This post covers Wireless Communications: Principles and Practices by Theodore S. Rapport.

Basic Ideas

• The 2G - Fundamental System Design

  

  • Most of today’s ubiquitous cellular networks use what is commonly called of technologies which conform to the second generation cellular standards.

    

  • Unlike first generation cellular systems that relied exclusively on FDMA/FDD and analog FM, second generation standards use digital modulation formats and TDMA/FDD and CDMA/FDD multiple access techniques.

  • The most popular second generation standards include three TDMA standards and one CDMA standard:

    • (a), global positioning system(GSM), which supports eight time slotted users for each 200 kHz radio channel and has been deployed widely by service providers in Europe, Asia, Australia, South America, and some parts of the US (in the PCS spectrum band only);
    • (b), also known as North American Digital Cellular (NADC), which supports three time slotted users for each 30 kHz radio channel and is a popular choice for carriers in North America, South America, and Australia (in both the cellular and PCS bands);
    • (c) a Japanese TDMA standard that is similar to IS-136 with more than 50 million users; and
    • (d) the popular 2G CDMA standard , also known as, which supports up to 64 users that are orthogonally coded and simultaneously transmitted on each 1.25 MHz channel. CDMA is widely deployed by carriers in North America (in both cellular and PCS bands), as well as in Korea, Japan, China, South America, and Australia.

  • The 2G standards mentioned above represent the first set of wireless air interface standards to rely on digital modulation and sophisticated digital signal processing in the handset and the base station.

  • Second generation systems were first introduced in the early 1990s, and evolved from the first generation of analog mobile phone system.

  • Today, many wireless service providers use both first generation and second generation equipment in major markets and often provide customers with subscriber units that can support multiple frequency bands and multiple air interface standards.

  • For example, in many countries it is possible to purchase a single tri-mode cellular handset phone that supports CDMA in the cellular and PCS bands in addition to analog first generation technology in the cellular band.

  • Such tri-mode phones are able to automatically sense and adapt to whichever standard is being used in a particular market.

  • Figure illustrates how the world subscriber base was divided between the major 1G and 2G technologies as of late 2001.

  • In many countries, 2G wireless networks are designed and deployed for conventional mobile telephone service, as a high capacity replacement for, or in competition with, existing older first generation cellular telephone systems.

  • Modern cellular systems are also being installed to provide fixed (non-mobile) telephone service to residences and businesses in developing nations—this is particularly cost effective for providing in countries that have poor telecommunications infrastructure and are unable to afford the installation of copper wire to all homes.

  • Since all 2G technologies offer at least a three-times increase in spectrum efficiency (and thus at least a 3X increase in overall system capacity) as compared to first generation analog technologies, the need to meet a rapidly growing customer base justifies the gradual, ongoing change out of analog to digital 2G technologies in any growing wireless network.

  • In mid-2001, several major carriers such as AT&T Wireless and Cingular in the US and NTT in Japan announced their decisions to eventually abandon the IS-136 and PDC standards as TDMA platform.

  • Simultaneously, international wireless carrier Nextel announced its decision to upgrade its iDen air interface standard to support up to five times the number of current users based on a data compression methodology using Internet protocol (IP) packet data. Most other carriers throughout the world had already committed to adopting a 3G standard based on either GSM or CDMA prior to 2001.

  • Decisions like these have set the stage for the inevitability of two universal and competing third generation (3G) cellular mobile radio technologies, one based on the philosophy and backward compatibility of GSM, and the other based on the backward compatibility of CDMA.

    

  • The 2.5G - Fundamental System Design

  • Since the mid 1990s, the 2G digital standards have been widely deployed by wireless carriers for cellular and PCS, even though these standards were designed before the widespread use of the Internet.

  • Consequently, 2G technologies use circuit-switched data modems that limit data users to a single circuit-switched voice channel. Data transmissions in 2G are thus generally limited to the data throughput rate of an individual user, and this rate is of the same order of magnitude of the data rate of the designated speech coders given in above Table.

  • Each of the 2G standards specify different coding schemes and error protection algorithms for data transmissions versus voice transmissions, but the data throughput rate for computer data is approximately the same as the throughput rate for speech coded voice data in all 2G standards.

  • It can be seen that all 2G networks, as originally developed, only support single user data rates on the order of 10 kilobits per second, which is too slow for rapid email and Internet browsing applications.

  • The technical specifications of the original GSM, CDMA, and IS-136 standards which originally supported 9.6 kilobits per second transmission rates for data messages are not discussed in detail here.

  • Even with relatively small user data rates, 2G standards are able to support limited Internet browsing and sophisticated short messaging capabilities using a circuit switched approach.

  • is a popular feature of GSM, and allows subscribers to send short, realtime messages to other subscribers in the same network by simply dialing a recipient’s cell phone number.

  • SMS first became popular in Europe through the dominant network of GSM service providers, and then became popular in Japan through the popular NTT DoCoMo PDC .

  • As of late 2001, SMS had not yet become widespread in the USA, since the wireless markets there are fragmented between many different types of technologies and network owners, and SMS presently only works between users of the same network.

  • In an effort to retrofit the 2G standards for compatibility with increased throughput data rates that are required to support modern Internet applications, new data-centric standards have been developed that can be overlaid upon existing 2G technologies.

  • These new standards represent technology and allow existing 2G equipment to be modified and supplemented with new base station add-ons and subscriber unit software upgrades to support higher data rate transmissions for web browsing, e-mail traffic, mobile commerce (m-commerce), and location-based mobile services.

  • The 2.5G technologies also support a popular new web browsing format language, called Wireless

  

  • As the name implies, High Speed Circuit Switched Data is a circuit switched technique that allows a single mobile subscriber to use consecutive user time slots in the GSM standard.

  • That is, instead of limiting each user to only one specific time slot in the GSM TDMA standard, HSCSD allows individual data users to commandeer consecutive time slots in order to obtain higher speed data access on the GSM network.

  • HSCSD relaxes the error control coding algorithms originally specified in the GSM standard for data transmissions and increases the available application data rate to 14,400 bps, as compared to the original 9,600 bps in the GSM specification.

  • By using up to four consecutive time slots, HSCSD is able to provide a raw transmission rate of up to 57.6 kbps to individual users, and this enhanced data offering can be billed as a premium service by the carrier.

  • HSCSD is ideal for dedicated streaming Internet access or real-time interactive web sessions and simply requires the service provider to implement a software change at existing GSM base stations.

  • General Packet Radio Service is a packet-based data network, which is well suited for non-real time Internet usage, including the retrieval of email, faxes, and asymmetric web browsing, where the user downloads much more data than it uploads on the Internet.

  • Unlike HSCSD, which dedicates circuit switched channels to specific users, GPRS supports multi-user network sharing of individual radio channels and time slots. Thus, GPRS can support many more users than HSCSD, but in a bursty manner.

  • The GPRS standard provides a packet network on dedicated GSM or IS-136 radio channels.

  • GPRS retains the original modulation formats specified in the original 2G TDMA standards, but uses a completely redefined air interface in order to better handle packet data access.

  • GPRS subscriber units are automatically instructed to tune to dedicated GPRS radio channels and particular time slots for “always on” access to the network.

  • When all eight time slots of a GSM radio channel are dedicated to GPRS, an individual user is able to achieve as much as 171.2 kbps (eight time slots multiplied by 21.4 kbps of raw uncoded data throughput).

  • Applications are required to provide their own error correction schemes as part of the carried data payload in GPRS.

  • As is the case for any packet network, the data throughput experienced by an individual GPRS user decreases substantially as more users attempt to use the network or as propagation conditions become poor for particular users.

  • Implementation of GPRS merely requires the GSM operator to install new routers and Internet gateways at the base station, along with new software that redefines the base station air interface standard for GPRS channels and time slots—no new base station RF hardware is required.

  • It is worth noting that GPRS was originally designed to provide a packet data access overlay solely for GSM networks, but at the request of North American IS-136 operators,

  • GPRS was extended to include both TDMA standards. As of late 2001, GPRS has been installed in markets serving over 100 million subscribers, and is poised to be the most popular near-term packet data solution for 2G TDMA-based technologies.

  • The dedicated peak 21.4 kbps per channel data rate specified by GPRS works well with both GSM and IS-136 and has successfully been implemented.

  • Enhanced Data rates for GSM (or Global) Evolution is a more advanced upgrade to the GSM standard, and requires the addition of new hardware and software at existing base stations.

  • Interestingly, EDGE was developed from the desire of both GSM and IS-136 operators to have a common technology path for eventual 3G high speed data access, but the initial impetus came from the GSM user community.

  • EDGE introduces a new digital modulation format, 8-PSK (octal phase shift keying), which is used in addition to GSM’s standard GMSK modulation.

  • EDGE allows for nine different (autonomously and rapidly selectable) air interface formats, known as (MCS), with varying degrees of error control protection. Each MCS state may use either GMSK (low data rate) or 8-PSK (high data rate) modulation for network access, depending on the instantaneous demands of the network and the operating conditions.

  • Because of the higher data rates and relaxed error control covering in many of the selectable air interface formats, the coverage range is smaller in EDGE than in HSDRC or GPRS.

  • EDGE is sometimes referred to as Enhanced GPRS, or EGPRS. EDGE uses the higher order 8-PSK modulation and a family of MCSs for each GSM radio channel time slot, so that each user connection may adaptively determine the best MCS setting for the particular radio propagation conditions and data access requirements of the user.

  • This adaptive capability to select the “best” air interface is called, whereby packets are transmitted first with maximum error protection and maximum data rate throughput, and then subsequent packets are transmitted with less error protection (usually using punctured convolutional codes) and less throughput, until the link has an unacceptable outage or delay.

  • Rapid feedback between the base station and subscriber unit then restores the previous acceptable air interface