Cellular Phone Technologies Comparision

Encoding and Multiplexing

Overview

With thousands of cellular phone calls going on at any given time within a city, it certainly would not work for everyone to talk on the came channel at once (as in CB and short-wave radios). Therefore, several different techniques were developed by cell phone manufacturers to split up the available bandwidth into many channels each capable of supporting one conversation. The following sections will discuss each technology and how it works.

Analog vs. Digital

While the distinction between analog and digital encoding is probably obvious to most readers, a short discussion is included for those who are not. Essentially, analog broadcasts audio as a series of continuously changing, voltage levels representing the amplitude of the voice conversation. When sent on the cell phone network using the standard frequency modulation (meaning voltage levels translate into frequency shifts) into channels separated by 30 kHz, we find that the amplitude can be effectively transmitted at 15 kHz due to Nyquist limitations.

Instead of sending data as various voltage levels, a digital signal quantizes the voltage levels into a number of bins (typically 28 or 256 representing an 8-bit encoding). These bins are encoded as a binary number and sent as a series of ones and zeros. This allows for digital compression in the encoding stage enabling voice to be sent at as little as 8000 bits per second.

FDMA

FDMA stands for "frequency division multiple access" and, though it could be used for digital systems, is exclusively used on all analog cellular systems. Essentially, FDMA splits the allocated spectrum into many channels. In current analog cell systems, each channel is 30 kHz. When a FDMA cell phone establishes a call, it reserves the frequency channel for the entire duration of the call. The voice data is modulated into this channel’s frequency band (using frequency modulation) and sent over the airwaves. At the receiver, the information is recovered using a band-pass filter. The phone uses a common digital control channel to acquire channels.

FDMA systems are the least efficient cellular system since each analog channel can only be used by one user at a time. Not only are these channels larger than necessary given modern digital voice compression, but they are also wasted whenever there is silence during the cell phone conversation. Analog signals are also especially susceptible to noise – and there is no way to filter it out. Given the nature of the signal, analog cell phones must use higher power (between 1 and 3 watts) to get acceptable call quality. Given these shortcomings, it is easy to see why FDMA is being replaced by newer digital techniques.

TDMA

TDMA stands for "time division multiple access." TDMA builds on FDMA by dividing conversations by frequency and time. Since digital compression allows voice to be sent at well under 10 kilobits per second (equivalent to 10 kHz), TDMA fits three digital conversations into a FDMA channel (which is 30 kHz). By sampling a person’s voice for, say 30 milliseconds, then transmitting it in 10 milliseconds; the system is able to offer 3 timeslots per channel in a round-robin fashion. This technique allows compatibility with FDMA while enabling digital services and easily boosting system capacity by three times.

While TDMA is a good digital system, it is still somewhat inefficient since it has no flexibility for varying digital data rates (high quality voice, low quality voice, pager traffic) and has no accommodations for silence in a telephone conversation. In other words, once a call is initiated, the channel/timeslot pair belongs to the phone for the duration of the call. TDMA also requires strict signaling and timeslot synchronization. A digital control channel provides synchronization functionality as well as adding voice mail and message notification. Due to the digital signal, TDMA phones need only broadcast at 600 miliwatts.

CDMA

CDMA stands for "code division multiple access" and is both the most interesting and the hardest to implement multiplexing method. CDMA has been likened to a party: When everyone talks at once, no one can be understood, however, if everyone speaks a different language, then they can be understood. CDMA systems have no channels, but instead encodes each call as a coded sequence across the entire frequency spectrum. Each conversation is modulated, in the digital domain, with a unique code (called a pseudo-noise code) that makes it distinguishable from the other calls in the frequency spectrum. Using a correlation calculation and the code the call was encoded with, the digital audio signal can be extracted from the other signals being broadcast by other phones on the network. From the perspective of one call, upon extracting the signal, everything else appears to be low-level noise. As long as there is sufficient separation between the codes (said to be mutually orthogonal), the noise level will be low enough to recover the digital signal. Each signal is not, in fact, spread across the whole spectrum (12.5 MHz for traditional cellular or 60 MHz in PCS cellular), but is spread across 1.25 MHz "pass-bands."

CDMA systems are the latest technology on the market and are already eclipsing TDMA in terms of cost and call quality. Since CDMA offers far greater capacity and variable data rates depending on the audio activity, many more users can be fit into a given frequency spectrum and higher audio quality can be provide. The current CDMA systems boast at least three times the capacity of TDMA and GSM systems. The fact that CDMA shares frequencies with neighboring cell towers allows for easier installation of extra capacity, since extra capacity can be achieved by simply adding extra cell sites and shrinking power levels of nearby sites. CDMA technology also allows lower cell phone power levels (200 miliwatts) since the modulation techniques expect to deal with noise and are well suited to weaker signals. The downside to CDMA is the complexity of deciphering and extracting the received signals, especially if there are multiple signal paths (reflections) between the phone and the cell tower (called multipath interference). As a result, CDMA phones are twice as expensive as TDMA phones and CDMA cell site equipment is 3-4 times the price of TDMA equivalents.

GSM

GSM stands for "Global System for Mobile Communications." GSM is mostly a European system and is largely unused in the US. GSM is interesting in that it uses a modified and far more efficient version of TDMA. GSM keeps the idea of timeslots and frequency channels, but corrects several major shortcomings. Since the GSM timeslots are smaller than TDMA, they hold less data but allow for data rates starting at 300 bits per second. Thus, a call can use as many timeslots as necessary up to a limit of 13 kilobits per second. When a call is inactive (silence) or may be compressed more, fewer timeslots are used. To facilitate filling in gaps left by unused timeslots, calls do "frequency hopping" in GSM. This means that calls will jump between channels and timeslots to maximize the system’s usage. A control channel is used to communicate the frequency hopping and other information between the cell tower and the phone. To compare with the other systems, it should be noted that GSM requires 1 Watt of output power from the phone.

Call Handoff

It is apparent that cells must somehow overlap, and when a user travels between cells, one cell must hand the call off to the other cell. The cells must also not interfere with each other. This is accomplished by giving each cell a slightly different chunk of the frequency spectrum (note that CDMA does not do this) and by measuring power levels. When the power level of the user begins to fade, the cell tower determines which cell is the closest cell. Upon finding this information, the current cell tower sends an over-the-air message to the new cell tower and to the cell phone. At this point, the new cell tower picks up the call and the old one drops the call as the cell phone switches frequencies. This type of handoff is called a "hard handoff" since the audio feed is lost for between 10 milliseconds and 100 milliseconds while the new tower picks up the signal. Often these "hard" handoffs fail when the new tower tries to pick the call up, leading to frequent dropped calls.

In most systems, each cell tower typically receives a 1.8 MHz frequency spectrum. In normal cellular systems that have a 12.5 MHz spectrum (not the high-band PCS systems that have more bandwidth), this allows for 7 cells before cells have to reuse frequencies. Generally, there are 1-2 cells and 10-20 miles separating cells using the same frequency in order to minimize interference.

Security

One of the largest problems in wireless communication is security. There are two worries: Other people listening into phone calls and other people illegally billing time to a user’s account (called "phone cloning").

Unfortunately, analog phones transmit in plain FM, and provide no security. For instance, a few years ago, Newt Gingrich had a cell phone conversation taped by someone using a simple police scanner, which is designed to receive police activity on the CB frequencies. Since analog phones have such weak security, the architects of digital technology designed digital phones with much more robust security.

Digital phones employ encryption to secure the phone and the conversation. Encryption is used in TDMA and CDMA to make sure that it is almost impossible to "latch" onto a conversation. The encryption works by picking a key that is used in an equation that compresses the audio. The encrypted key is sent to the cell tower so the cell tower knows how to decode the conversation. Therefore, even if the person with the scanner finds the channel and time slice you are using, they would need to find the encryption code to make sense of the signal. It is also important to mention that CDMA also uses its modulation code to provide increased security, resulting in over four billion possible encryption codes. Cell phones also must be protected from cloning. By encrypting the cell phone number and related information when sending the information to the switch, cloning is prevented.

Source:

How Cellular Phone Technologies Compare Best of the Net Very good and concise explanation of how cell phone networks work and what are the differences between CDMA, TDMA, FDMA and GSM.