Evolution Of Cellular Networks Information Technology Essay

The wireless telegraph is not difficult to understand. The ordinary telegraph is like a very long cat. You pull the tail in New York and it meows in Los Angeles. The wireless is the same, only without the cat.- Albert Einstein.

1.1 SECOND GENERATION (2G) CELLULAR NETWORKS

Most of today’s ubiquitous cellular networks use what is commonly called second generation or 2G technologies. 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 standards

Global System Mobile (GSM), which supports eight time slotted users for each 200 KHz radio channel and has been deployed widely in the cellular and PCS bands by service providers in Europe, Asia, Australia, South America, and some parts of the US (in the PCS spectrum band only)

Interim Standard, also known as North American Digital Cellular (NADC) or US DIGITAL CELLULAR (USDC), 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);

Pacific Digital Cellular

The popular 2G CDMA standard Interim Standard 95 Code Division Multiple Access (IS-95) also known as cdmaOne, 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.

2G systems were first introduced in the early 1990s, and evolved from the first generation of analog mobile phone systems (e.g. AMPS, ETACS, and JTACS). 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 tri-mode phones are able to automatically sense and adapt to whichever standard is being used in a particular market. [3]

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. 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, on going changes out of analog to digital 2G technologies in any growing wireless networks. [5]

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. 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.

1.2 EVOLUTION TO 2.5G MOBILE RADIO NETWORKS

Since the mid 1990s, the 2G digital standards have been widely deployed by wireless carriers for cellular and PCS. 2G technologies use circuit-switched data modems that limit data users to a signal circuit-switched voice channel. Data transmissions in 2G are generally limited to the data throughout rate of an individual user. Each of the 2G standards specify different coding schemes and error protection algorithms for data transmission 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. From inspection it can be seen that all 2G networks, as originally developed, only support single user data rates on the order of 10 kbps, 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 kbps transmission rates for data messages.

These new standards represents 2.5G 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 rates transmission 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 Application protocol (WAP). [3]

1.3 EVOLUTION FOR 2.5G TDMA STANDARDS

Three different upgrade paths have been developed for GSM carrier, and two of these solutions also support IS-136. These options provide significant improvements in Internet access speed over today’s GSM and IS-136 technology and support the creation of new Internet-ready cell phones.

HSCSD for 2.5G GSM

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 to only one specific time slots 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 and increase 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. HSCSD is ideal dedicated streaming Internet access or real-time interactive web sessions and simply requires the service provider to implement a software changes at existing GSM base stations. [3]

GPRS for 2.5G GSM and IS-136

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. 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 burst manner. Similar to the Cellular digital packet data (CDPD) standard developed for North American AMPS systems in early 1990s 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.

EGDE for 2.5G GSM and IS-136

EDGE, which stands for Enhanced Data rates for GSM (or global) Evolution is a more advanced upgrade to the GSM standards, and requires the addition of new hardware and software at existing base stations. EDGE was developed from the desire of both GSM and IS-136 operates to have a common technology path for eventual 3G high speed data access.

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 multiple modulation and coding schemes (MCS), with varying degrees of error control protection. 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. EGDE is sometimes referred to as Enhanced GPRS, or EGPRS. The adaptive capability of EDGE to select the “best” air interface is called incremental redundancy, 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 state, which presumably at an acceptable level but with minimum required coding and minimum bandwidth and power drain. [3]

1.4 IS-95B for 2.5G CDMA

Unlike the several GSM and IS -136 evolution paths to high speed data access, CDMA (often called cdmaOne) has a single upgrade path for eventual 3G operation. The interim data solution for CDMA is called IS-95B is already being deployed worldwide, and provides high speed packet and circuit switched data access on a common CDMA radio channel by dedicating multiple orthogonal user channels (Walsh functions) for specific users and specific purposes. Each IS-95 CDMA radio channel supports up to 64 different users channels. The original IS-95 throughput rate specification of 9600 bps was not implemented in practice, but was improved to the current rate of 14,400bps as specified in IS-95A. The 2.5G CDMA solution, IS-95, supports medium data rate (MDR) service by allowing a dedicated user to command up to eight different user Walsh codes simultaneously and in parallel foe an instantaneous throughput of 115.2 kbps per user (8*14.4 kbps). However only about 64 kbps of practical throughput is available to a single user in IS-95B due to the slotting techniques of the air interface

IS-95B also specifies hard handoff produces that allow subscriber units to search different radio channels in the network without instruction from the switch so that subscriber units can rapidly tune to different base stations to maintain link quality. Prior to IS-95B, the link quality experienced by each subscriber had to be reported back to the switch through the serving base station several times per second, and at the appropriate moment, the switch would initiate a soft-handoff between the subscriber and candidate base stations. The new hard handoff capability of IS-95B is more efficient for multiple channel systems now being used in today’s more congested CDMA markets. [3]

1.5 THIRD GENERATION (3G) WIRELESS NETWORKS

3G systems promise unparalleled wireless access in ways that have never been possible before Multi-megabit Internet access, communications using Voice over Internet Protocol (VoIP), voice activated calls, unparalleled networks capacity, and ubiquitous “always-on” access are just some of the advantages being touted by 3G developers. Companies developing 3G equipment envision users having the ability to receive live music, conduct interactive web sessions, and have simultaneous voice and data access with multiple parties at the same time using a single mobile handset, whether driving, walking, or standing still in an office setting.

The International Telecommunications Union (ITU) formulated a plan to implement a global frequency band in the 2000 MHz range that would support a wireless communication standard for all countries throughout the world. This plan, called International Mobile Telephone 2000 (IMT-2000), has been successful in helping to cultivate active debate and technical analysis for new high speed mobile telephone solutions when compared to 2G. However, the hope foe a single worldwide standard has not materialized, as the worldwide user community remains spilt between two camps: GSM/IS-136/PDC and CDMA. [3]

The eventual 3G evolution for 2G CDMA systems leads to cdma2000, several variants of CDMA 2000 are currently being developed, but they all are based on the fundamentals of IS-95 and IS-95B technologies. The eventual 3G evolution for GSM, IS-136, and PDC systems leads to Wideband CDMA (W-CDMA), also called Universal Mobile Telecommunication Service (UMTS). W-CDMA is based on the network fundamentals of GSM, as well as the merged versions of GSM and IS-136 through EDGE. It is fair to say that these two major 3G technology camps, cdma2000 and W-CDMA, will remain popular throughout the early part of the 21st century. [3]

1.6 3G W-CDMA (UMTS)

The Universal Mobile Telecommunication System (UMTS) is an air interface standard that has evolved since late 1996 under the auspices of the European Telecommunication Standards Institute (ETSI). UMTS was submitted by ETSI to ITU’s IMT-2000 body in 1998 for consideration as a world standard. At that time, UMTS was known as UMTS Terrestrial Radio Access (UTRA), and was designed to provide a high capacity upgrade path for GSM. Around the turn of the century, several other competing wideband CDMA (W-CDMA) proposals agreed to merge into a single W-CDMA standard, and this resulting W-CDMA standard is now called UMTS. UMTS, or W-CDMA, assures backward compatibility with second generation GSM, IS-136, and PDC TDMA technologies, as well we all 2.5G TDMA technologies. The network structure and bit level packaging of GSM data is retained by W-CDMA, with additional capacity and bandwidth provided by a new CDMA air interface. [3]

The 3G W-CDMA air interface standard had been designed for “always-on” packet-based wireless service, so that computers, entertainment devices, and telephones may all share the same wireless network and be connected to the Internet, anytime , anywhere. W-CDMA will support packet data rates up to 2.048 Mbps per user (if the user is stationary), thereby allowing high quality data, multimedia, streaming audio, streaming video, and broadcast-type services to consumers. Future versions of W-CDMA will support stationary user data rates in excess of 8Mbps. W-CDMA provides public and private network features, as well as videoconferencing and virtual home entertainment (VHE). W-CDMA requires a minimum spectrum allocation of 5 MHz, which is an important distinction from the other 3G standards. With W-CDMA, data rates from as low as 8 kbps to as high as 2 Mbps will be carried simultaneously on a single W-CDMA 5MHz radio channel, and each channel will be able to support between 100 and 350 simultaneous voice calls at once, depending on antenna sectoring, propagation conditions, user velocity, and antenna polarizations. W-CDMA employs variable/selectable direct sequence spread spectrum chip rate that can exceed 16 Mega chips per second per user. A common reel of thumb is that W-CDMA will provide at least a six times increase in special in spectral efficiency over GSM when compared on a system wide basis. [3,5]

1.7 3G cdma2000

The cdma2000 vision provides a seamless and evolutionary high data rate upgrade path for current users of 2G and 2.5G CDMA technology using a building block approach that centers on the original 2G CDMA channel bandwidth of 1.25MHz per radio channel. The improvements in cdma2000 1X over 2G and 2.5G CDMA systems are gained through the use of rapidly adaptable base band signaling rates and chipping rates for each user (provided through incremental redundancy) and multi-level keying with in the same gross framework of the original cdmaOne standard. No additional RF equipment is needed to enhance performance- the changes are all made in software or in base band hardware. To upgrade from 2G CDMA to cdma2000 1X, a wireless carrier merely needs to purchase new backbone software and new channel cards at the base station, without having to change out RF system components at the base station.

Cdma2000 1*EV is an evolutionary advancement for CDMA originally developed by Qualcomm, Inc, as a proprietary high data rate (HDR) packet standard to be overlaid upon existing IS-95, IS-95B, and cdma2000 networks, Qualcomm later modifies its HDR standard to be compatible with W-CDMA as well, and in august 2001, ITU recognized cdma2000 1*EV as part of IMT-2000. cdma2000 1*EV provides CDMA carriers with the option of installing radio channels with data only (cdma2000 1*EV-DO) or with data and voice (cdma2000 1*EV-DV). Using cdma2000 1*EV technology, individual 1.25MHz channels may be installed in CDMA base stations to provide specific high speed packet data access within selected cells. The cdma2000 1*EV-DO option dedicates the radio channel strictly to data users, and supports greater than 2.4 Mbps of instantaneous high-speed packet throughput per user on a particular CDMA channel, although actual user data rates are typically much lower and are highly dependent upon the number of users, the propagation conditions, and vehicle speed. Typical users may experience throughputs on the order of several hundred kilobits per second, which I is sufficient to support web browsing, email access, and m-commerce applications. Cdma2000 1*EV-DV supports both voice and data users, and can offer usable data rates up to 144 kilobits per second with about twice as many voice channels as IS-95B. [3,5]

The ultimate 3G solution for CDMA relies multicarrier techniques that gang adjacent cdmaOne radio channels together for increased bandwidth. The cdma2000 3*RTT standard uses three adjacent 1.25 MHz radio channels that are used together actual throughput depends upon cell loading, vehicle speed, and propagation conditions. Three non-adjacent radio channels may be operated simultaneously an din parallel as individual 1.25 MHz channels (in which case no new RF hardware is required at the base station), or adjacent channels may be combined into a single 3.75 MHz super channel (in which case new RF hardware is required at the base station). With peak user data rates in excess of 2 Mbps, it is clear that cdma2000 3X has a very similar user data rate throughput goal when compared to W-CDMA (UMTS). Advocates of cdma2000 claim their standard gives a wireless service provider a much more seamless and less expensive upgrade path when compared to W-CDMA, since cdma20 cdma2000 allows the same spectrum, bandwidth, RF equipment, and air interface framework to be used at each base station as the 3G upgrades are introduced over time. [3]

1.8 3G TD-SCDMA

IN China, GSM is the most popular wireless air interface standard, and the wireless subscriber growth in China is unmatched anywhere in the world. The China Academy of Telecommunication Technology (CATT) and Siemens Corporation jointly submitted an IMT-2000 3G standard proposal in 1998, based on Time Division Synchronous Code Division Multiple Access (TD-SCDMA). This proposal was adopted by ITU as one of the 3G options in late 1999.

TD-SCDMA relies on the existing core GSM infrastructure and allows a 3G network to evolve through the addition of high data rate equipment at each GSM base station. TD-SCDMA combines TDMA and TDD techniques to provide a data-only overlay in an existing GSM network. Up to 384 kbps of packet data is provided to data users in TD-SCDMA [TD-SCDMA]. The radio channels in TD-SCDMA are 1.6 MHz in bandwidth and rely on smart antennas, spatial filtering, and joint detection techniques to yield several times more spectrum efficiency than GSM. A 5 millisecond frame is used in TD-SCDMA, and this frame is subdivided into seven time slots which are flexibly assigned to either a single high data rate user or several slower users. By using TDD, different time slots within a single frame on single carrier frequency are used to provide both forward channel and reverse channel transmissions. For the case of asynchronous traffic demand, such as when a user downloads a file, the forward link will require more bandwidth than the reverse link, and thus more time slots will be dedicated to providing forward link traffic. TD-SCDMA proponents claim that the TDD feature allows this 3G standard to be very easily and inexpensively added to existing GSM systems. [3]

1.9 WLAN

Gfeller at the IBM laboratories in Switzerland first introduced the idea of WLAN in 1970s. In 1987 WLAN started as 802.4L standard and 802.11 in 1889. Currently we have 802.11 a,b,e,g,h,i,j,k standards. Channels are specified in 802.11b standard for 2400-2483.5 MHz. This figure shows a typical wireless LAN configuration, which includes an Access Point and PC cards in notebook or palmtop computers. [1]

Figure: Architecture of WLAN [1]

WLAN uses CSMA/CD (Carrier sense multiple access/collision avoidance) mechanism. The receiver reads the peak voltage on cable and compares it with a threshold. It supports three physical layers DSSS, FHSS, DFIR. It works on two frequency hopping techniques i.e. DSSS and FHSS. The protocol layers of IEEE 802.11 are shown below.

[1]

Where PLCP is physical layer convergence protocol and PMD is physical medium dependent. LLC provide an interface to higher layers and perform flow and error control

MAC layer gives access mechanism. MAC Management provides roaming in ESS and power management and security. PLCP provides carrier sensing assessment. PMD gives modulation and coding. Physical Layer Management tunes channel. Station Management: interacts with MAC and Physical layer.

1.9.1 Distributed Coordination Function (DCF)

It uses CSMA/CA algorithm based on Inter frame Space (IFS).

1) If the medium is idle, the station waits to see if the medium is idle for a time equal to IFS. If so, it may transmit immediately.

2) If the medium is busy, the station defers transmission and continues to monitor the medium.

3) Once the current transmission is over, the station delays another IFS. If the medium remains idle for this period, then it back off the random amount of time and again sense the medium. If the medium is still idle, it may transmit. During the back off time, if the medium becomes busy, the back off timer is halted and resumes when the medium is idle. Additive increase, Multiplicative decrease is a good algorithm for this purpose. [1,2]

1.9.2 MAC Management

Beacons are sent periodically (every 100ms) by AP to establish time sync. (TSF) and maintain connectivity or associations. Beacons contains BSS-ID used to identify the AP and network, traffic indication map (for sleep mode), power management, roaming. RSS measurements are based on the beacon message. AP and mobile devices form “associations” and mobile device “registers” with AP. After registering can mobiles send/receive DATA.

This figure shows the handover principle of WLAN, when two signals appear in the same coverage, the receiver will switch its frequency to select the strongest signal. When the WLAN is constructed, it is necessary to consider the signal superposition phenomenon.

[1]

Handover of Frequency

1.10 SUMMARY

Wireless network standards have been aimed at specific market regions such as North America, Europe and Asia. In general wireless networks can be divided into WLAN, Wireless personal area networks (WPAN), Wireless metropolitan area networks (WMAN) and wireless wide area networks (WWAN), including cellular and satellite networks. They are summarized in table below. [6]

NETWORK

STANDARD

DATA RATE

BAND

Cellular Network

GSM (2G)

HSCSD(2G+)

GPRS (2.5G)

EDGE (2.75 G)

UMTS (3G)

9.6 kbps

14-42 kbps

14-128 kbps

128-384 kbps

Up to 2 Mbps

900/1800/1900 MHz

1900-2025 MHz

WLAN

IEEE 802.11 b

IEEE 802.11 a

1, 2, 5.5, 11 Mbps

1-54 Mbps

2.4 GHz

5 GHz

BLUETOOTH

IEEE 802.15.1

721 kbps (BT 1.1)

2-20 Mbps (BT 2.0)

2.4 GHz

WPAN

IEEE 802.15.3

11-55 Mbps

2.4 GHz

WMAN

IEEE 802.16a

75 Mbps

2-11 GHz

WiMAX

IEEE 802.16c

134 Mbps

10-66 GHz

WWAN

IEEE 802.20

2.25 – 18 Mbps

<3.5 GHz

HIPERLAN

HIPERLAN 1

HIPERLAN 2

HIPERLAN 3

HIPERLAN 4

23.5 Mbps

1-54 Mbps

25-100 Mbps

Up to 155 Mbps

5 GHz

40.5 – 43.5 GHz

17 GHz

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