3rd Generation Wireless Systems

3rd Generation Wireless Systems

       The IMT-2000 standard for third-generation wireless systems is presently under development by the ITU-R Task Group 8/1 (R stands for radio). The ITU plans to complete this task by the end of 1999, hence the name IMT-2000 standard [3GW21,3GW8].

      The IMT-2000 standard will offer users transmission rates up to 2 Mbits/sec, global roaming and interoperability, and a much wider array of wireless services. These should be contrasted with the second-generation wireless systems, which provide mainly voice service, some limited data communication capability (e.g., e-mail), and lack of interoperability due to the use of different systems and standards around the world. The transmission rate for second-generation systems is on the order of only tens of kilobits per second. Third-generation systems have the potential to offer users high data rate services such as web browsing (usually involving web pages with multimedia content) and even applications such as video teleconferencing. In the interim, and until systems based on the IMT-2000 standard are deployed some time between 2002 and 2006, the GSM operators plan to offer an enhanced version of the GSM system, called the EDGE system, with service rates up to 384 Kbps. EDGE employs a more spectrally efficient modulation scheme than GSM, as well as some other advanced techniques, but it is not as radical a departure from GSM as are the proposed third-generation systems. Some call EDGE a 2.5-generation system.

      The deadline for submission of proposals for the air interface of the IMT-2000 system to the ITU was June 30, 1998. By that date fifteen RTT (Radio Transmission Technologies) proposals had been submitted, ten on terrestrial systems and five on satellite systems. A sixth satellite system, the Iridium system, was submitted to ITU after the expiration of the June 30 deadline. The Iridium proposal was accepted by ITU, and this brought the total number of RTT proposals under consideration to sixteen. These proposals were developed by national and regional standardization bodies and industry consortia in a handful of countries around the world. Between June 30 and September 30, 1998, these sixteen proposals were evaluated by ten evaluation bodies around the world. Not all evaluation groups evaluated all the sixteen proposals. As a matter of fact, all the evaluations and comparisons were based on the self-evaluations of the proposals made by the proposers themselves. No evaluation body had the time or resources to implement and simulate the different systems and carry out a more in-depth and unbiased evaluation of the proposals.

The period between September 30, 1998 and March 31, 1999 has been designated by the ITU as the time during which harmonization of different proposals is to take place, and a single set of key characteristics for the IMT-2000 standard is to emerge.

 The following nine months until the end of this millennium have been reserved for further developing and refining the IMT-2000 standard until it is fully developed. During the year 2000, some field testing of the IMT-2000 standard is supposed to take place. Japan has the most ambitious goal of fully deploying an IMT- 2000 system in 2001. Europe and the US are not expected to deploy such systems until about 2005. In the remainder of this section, we present an overview of the terrestrial and satellite RTT proposals, go over various inhibitors to success, and describe and motivate NIST work plans and strategy as far as the IMT-2000 and future standards for wireless communication systems are concerned.

1. IMT-2000 Terrestrial Proposals: Ten RTT (radio transmission technology) terrestrial proposals were submitted to ITU, four from the US, two from Europe, two from Korea, and one each from Japan and China. Only the following four RTT proposals are the major contenders for the IMT-2000 standard: the cdma2000 and the UWC-136 RTT proposals from the US, the W-CDMA (or UMTS Terrestrial Radio Access (UTRA)) from Europe, and the W-CDMA RTT proposal from Japan. The UWC- 136 RTT proposal, which is an extension of the IS-136 standard, is the only TDMAbased proposal among the four. The other three are all based on wide-band CDMA. In fact, the UTRA and the W-CDMA proposals are so similar that with most likelihood they will merge into a single proposal, a very strong force for the US proposals to contend with. Therefore, the term W-CDMA will be used to denote this joint effort underway by the 3G Partnership Program (3GPP) Consortium. In the interest of brevity, the other six RTT proposals submitted to the ITU will not be discussed, except to mention in passing that the remaining two RTT proposals from the US have merged into a single RTT proposal called the Wideband Packet CDMA (WP-CDMA) system, which is much closer to the W-CDMA system than to the cdma2000 system.
     All these cellular systems use 5MHz channels within each cell. The channel bandwidth is strongly (but not entirely) dependent on a parameter called the chip rate in a direct-sequence CDMA (DS-CDMA) system. A major difference between the cdma2000 and the W-CDMA systems is that the former uses a chip rate of 3.6864MHz and the latter uses a chip rate of 4.096MHz. The chip rate chosen for cdma2000 is three times that used in the IS-95 system, and this was done for the purpose of backward compatibility with IS-95. In case of W-CDMA, the chip rate was chosen so that it would be backward compatible with GSM and the system in use in Japan. Of course, there are many other differences between the two systems, but perhaps none as divisive as the choice of chip rate. All three systems, and all other systems proposed to ITU for that matter, offer users a variety of transmission rates up to 2Mbits/sec. In fact, all sixteen RTT proposals (including the six satellite RTT proposals) meet all the ITU requirements for the IMT- 2000 standard. Whether a single standard will emerge from the ITU standardization effort depends on a number of factors, some of which are not technical. For example, the chip rate difference is due to the strong desire of the cellular system operators (service providers) to protect their investment in second-generation systems. Each operator would like to ensure that the IMT-2000 standard is, to the maximum extent possible, backward compatible with its second-generation system. Of course, it is not possible to keep everybody happy, because, together with the desire for having a system capable of offering worldwide roaming, that would mean expensive base stations and mobile user handsets as well as possibly an inefficient use of the frequency spectrum. This, and some other obstacles in way of having a single IMT-2000 standard for the whole world, is addressed later in this section.

2. IMT-2000 Satellite Proposals: A key difference between a terrestrial wireless system and a satellite wireless system [3GW22] is that it is simpler to provide global coverage with the latter. So, global roaming is a major selling point for the satellite systems. The drawbacks are the price of the service, the delay in communicating back and forth with a satellite in the earth orbit, and perhaps the reduced overall system capacity. It is very expensive to launch satellites to put base stations in the orbit. The satellite phones are also substantially more expensive than their terrestrial counterparts. The price difference depends on how high the satellite is in the orbit. The round trip delay of communicating with the satellite can lower the quality of service in real-time applications such as voice conversations and video teleconferencing. The reduction in capacity is due to the fact that a down-link satellite beam will cover a large geographic area on the earth. That makes frequency reuse difficult. Therefore, despite the huge investment of a number of companies in developing and deploying satellite systems for commercial cellular phone users, it is not expected that such systems would have nearly as large a market penetration as the terrestrial systems. It is sufficient to mention that users are expected to be charged anywhere from $3 to $6 per minute for phone calls made on a satellite mobile phone, and this is about ten times more expensive than the rate on terrestrial cellular systems. Therefore, mobile satellite phones will be attractive to users needing telecommunication capability and network connectivity in remote areas of the world or places that do not even have wireline phone service. This means that the market will require a class of customers for which price is not an issue or will require penetration into developing countries that do not have phone service. The percentage of the population in such countries that can afford using a satellite mobile phone and the extent of usage remains to be seen.

        There is no strong reason that the six satellite RTT proposals should harmonize and merge into a single standard. Since they offer global coverage, it is perfectly OK for them to coexist as long as they can share the available frequency band. Given the high price of launching satellites and maintaining these systems, this is going to be a field for a small number of large players. From a business point of view, it may turn out that only one or two systems survive. For the satellite system operators, the ITU standardization process is like a seal of approval that may increase their chances of grabbing a larger share of the market. This seal of approval is not as crucial for them as it is for terrestrial wireless system operators.