Multiple Access Schemes for Mobile Phones

In the late 1800’s, a Scottish physicist named James Clerk Maxwell formulated a principle that would forever change our world. Maxwell was able to show that the generalized forms of the laws of electricity and magnetism–the laws of Coulomb, Gauss, Biot-Savart, Ampere, and Faraday–suggested the existence electromagnetic (EM) waves [1]. Electromagnetic waves have both an electric and magnetic field component that propagate through space, similar to how a sound wave propagates through air or water. Maxwell’s theory has since proven true and has been put to great use. His work catalyzed the development of EM wave transmitters and receivers, eventually leading to the creation of mobile phones.

For better or worse, the world entered the mobile telephony era in the 1980’s and hasn’t looked back. The transmission and reception of EM waves are only a part of how the mobile phone networks allow the public to make calls and send text messages (see Fig. 1). The information, whether voice, word, or image, must be encoded and decoded in such a way that the user receives only what he or she was intended to receive. This is accomplished through what are known as multiple access schemes. Numerous such schemes have been developed, each with their own advantages and disadvantages. In the design of a cellular communication system, engineers must consider bandwidth and how many subscribers it can support, which thereby determines how much information can be sent over a given time period. In the world today, there are two multiple access schemes that play an important role in current and future mobile technologies: the Global System for Mobile communication (GSM) and the Code Division Multiple Access scheme (CDMA).

The roots of CDMA technology are in the military field and navigation systems. Originally developed to counteract intentional radio jamming, it was later proved to be suitable for cellular communications [2]. In 1949, John Pierce wrote a technical memorandum that described a multiple access system that used a common medium that carries a coded signal that didn’t need to be synchronized. Later that year, Claude Shannon and Robert Pierce developed the basic operational ideas for the CDMA scheme. Following these developments, other theoretical and technological discoveries were made that led to Qualcomm’s investigation into the use of CDMA techniques, beginning with the introduction of narrowband CDMA IS-95 standards in July of 1993 [2].

In 1982, the Groupe Special Mobile was established by the European Conference of Postal and Telecommunications Administrations (CEPT) to create and implement a new common standard for mobile phone networks in Europe. This new standard would be implemented in the twelve countries that signed on to CEPT [3]. The GSM standard used a digital system to represent and transmit voice and user rate data through a Time Division Multiple Access scheme (TDMA). Since GSM provided a common system for which products could be designed, a low-cost environment in which to operate, and was usable over a large geographical region, this new standard excited both the manufacturing industry and network operators. At its initial launch in 1992, GSM was renamed Global System for Mobile Communication; it has been implemented in more than 100 countries in the world, including the United States, Australia, and South Africa.

The CDMA scheme is used primarily in the United States, and in some Asian countries. Major mobile telephony providers Sprint PCS and Verizon Wireless implement a version of the CDMA system. In the US, commercial networks use a version of the GSM system that operates at higher frequency ranges than the European system due to bandwidth allocations already in place. Companies using GSM include AT&T Wireless and Cingular Wireless [4].

While both the CDMA and GSM schemes represent digital data, the methods in which they encode and transmit the information are strikingly different. GSM uses a version of time division multiple access (TDMA) to divide up frequencies in the electromagnetic spectrum for multiple users to access the mobile system [5]. In its implementation, CDMA utilizes a version of frequency division multiplexing. To understand these two multiple access schemes (CDMA and GSM), the generation systems of frequency division multiple access (FDMA) and TDMA need to be discussed briefly.

Frequency division multiple access is a common access scheme, not only found in mobile networks. In FDMA, a frequency band is divided into several smaller channels of equal bandwidth. This allows the mobile network to send a separate conversation on each of the smaller channels [5]. An analogy of this would be standing in a room full of people while each person is talking directly to only one other person the entire time. FDMA is used for mobile radio, but it is not a practical as the main access scheme for mobile phone networks. In order for a cellular base-station to receive and transmit the FDMA signal, each channel would need its own transceiver unit. In addition, a frequency band is limited in the number of channels it can be divided up into, limiting the number of users a system can accommodate.

Time division multiple access is one of the preferred second-generation (2G) schemes used today for mobile networks [5]. The TDMA technique takes a time period and divides it into several shorter time slots. As each slot passes, a set of data for a specific user is transmitted and received. At the same time, these time slots are divided further using a FDMA scheme, which creates frequency sub-bands. In relation to our previous analogy: the people in the room take turns speaking for equal intervals of time. However, TDMA requires a higher degree of synchronization between the base-stations and mobile phones. The receiving unit must be able to determine at what time period it is to decode the transmission. TDMA also suffers from what is known as frequency-selective co-channel interference. In varying degrees, EM signals always interfere with each other. The signals are sent in bursts, meaning that when they do interfere with each other, some time slots might be clear while others may possess significant interference. This is solved by using a frequency-hopping procedure in which a user’s time slot is sent on one frequency channel at one point in time and another the next.

In comparison, code division multiple access (CDMA) is different than both FDMA and TDMA. In CDMA, the digital signal is encoded with a user-specific code and then transmitted. The result is that the bandwidth of the signal is increased; this is called spreading. When the system transmits the signal, it sends multiple signals out onto a signal channel. Upon receiving the spread spectrum (SS) signal, the receiving device filters the signal, utilizing a user-specific code to isolate the signal meant for that user. This requires less synchronization with the base-stations than with the TDMA scheme. FDMA is used to increase the number of SS signals that can be transmitted by dividing up the frequency band. In the case of CDMA, the people in our room are all talking at once and each person can understand only the person they are talking to; all other conversations are filtered out. This obviously increases the privacy of the signals, since the receiver must know the specific code to recover the original signal [2]. While synchronization with the base-stations is not as rigorous in comparison with TDMA, the received signal and the locally created decoding signal must be synchronized inside the mobile device. Also, there is what is known as the “near-far” problem, in which base-stations receive signals from closer mobile phones at a higher power than those that are farther away. This difference in power causes interference in the CDMA scheme but can be compensated for by using a power control algorithm which adjusts all incoming signals so that they are received at the same power levels.

In the United States, GSM operates on the 1900 megahertz (MHz) band. Elsewhere in the world, GSM operates on the 900 and 1800 MHz bands (see Fig. 2). As mentioned in the operation concept behind GSM, the radio frequency bands are divided into 200 kilohertz (kHz) channel, each of which can accommodate 8 users through time division multiplexing [4]. GSM was originally created with a short message service (SMS), which allows messages of fewer than 160 characters to be sent from user to user, commonly referred to as text messaging. An example text message would be: Where are we meeting? In recent years, abbreviations and acronyms have been developed by mobile phone subscribers to speed up the rate at which they can type messages on the number pad. An example of this is: R U at the PRT?, which translates to “Are you at the party?” The text messages are transmitted and received the same way voice conversations take place, allowing for an alternative form of cellular communication. The size of these messages are often small enough that they can be sent all at once. In our familiar example, instead of speaking one at a time, each person hands the other a slip of paper with a short message.

Data transfer is also possible through the use of circuit-switched data (CSD), which allows for transfer rates of up to 14.4 kilobits per second (kbps). This data transfer system has been changed to High Speed Circuit Switched Data (HSCSD) and General Packet Radio Service (GPRS). These two transfer systems allow for transfer rates up to 57.6 kbps.

CDMA systems operate in the 850 MHz and 1900 MHz bands. These bands are divided into 1.25 MHz channels, which up to 64 users can share at one time. Unlike GSM, CDMA was not originally designed with a text-based message system. However, the first commercial version of CDMA, IS-95A, was capable of transfer rates up to 14.4 kbps. The newer scheme, IS-95B, which was completed in mid-1997, increased the data rate up to 64 kbps [4]. The table provides a brief summary of the operating capabilities of current and future GSM and CDMA systems.

As the number of mobile phone users increases, the need for a new access scheme capable of handling a larger number of subscribers is apparent (see Fig. 3). Not only will the new systems be able to handle more users, but system upgrades are needed to accommodate higher data rates for a more complete information interchange, like video conferencing. Initially, the goal of third generation (3G) wireless was to create an international standard which all systems would be upgraded to meet [4]. However, two different systems have been proposed for the 3G upgrade: Universal Mobile Telecommunications System (UMTS) and CDMA2000.

The CDMA2000 scheme will begin with an upgrade from the current CDMA systems of IS-95A and B to CDMA200 1x, which will support data transfers of 144 kbps. The next upgrade in the code-division system will be to the 1xEV-DO version. This upgrade will require 1.25 MHz carrier channels and will provide a downlink rate of 2.4 Mbps, but only 153 kbps for uplinks [4]. The final version of the 3G CDMA system is promised to have transfer rates up to 3 Mbps.

Mobile phones and related mobile technology and services have become an important part of our modern culture. Both the Global System for Mobile Communication and Code Division Multiple Access schemes have impacted our current and future mobile communications technologies. We are already beginning to see the emergence of 3G systems with the addition of internet access and limited video conferencing to some of the major cellular operators. While there are different multiple access schemes operating in the world, within the next decade a single international standard will be agreed upon and implemented, making our world more efficient.