interface can be encrypted. The speech is coded at 13kbps over the air interface. Using EFR (Enhanced Fullrate Coding) the voice quality approaches the land line quality. Recent developments like AMR (adaptive multi-rate coding) allow speech coding and channel coding to be dynamically adjusted giving acceptable performance even in case of bad radio conditions. The GSM network supports automatic handovers. Since the mobiles are not transmitting or receiving at all times battery consumption can be conserved. Further using DTX and DRX (Discontinuous transmission and reception, mobile transmits or receives only when there is a voice activity detection) batter power can be conserved even more – a highly desirable characteristic of any mobile system. Also since the mobile is not transmitting or receiving at all times, this allows the mobile to listen to control channels and to provide useful information about other channels back to the cell.

Recent developments and initiatives include:

  • GSM Association together with the Universal Wireless Communications Consortium (UWCC), which represents the interests of the TDMA community, are working towards inter-standard roaming between GSM and TDMA (ANSI-136) networks.
  • The majority of European GSM operators plan to implement general packet radio system (GPRS) technology as their network evolution path to third-generation
  • MExE will allow operators to provide customized, user-friendly interfaces to a host of services from GSM, through GPRS and eventually UMTS. The first implementations of MExE are expected to support the wireless application protocol (WAP) and Java applications. MExE can extend the capabilities that currently exist within WAP by enabling a more flexible user- interface, more powerful features and security.
  • GSM cordless telephony system to provide a small home base station to work with a standard GSM mobile phone in similar mode to a cordless phone. The base station would be connected to the PSTN.
  • Number portability will allow customers to retain their mobile numbers when they change operators or service providers
  • Location services to standardize the methods for determining a GSM subscriber’s physical location
  • Tandem free operation where the compressed speech is passed unchanged over the 64 kbps links between the transcoders, hence improving the voice quality.

Time Division Multiple Access (TDMA) IS-136 Technology

TDMA is so named because frequency bands available to the network are divided into time slots, with each user having access to one time slot at regular intervals. Three users share a 30 kHz bandwidth (IS 136) by splitting a 30 kHz carrier into 3 time slots. TDMA was first specified as a standard in EIA/TIA Interim Standard 54 (IS-54). IS-136, an evolved version of IS-54, is the United States standard for TDMA for both the cellular (850 MHz) and PCS (1.9 GHz) spectrums. TDMA IS-136 is an evolved form of TDMA IS-54. Unlike IS-54, IS-136 utilizes time division multiplexing for both voice and control channel transmissions. Digital control channel allows residential and in-building coverage, dramatically increased battery standby time, several messaging applications, over the air activation and expanded data applications (GSM also has the same characteristics). The digital control channel allows for the creation of micro cell applications making it suitable for wireless PBX and paging applications. TDMA networks transmit at a higher data rate on a relatively low bandwidth channel resulting in chances of co-channel interference. As described above for GSM, the time slot structure allows the mobiles to conserve battery power and to collect information about other channels. IS-136 specifies a “sleep mode” that instructs the compatible cellular phones to conserve power. IS-136 handsets are not compatible with IS-54.

TDMA makes more efficient use of available bandwidth than the previous generation analog technology. TDMA IS-136 exists in North America at both the 800MHz and 1900MHz bands. IS-136 TDMA normally co-exists with analog channels on the same network. One advantage of this dual-mode technology is that users can benefit from the broad coverage of established analog networks while IS-136 TDMA coverage grows within, and at the same time take advantage of the more advanced technology of IS-136 TDMA where it exists. TDMA networks have increased the capacity of the analog networks (using the same bandwidth) by 3 times.

The universal wireless communication consortium (UWCC) is a group of more than 100 telecom carriers and vendors of mobile products and services, which focuses on efforts to develop services based on IS-136 TDMA and IS-41 WIN. The Global TDMA Forum (GTF) of UWCC focuses on both technical and market led developments. IS-136 revision A has introduced several new features like Adaptive channel allocation (ACA) depending on the instantaneous channel quality determined by the level of interference, the Private System Identification (PSID) which allows development of large scale corporate private systems either as multi location or in-building closed user groups, two way short text messaging (SMS, 256 chars) etc. IS-136 revision B, as standard includes all IS-136+ proposals from the UWC-136 RTT proposal for voice and circuit switched features. Notable features are packet data service, mobile assisted handoff, improved SMS and intelligent roaming.

Major US carriers using TDMA are AT&T; Wireless Services, Bell South and Southwestern Bell.

Code Division Multiple Access (CDMA) Technology (IS-95) (cdmaOne)

The CDMA technology used in North America is based on the IS-95 protocol standard first developed by QUALCOMM. CDMA differs from the other two technologies by its use of spread spectrum techniques for transmitting voice or data over the air. Rather than dividing RF spectrum into separate user channels by frequency slices or time slots, spread spectrum technology separates users by assigning them digital codes within the same broad spectrum. Advantages of CDMA technology include high user capacity and immunity from interference by other signals. Like TDMA IS-136, CDMA operates in the 1900-MHz band as well as the 800 band.

Work on developing the CDMA standard is conducted mainly by the CDMA Development Group (CDG), a consortium of the main CDMA manufacturers and operators formed to standardize and promote CDMA technology. Whilst work to develop CDMA as a third-generation technology has attracted a great deal of attention over recent months, the CDG has also been working to improve the current performance of CDMA as a second-generation technology. The CDMA Development Group (CDG) has formally adopted the cdmaOne name and logo as a technology designator for all IS-95-based CDMA systems. The term represents the end-to-end wireless system and the necessary specifications that govern its operation. cdmaOne incorporates the IS-95 CDMA air interface, the ANSI-41 network standard for switch interconnection and many other standards that make up a complete wireless system.

The CDMA technology, used in the Interim Standard IS-95, maximizes spectrum efficiency and enables more calls to be carried over a single 1.25 MHz channel. In a CDMA system each digitized voice is assigned a binary sequence that directs the proper response signal to the corresponding user. The receiver demodulates the signal using the appropriate code. The resulting audio signal will contain only the intended conversation, eliminating any background noise. This allows more calls to occupy the same space in the communication channel, thereby increasing capacity. As a simple, example let us assume a user is talking into a mobile phone on a CDMA network. The transmitted portion of a voice signal has frequency components from approximately 300~3400 Hz. This analog signal is digitally encoded, using QPSK (Quadrature Phase Shift Keying), at 9600 bps. The signal is then spread to approximately 1.23 Mbps using special codes that add redundancy. Some of these codes include a device ID that is unique to the phone (like a serial number). Next the signal is broadcast over the channel. When broadcast, the signal is added to the signals of the other users in the channel. On the receiving end, the same code is used to decode the incoming signal. The 9600 bps signal is obtained and the original analog signal is reconstructed. When the same code is used on another user’s signal, the redundancy is not removed and the signal remains at 1.23 Mbps.

The problems are the quality of reception and voice squeakiness. To address this major PCS carriers are using 13 kbps vocoders instead of 10 kbps. This improves quality but at the cost of capacity. The technology has been widely adopted by major cellular and PCS carriers in the United States and also internationally. CDMA networks provide operators with reliable digital systems that offer higher capacity, large coverage area and improved voice quality and above all a good 3G upgrade path, CDMA 2000 (I’ll discuss this later). It also offers simplified system planning — through the use of the same frequency in every sector of every cell.

Factors contributing to CDMA’s capacity gains are:

  • Frequency reuse
  • Soft handoffs
  • Power control,
  • Variable rate vocoders

Some of the benefits of using cdmaOne are:

  • Capacity gains of eight to ten times that of AMPS analog systems
  • Improved call quality, with better and more consistent sound as compared to AMPS systems
  • Simplified system planning through the use of the same frequency in every sector of every cell
  • Enhanced privacy through the spreading of voice signals
  • Improved coverage characteristics, allowing for fewer cell sites
  • Increased talk-time for portables

cdmaOne technology improves quality of service through the use of soft handoffs, which greatly reduce the number of dropped calls and ensure a smooth transition between cells. In soft handoff, a connection is made to the new cell while maintaining the connection with the original cell. This transition between cells is one that is almost undetectable to the subscriber. cdmaOne technology also takes advantage of multipath fading to enhance communications and voice quality. Using a rake receiver and other improved signal-processing techniques, each mobile station selects the three strongest multipath signals and coherently combines them to produce an enhanced signal.

The cdmaOne data capabilities are based on IS-95A, which can provide data speeds of 14.4kbit/s. IS-95B and IS-95C are designed to enhance CDMA’s data capability. IS-95B can provide data speeds of up to 64kbit/s by aggregating existing channels. IS 95-B can provide these enhanced data rates through software upgrades only. IS-95C aims to offer a minimum of 24.4kbit/s per channel and aggregated data speeds of more than 115kbit/s. It is expected that IS-95C will define CDMA’s capability as a third-generation system. CDMA already supports asynchronous data and faxing (IS-99) and has standardized packet data (IS-657).

The major development initiatives being taken by the CDG for 2G CDMA systems enhancements include Enhanced roaming enables transparent roaming across cellular and PCS networks, with selection of networks and location services. Enhanced roaming will provide roaming between CDMA systems similar to that on GSM: registration, authentication and credit-checking are automatically carried out between the networks without users having to do anything more than switch on their mobiles. Roaming agreements will still be needed between operators.

Mobile Wireless Market: Technology Forecasts

In the last few years the traffic increases in the mobile networks and the number of subscribers has been greater than anticipated. Infact in many places the mobile penetration has grown to more than 50%. In this section I’ll talk about the existing market for the 2G technologies as well as forecasts for these technologies and the third generation networks.

What looks clear is that after coexisting with the digital technologies for a few years (3-5) we’ll see a disappearance of the analogue technologies. According to the latest updates from [2], the number of digital cellular subscribers is expected to double from around 624 million in early 2001 to over 1.14 billion at the beginning of 2003 and may move up to 1.62 billion at the beginning of 2005. At the same time we’ll see the number of analogue cellular subscriptions falling from 80 million at the beginning of 2001 to 37 million by the beginning of 2003 and 14 million by the beginning of 2005.

The current second generation technologies, thanks to the continuos improvements and 2.5G overlays should be viable in the medium term and continue to win market share for at least next five years. Also, the 3G technologies should not have any major impact till 2003 and then coexist with 2G technologies for another 2 to 3 years before gaining prominence. 3G related work is still going on and Japan could be among the very first countries with commercial 3G roll out, as early as 2002. The next graphic shows a distribution of cellular subscribers across various regions of the world and the forecasts for future.

According to the coverage and the subscriber numbers world wide, GSM comes out as a clear winner in digital technologies. Today 89% of all cellular subscribers are using digital technology and around 65% of these are GSM subscribers, with CDMA and D-AMPS having around 13-14 % of digital subscribers each.

As the number of digital subscribers grows by almost 100 % by 2003 the subscriber ratio is expected to remain the same. 3G should begin showing it’s presence but with a very small number of subscribers till 2003 and a subscriber base of under 2.5% till 2005.

The Standards Debate

Well we have so far seen the various cellular mobile wireless technologies, their features, capabilities, market penetration etc. So, which is the best standard. There are no clear answers. The answers must be looked for by keeping in perspective the geographical location, the future plans of the mobile service providers, the evolution options available for the operator, investments made so far, and the work being done the individual technology front by the corresponding bodies.

The debate as to which standard is superior basically revolves around two basic underlying technologies, TDMA and CDMA. Time division multiplex systems like GSM have the advantage of having very big market penetration, being in operation (successfully) for so many years, easy and cheap upgrade to packet based data services, almost global roaming, recent developments towards roaming between TDMA and GSM networks and the possibility of a common air interface and subsequent evolution towards 3G after implementing EDGE. These technologies have evolved and matured successfully over so many years of existence. The proponents of TDMA claim that their networks are more rugged compared to the CDMA technology, which in their view is not matured and suffers from problems like deteriorating speech quality with increase in traffic load and voice squeakiness.

CDMA proponents on the other hand claim substantial improvements in capacity, security and speech quality. They claim that a lot more users can be supported over the same bandwidth in CDMA as compared to TDMA where the numbers are fixed in case of TDMA networks. Another front where CDMA claims an edge and is a valid claim is the comparatively much clearer, simple and well defined evolutionary path to 3G (I’ll touch upon this later in the article). 3G networks use CDMA in the air interface and so CDMA networks like IS-95 have advantage on this front. Also, although the evolution path to 2.5G may be cheaper for GSM/TDMA, the total costs involved in moving to 3G will be substantially higher in case of GSM/TDMA.

CDMA scores over TDMA on the following fronts:

  • A CDMA system uses a combination of frequency division and code division to provide multiple user access. Although the capacity of a CDMA system is not unlimited, its limitations are considerably higher than those of a TDMA system. It can provide 8~10 times more users than traditional FDMA/CDMA
  • Because a number of transmissions are possible over the same bandwidth, the frequency reuse in CDMA networks can be very high.
  • Better signal quality
  • Privacy of coded digital communications
  • Easy addition of more users. But “soft” capacity limit – Additional users add more noise to the cell

If CDMA is superior technology then why do the figures in the market forecasts section indicate other wise? Why are service providers going towards a supposedly “inferior” technology? For any technology to succeed in the market there are two essential requirements, the capacity of the system and the ease to implement the system. A major concern about CDMA is the fact that it has very little field experience, where as TDMA systems have been operable for quite some time. Fast “time to market” is essential to companies due to the phenomenal pace of today’s wireless communications market, and many providers choose to invest in TDMA systems that have already been developed and proven. The very fact that CDMA technology is so new and unproven makes it a big risk for companies to invest in when there is the option of going with a time-proven technology such as TDMA. Also existing service providers who have already spent lots of monies into TDMA networks would like to protect their investments. Continuos developments and improvements in the GSM networks for example has added value to their investments and has so far been able to keep up with the demands for higher capacities and data applications.

So, as reflected in the technology forecasts in this article, the share of these technologies in the 2G/2.5G market will remain almost the same, while the demand and numbers continue to increase. CDMA would have a much larger influence by means of 3G technologies over the next 5-10 years.

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