1st & 2nd Generation Wireless Systems

1st Generation Wireless Systems

The commercial use of mobile phones started in the US when AT&T developed the AMPS (Advanced Mobile Phone System) system in late 1970’s [3GW9, 3GW1]. AMPS went into service first in Chicago area in 1983. The AMPS system is an analog communication system, which uses frequency modulation (FM) to transmit speech and frequency shift keying (FSK) to transmit important network control information in digital form [3GW10]. The AMPS system was designed primarily for carrying two-way voice conversations. Surprisingly, AMPS remains in use almost sixteen years after its introduction into the market. In fact, AMPS still enjoys 11% share of the cellular communications market in the US. Other analog mobile systems were introduced in Europe and Japan a few years earlier than AMPS was deployed in the US [3GW11]. However, unlike in the US, where AMPS was the only system introduced, European countries developed several incompatible systems. Lack of interoperability soon proved to be a major problem for many European users who regularly traveled to other countries within Europe for business and pleasure. The number of cellular users and the geographic coverage of the system increased steadily, but modestly, in the US. 

 2nd Generation Wireless Systems

In the 1980’s the Europeans recognized the need for a second generation of cellular systems based on digital modulation and transmission schemes, which promised a more efficient use of the available frequency spectrum. This translates into increased total communication capacity and hence the capability to offer cellular service to a larger number of users per unit area. To avoid the interoperability problems encountered with the first-generation systems, the European countries agreed to jointly develop a common system for the entire continent. The result was the Global System for Mobile Communications (GSM), which was first commercially deployed in 1991 [3GW12, 3GW2]. GSM uses many features made possible by adoption of digital technology, such as digital voice compression and encryption. GSM also introduced many innovations in network level architectures and services. The multiple access mechanism in GSM is Time Division Multiple Access (TDMA), where each user in a cell transmits and receives information in time slots allocated to that user. The multiple access mechanism in the first-generation systems is Frequency Division Multiple Access (FDMA), where each user is allocated two bands or channels in the frequency spectrum, one for transmitting information and one for receiving it. GSM was a major success for Europe by all accounts, allowing users to roam all over the continent and yet be able to use their mobile phones. The success of GSM spread throughout the world.

GSM now has the largest share of the market in the world with 64% of the user population using it [3GW2]. This was followed by the development of the digital cellular IS-54 standard [3GW13] in the US, which incorporated several important GSM network control innovations [3GW10].

The Japanese digital cellular standard, referred to as Pacific Digital Cellular (PDC), was designed soon after IS-54 was published [3GW14]. However, the Japanese soon found themselves with a system incompatible with AMPS, GSM, and IS-54. This system never found market acceptance outside of Japan.

During the almost ten-year development period of GSM in Europe, the Americans were reasonably satisfied with AMPS and there was no dire need for developing a new generation of mobile phones. This gradually changed when the number of users increased to a point where the AMPS system was no longer capable of providing the needed capacity. Of course, one can increase the capacity to a certain extent by making the cells smaller and having a larger frequency reuse. This was only a temporary solution and it soon became clear that a second-generation system had to be developed. This was the beginning of a lively, fierce, and often controversial debate on the access mode for the new system. A digital transmission technology called spread spectrum communications, developed for the US military more than 40 years ago [3GW15], was at the center of these discussions [3GW3]. In a spread spectrum system, a user spreads its transmitted signal over a frequency band much wider than the actual symbol bandwidth. This makes the signal look like noise, making eavesdropping difficult. The intended receiver, having knowledge of the spreading code or the frequency-hopping pattern used, is able to demodulate the signal. The privacy and security of spread spectrum communications makes it suitable for military communications.

As far as  mercial use is concerned, spread spectrum is used with a multiple access scheme called Code Division Multiple Access (CDMA) [3GW15, 3GW16], where each transmitting user spreads its signal over the entire available spectrum using its individual code. This turns out to be a very flexible and efficient use of the frequency spectrum, where the users can come and go any time into the system and only slightly change the "noise level" as far as the other users are concerned. In addition, the spread spectrum system very naturally exploits the fact that speech consists of almost 40% silence, where there is nothing to be transmitted. This statistical multiplexing gain translates into increased capacity for the system. A TDMA system can also take advantage of this large silence content in speech signals. So, the big debate in the US was about which system would offer a larger overall capacity for the available frequency bandwidth. Qualcomm, which first commercialized spread spectrum systems, claimed through various analyses that CDMA systems offered much higher capacity than TDMA systems. In the opposing camp, Ericsson and some other companies consistently rejected Qualcomm’s claim. The main difficulty was that TDMA and CDMA systems are complicated, and it is difficult to settle the capacity question in a decisive manner. This discussion raged in the US for a few years, leading to the introduction of the TDMAbased IS-54 and CDMA-based IS-95 [3GW17, 3GW5] standards in the US in 1992 and 1993, respectively. IS-136, a revised version of IS-54 with a digital control channel specification, was published in 1994 [3GW4], followed by IS-136A in 1996 [3GW18]. These second-generation systems were introduced in the US later than the GSM system’s introduction in Europe. In addition, the US market was fragmented into three parts, these two second-generation digital systems and the analog, first-generation AMPS system. In a sense, the situation in the US for second-generation systems is like what Europeans experienced at the time of first-generation systems: a multitude of systems and the resultant interoperability problems. This worked to GSM’s advantage and remarkable success with 140 million GSM users worldwide now. IS-136 and IS-95 have about 93 million and 12 million users, respectively.

Yet another family of second-generation wireless systems is that of Personal Communication Networks (PCNs) or Personal Communication Services (PCS). The objective of PCS or PCNs is to provide ubiquitous wireless communications coverage, enabling users to access telephone networks for different types of communication needs, without regard for the location of the user or the location of the information being accessed [3GW11]. It is clear that this definition has much overlap with that for mobile phones. In fact, according to [3GW10], PCS presently means different things to different people! In the US, the FCC issued PCS licenses in the 900 MHz band for “narrowband services” and near 1900 MHz band for “wideband services” in 1995, 1996, and 1997 without placing any restrictions on the air interface and system architecture to be used or nature of services to be offered. PCS license holders began offering services in 1995. The industry, which not surprisingly includes the largest cellular companies in the US, and the standards organizations, has adopted seven technical approaches to wideband PCS. Suffice it to say that the distinctions between cellular systems and PCS will most likely disappear as third-generation wireless systems are developed and deployed.

The need for wireless data communication services has been recognized for many years. Today there is 4% data and 96% voice traffic in wireless systems. The volume for data traffic, however, is expected to rise sharply due to the use of the Internet and the worldwide web. By 2005, some industry sources predict that there will be 70% data and only 30% voice traffic. Furthermore, the number of wireless phone users has been sharply increasing over the past few years and this trend is expected to continue for at least the next decade. Some industry sources expect that there will be more wireless phones in the world than wired phones by 2003. The number of wireless users is expected to hit 800 million by 2003. These drivers have made it more important than ever to use the precious available frequency band as efficiently as possible. From a capacity point of view, it is now generally accepted that the CDMA system is more efficient than the TDMA system, although the gap between the two is not nearly as wide as Qualcomm had originally claimed. While flexibility and ease of engineering may partly explain the higher relative use of TDMA systems, CDMA systems are becoming increasingly refined. Progress has also been made in the development of other signal

processing techniques and concepts for use in tomorrow's wireless systems. These include smart antennas and diversity techniques, better receivers, and hand over and power control algorithms with higher performance. Therefore, the search for thirdgeneration wireless systems began a few years ago under the guise of Future Public Land Mobile Telecommunication System (FPLMTS) [3GW6] by the International

Telecommunications Union. The name was later changed to International Mobile Telecommunication System 2000 (IMT-2000) in 1995 [3GW19, 3GW20]. The European efforts in this area started as the RACE (Research and Development of Advanced Communication Technologies) research program for development of the Universal Mobile Telecommunication System (UMTS) [3GW19, 3GW20, 3GW7].