Multiple Access Technologies for B3G Wireless

Some general discussions on multiple access technologies were given in some of the earlier chapters,
such as Chapters 2 and 6. In this chapter, the multiple access technologies suitable for beyond 3G
wireless communications are discussed. Before proceeding to the discussions, we would like to take
a brief look at the history of wireless communications under the context of the multiple access
technologies.
Multiple access is always an important issue that should be addressed carefully in the design of
any wireless communication systems. Many peculiar properties of a wireless transmission medium, as
discussed in Chapter 2, make it critical to choose appropriate multiple access technologies, ensuring
an efficient and yet fair sharing of precious radio spectrum resources in a wireless communication
system.
In Chapter 3, we discussed a variety of 3G standards for mobile cellular communication systems.
It was seen from the discussions that the evolution of multiple access technologies has been driven by
the need to deliver increasingly high-data-rate and multimedia services. The first-generation mobile
cellular systems operated mainly on analog electronics based on the Frequency-Division Multiple
Access (FDMA) technology. At that time, the mobile cellular systems were voice application oriented.
The ultimate requirements of the systems then were to provide customers with a satisfactory voice
quality at a reasonable cost. The simple idea of separating users via different frequency channels
could hardly offer a very high capacity to bring down the operation cost of mobile cellular systems.
Analog radio technology was not concerned with the issue of data transmission rate, and thus only
voice channels per MHz were an important merit parameter of the systems.
It so happened that the demand for mobile voice communications effectively saturated the capacity
of the entire analog cellular networks. This was a strong push to search for a new and more effective
multiple access technology to replace the legacy FDMA technology, as an effort to support more users
within a limited radio spectrum bandwidth. Time-Division Multiple Access (TDMA) was put forward
as the right choice to meet the needs of the second-generation mobile cellular systems, which were
proposed as an effort to make international roaming possible. The TDMA scheme works on digital
mobile cellular architectures, which also become much more complex than FDMA-based systems.
Unlike FDMA, the TDMA technology works on the idea that the transmission time in a cell is divided
into many frames, each of which consists of many short time slots. The signal transmissions from all
users need to be synchronized in time, and every active user is assigned a particular time slot in the frame for its transmission. A specific user will transmit only at a time slot assigned to it, and should
refrain from transmission until the same slot appears in the next frame, and so on. Therefore, the
number of users a cell can support is exactly equal to the number of time slots available in a complete
signal frame. The on-and-off transmission nature in each user makes it easier for a TDMA system to
adopt digital transmission technologies. The Global System for Mobile (GSM) communication and
IS-54B (and later IS-136) standards were proposed on the basis of TDMA technologies.
It is noted that the IS-136 system was proposed at almost the same time as the IS-95A, which
took a very different path from that of GSM and IS-136 systems. The IS-95A standard adopted Code
Division Multiple Access (CDMA) technology as its multiple access scheme. CDMA technology was
developed from Spread Spectrum transmission technology, which had been used mainly in military
applications for a long time before the 1970s. As discussed in Subsection 2.3.3, CDMA makes use
of the orthogonality or the quasi-orthogonality of signature codes to divide users in a cell. Thus,
different users should be assigned different codes, which should maintain acceptably low crosscorrelation
functions (CCFs) between any two codes. Like TDMA technology, CDMA should also
be implemented by digital technology, and should provide many unique desirable features that are
otherwise impossible when using other multiple access technologies, as discussed in Subsection 2.3.3.
CDMA technology has become a prime multiple access technology in third-generation wireless
communication systems. Almost all 3G standards submitted to ITU as candidate proposals of the
IMT-2000 system chose CDMA as their multiple access technology. Three major 3G standards,
CDMA2000, WCDMA, and TD-SCDMA have been discussed in Sections 3.1, 3.2, and 3.3 respectively.
It is to be noted that the CDMA technologies used in all the 3G standards share almost the same
core technologies as those introduced by IS-95A. There was no technological revolution in them. It
is regretful to say so here, but it is true. For instance, the channelization codes used in WCDMA and
TD-SCDMA standards are Orthogonal Variable Spreading Factor (OVSF) codes, which is, in fact,
exactly the same as the Walsh-Hadamard codes used by the IS-95A. Therefore, many people agree
that the proposals for 3G standards were made in too short a time frame to choose technologically
right solutions. In other words, it has been suggested by many people that the development of the
3G standards were somehow driven by politics rather than by technologies. The triggering factor was
the competition between two Asian countries, Japan and Korea, who have a history of enmity with
each other. Japan worried about the fact that Korea was one step ahead in developing CDMA-based
technologies, and thus pushed hard on Europe to jointly propose a WCDMA standard in a hurry.
The worldwide 3G standardization activities might be a different story if Korea had not decided to
purchase Qualcomm CDMA technologies in the early 1990s.
After having reviewed the 1- to 3G mobile cellular standards from a perspective of multiple
access technologies, we would like to talk about the scenarios beyond 3G wireless applications. In
fact, the services for all 3G systems have been very different from those offered in the 2G systems,
which concentrate on voice-centric applications. 3G networks should carry a lot of multimedia traffic
or contents, such as videoconferencing, digital TV broadcast, DVD quality interactive gaming, and so
on. Therefore, it is foreseeable that a direct impact of multiple access technologies on the B3G systems
is the capability of ensuring efficient and fair sharing of a limited radio spectra among concurrent
wideband transactions. Therefore, it is very challenging to design a multiple access scheme for
future B3G wireless applications. More discussions on B3G multiple access technologies for wireless
communications can also be found in [551–561, 707, 764–766, 769–771].

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