Wireless & mobile communication: THE PATH THUS FOR EVOLUTION OF wireless & mobile technology

This paper is organized as follows. Section II provides brief prologue of 1G. Section III introduces 2G with GSM, CDMA One. Section IV describes evolution of TDMA based 2G systems to 3G systems focusing on HSCSD and 2.5G mobile technologies such as GPRS, EDGE. Section V thrashes out the evolution of 2G CDMA One to 3G CDMA2000. Section VI visualizes 4G mobile communication systems stating limitation of 3G and future access technology (MC-CDMA) and its application. Section VII launches a synoptic account of this paper by comparing different generations and finally section VIII winds with conclusion.

The first generation systems suffered from poor voice quality, poor battery life, large phone size, less security, frequent call drops, limited capacity and poor handoff reliability between cells.

Despite its limitations, first generation systems were very much successful at that time. However, we have seen 2G & 3G already replaced them and in near future, nothing is impossible for 4G wireless to grab their position.

III. SECOND GENERATION (2G)
2G was the switching revolution of mobile communication from analog to digital. Global System for Mobile (GSM) communication was one of the main attractive sides of 2G. It introduces the concepts of Subscriber Identity Module (SIM) cards. CDMA was the follower of the GSM. The main aim in the design of the 2G system was the maximization of the system capacity measured the number of user per spectrum per unit area.
A. Global System for Mobile Communication (GSM-2G)

GSM-2G was the first digital wireless technology. In addition to voice, GSM supports low rates for data services (up to 9.6 Kbps) and Short Message Services (SMS). The GSM standard recommends the application of Gaussian Minimum Shift Keying (GMSK) modulation. GMSK was developed to reduce susceptibility to radio noise, reduce bandwidth and limit power to increase battery life for mobile users. These result in increased frequency reuse, better voice quality and higher data rates. To support multiple accesses by users, GSM uses a combination of FDMA and TDMA.

However, there are drawbacks to the current GSM: (a) The GSM is a circuit switched, connection oriented technology, causing inefficiency in usage of bandwidth and resources. (b) The GSM does not support high data rates, e.g. video. (c) Only basic messaging services such as SMS can be supported. (d) The GSM networks are not compatible with common networks like TCP/IP because of differences in network hardware, software and protocols.
B. CDMAOne, ITU IS-95 (2G):

CDMA One (ITU IS-95 system) is first version of CDMA. Originally designed for voice, changes specified in IS-95A now allow packet data rates up to 14.4 Kbps. The major difference between CDMA and other 2G technologies is the modulation scheme. CDMA, developed by Qualcomm, uses a Spread Spectrum Technology that distributes a signal across a wide frequency (1.25 MHz) channel.

The IS-95 system operates in two bands. The Frequency Division Duplex (FDD) method is applied in both of them. For the first band, Band class 0, 824-849 MHz is for downlink and 869-894 MHz for uplink. For second band, Band class 1, 1930-1990 MHz is for downlink and 1850-1910 MHz for uplink. In transmission from a base station to mobile stations there exist four physical channels: namely, pilot, synchronization, paging and traffic. Direct Sequence Spread Spectrum (DS-SS) transmission at the rate 1.2288 Mchip/s is applied on all channels. The operation of all base stations is synchronized with respect to the system clock by the Global Positioning System (GPS). All mobiles use the same channel and are separated through orthogonal sequences, called Walsh codes.

CDMA uses a sophisticated RF power control mechanism to increase capacity, adaptive power control techniques to overcome the Near-Far Problem.

Time Division Multiple Access (TDMA) was originally specified as IS-54. IS-54 retains the 30 kHz channel spacing of AMPS to facilitate evolution from analog to digital systems. Each user is given a small slice of airtime (timeslot). This allows all users to share the available bandwidth. TDMA uses the same frequency band and channels as 1G system, but it provides increased capacity and improved performance.

IS-136 is a frequency-shifted derivative of IS-54 for PCS bands. It includes a digital control channel (DCCH), which uses a 48.6 kbps modem. With increased signaling rate, IS-136 supports short message services (SMS), users groups, wireless private Branch exchange (PBX), and sleep modes to reduce mobile power consumption and, therefore, extend battery life.
IV. EVOLUTION OF TDMA-BASED 2G SYSTEMS TO 3G SYSTEMS
A. High-Speed Circuit Switched Data (HSCSD) in GSM

The aim of HSCSD is to provide a mixture of services with different air interface user rates by a single physical layer structure. It is a feature that enables the co-allocation of multiple full-rate traffic channels (TCH-F) of GSM into a HSCSD configuration. Ushering faster data rates into the mainstream is the new speed of 14.4 kbps per time slot and HSCSD protocols that approach wire-line access rates of 57.6 kbps by using multiple 14.4 kbps time slots. For end-users, HSCSD enables faster Web browsing, file downloads, mobile video conference, vertical applications, telematics, and bandwidth-secure mobile LAN access.

GPRS provides a packet data service for GSM where time slots on the air interface can be assigned to GPRS over which packet data from several mobile stations is multiplexed. Services of GPRS have been developed to reduce connection set up time and allow an optimum usage of radio resources.

GPRS architecture for GSM is shown.

Figure 1. GPRS architecture for GSM

SOURCE: http://www.gsmworld.com/

GPRS can be implemented in the existing GSM systems. It requires only minor changes in an existing GSM network.

The GPRS introduces two nodes, the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN). The Home Location Register (HLR) is enhanced with GPRS subscriber data and routing information. Independent packet routing and transfer within the Public Land Mobile Network (PLMN) is supported by a new logical network node called the GPRS Support Node (GSN).

Two types of services are provided by GPRS: a) Point-to-point [PTP] b) Point-to-Multipoint [PTM].

Although the specific operations and components of a cellular telephone system Figure 2a. 2G system architecture

depend on the specific technology employed (FDMA, TDMA, CDMA, as discussed) in Section III, the basic architecture of a cellular network is essentially common standards.

Within the cell, the user communicates with the system via Base Transceiver Station (BTS). Each cell has one BTS, a tower which contains radio transceivers and antenna, a processor, channel cards. Each BTS is controlled by a Base Station Controller (BSC); A BSC controls the BTSs, including managing call handoffs and administering radio resource. The Mobile Switching Center (MSC) provides connections to PSTN as well as data network via an Internetworking Function (IWF). It also support all the functions required to manage the mobile user, including device registration, location updating, and call handoff, call setup & managing connections to BSCs and other MSCs.
B. The CDMA2000 (1XRTT, 3XRTT)

The CDMA 2000 is a trademark of Telecommunications Industry Association (TIA). The CDMA2000 standard is actually comprised of two phases: 1xRTT and 3xRTT (RTT – Radio Transmission Technology). The 1xRTT is sometimes referred to as Phase I of the CDMA2000 3G and 3xRTT as Phase II 3G. 1xRTT offers a doubling of voice capacity over IS-95, and will allow data speeds of up to 384 Kbps (theoretically).

Figure 2b. 1 x RTT architecture

Some main challenges are 1. Multimode user terminals, 2.Wireless System Discovery and Selection, 3.Terminal Mobility: Location Management and Vertical Hand-Off management, 4.Personal mobility, 5. Security and privacy, 6.Fault tolerance, 7. Billing System.

OFDM has the capability to cancel multi-path distortion in a spectrally efficient manner. Rapid variation in channel characteristics are caused by multi-path and Doppler spread (due to the different speeds of mobile). Sometimes these time varying channels are characterized by very good SNR (Signal to Noise Ratio), but worse SNR at other times. So a fixed modulation technique cannot achieve the best spectral efficiency as the system has to be built with a modulation scheme considering the worst case scenario. Hence during good channel conditions the system would not be able to obtain the best possible spectral efficiency. This is where adaptive modulation shows its role. Adaptive modulation techniques takes advantage of the time varying channel characteristics and adjust the transmission power, data rate, coding and modulation scheme for the best spectral efficiency.

These will be some applications of 4G:

(a) Virtual Presence, (b) Virtual navigation, (c) Tele-Medicine, (d) Tele-geoprocessing applications, (e) Crisis management, (f) Education.

Although 4G wireless technology offers higher data rate and ability to roam across multiple heterogeneous wireless networks, several issues require further investigations and researches. 4G is expected to be launched by 2010 and the world is looking forward for the most intelligent technology that would connect the entire globe

[2] Vijay K. Garg, Wireless Network Evolution: 2G to 3G, Prentice Hall PTR, 2002.