CDMA, GSM and AMPS mobile network technologies
Telstra (Australia) has announced the introduction of a CDMA digital mobile phone network from mid-1999 to eventually replace the existing analogue AMPS network.
The difference between each technology is not at all obvious, so the following extracts from a number of documents have been put together to explain some interesting aspects of each technology. For more information, use the links provided.
Telstra selects CDMA for new mobile network
Telstra announced today its new national mobile network that will replace the old analogue (AMPS) network would be based on Code Division Multiple Access (CDMA) technology.
Spectrum allocation: 800MHz
Code Division Multiple Access (CDMA) is a unique RF access technology that separates subscriber calls from one another by code instead of time or frequency. As a result, all available CDMA frequencies can be used in every cell, thereby increasing the total number of available voice channels and the overall system capacity.
What is CDMA (Code Division Multiple Access)?
CDMA is a "spread spectrum" technology, which means that it spreads the information contained in a particular signal of interest over a much greater bandwidth than the original signal.
A CDMA call starts with a standard rate of 9600 bits per second (9.6 kilobits per second). This is then spread to a transmitted rate of about 1.23 Megabits per second. Spreading means that digital codes are applied to the data bits associated with users in a cell. These data bits are transmitted along with the signals of all the other users in that cell. When the signal is received, the codes are removed from the desired signal, separating the users and returning the call to a rate of 9600 bps.
Principles of CDMA
THE GOAL OF SPREAD SPECTRUM is a substantial increase in bandwidth of an information-bearing signal, far beyond that needed for basic communication. The bandwidth increase, while not necessary for communication, can mitigate the harmful effects of interference, either deliberate, like a military jammer, or inadvertent, like co-channel users. The interference mitigation is a well-known property of all spread spectrum systems. However the cooperative use of these techniques in a commercial, non-military, environment, to optimize spectral efficiency was a major conceptual advance.
SPREAD SPECTRUM systems generally fall into one of two categories: frequency hopping (FH) or direct sequence (DS). In both cases synchronization of transmitter and receiver is required. Both forms can be regarded as using a pseudo-random carrier, but they create that carrier in different ways.
FREQUENCY HOPPING is typically accomplished by rapid switching of fast-settling frequency synthesizers in a pseudo-random pattern. The references can be consulted for further discussions of FH, which is not a part of commercial CDMA.
CDMA uses a form of direct sequence. Direct sequence is, in essence, multiplication of a more conventional communication waveform by a pseudonoise (PN) ±1 binary sequence in the transmitter.
Leap Wireless International Operating Company OzPhone Pty. Ltd. Acquires Additional Spectrum in Australia OzPhone Plans to Offer Service in Australia in Second Half of 1999.
Wireless communications carrier Leap Wireless International, Inc. today announced that one of its operating companies, OzPhone Pty. Ltd., has acquired two additional cellular licenses in Australia. The properties, Remote West and Remote Queensland, are in addition to the eight licenses OzPhone purchased in Australia in May 1998. OzPhone's portfolio now totals ten licenses covering 6 million potential customers (POPs) in Brisbane, Perth, Cairns, Mackay, Maryborough, Grafton, Tasmania, Regional West, Remote West and Remote Queensland regions. OzPhone expects to provide digital wireless services using Code Division Multiple Access (CDMA) technology starting in the second half of 1999.
Multiple access and channel structure
Since radio spectrum is a limited resource shared by all users, a method must be devised to divide up the bandwidth among as many users as possible. The method chosen by GSM is a combination of Time- and Frequency-Division Multiple Access (TDMA/FDMA). The FDMA part involves the division by frequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart. One or more carrier frequencies are assigned to each base station. Each of these carrier frequencies is then divided in time, using a TDMA scheme. The fundamental unit of time in this TDMA scheme is called a burst period and it lasts 15/26 ms (or approx. 0.577 ms). Eight burst periods are grouped into a TDMA frame (120/26 ms, or approx. 4.615 ms), which forms the basic unit for the definition of logical channels. One physical channel is one burst period per TDMA frame.
Global System for Mobile communication
GSM (Global System for Mobile communication) is a digital mobile telephone system that is widely used in Europe and other parts of the world. GSM uses a variation of time division multiple access (TDMA) and is the most widely used of the three digital wireless telephone technologies (TDMA, GSM, and CDMA). GSM digitizes and compresses data, then sends it down a channel with two other streams of user data, each in its own time slot. It operates at either the 900 Mhz [the current Australian allocation] or 1800 MHz frequency band.
GSM is the de facto wireless telephone standard in Europe and, according to IEEE Spectrum, is being used in more than 150 networks in 86 countries. Since many GSM network operators have roaming agreements with foreign operators, users can often continue to use their mobile phones when they travel to other countries.
Why can't I use my phone more the 35 km's from a base station?
Basically once the Base Station is more then 35 km's away the TDMA signalling fails because of the time required to reply.
To allow maximum number of users access, each band is subdivided into 124 carrier frequencies spaced 200KHz apart, using FDMA techniques. By applying TDMA techniques, each of these carrier frequencies is further subdivided into time slots which provide each user with the carrier frequency for approximately 0.577ms. This equates to approx 217 jumps per second, but amongst a very small frequency range so encryption is a must for proper security of calls. In fact it is not exactly that, it is hopping 13 times every 60 ms, which gives 13/0.06 per second. 0.577ms = 13 frames/60 ms /8 time slots
As soon as you get more then 35km from the base station, the phone cannot respond in time (eg in that time slot) so the base station starts handling another call.
What is the difference between GSM, DCS and PCS?
Really nothing at all. All DCS and PCS are is a different technical implementations of the GSM standard - namely GSM 900, DCS 1800 and PCS1900. All of these systems use GSM technology, but they operate at different frequencies. GSM900 operates at 900MHz , DCS1800 operates at 1800 MHz and PCS 1900 operates at 1900 Mhz.
Spectral allocations are in the 800-900 MHz region.
Several hundred channels are available within the spectrum allocation. One channel of one base station is used for each conversation. Upon handoff, the subscriber station is directed via messaging to discontinue use of the old channel and tune to the new one, on which it will find the new cell.
AMPS (Advanced Mobile Phone Service)
AMPS (Advanced Mobile Phone Service) is a standard system for analog signal cellular telephone service in the United States and is also used in other countries. It is based on the initial frequency spectrum allocation for cellular service by the Federal Communications Commission (FCC) in 1970. Introduced by AT&T in 1983, AMPS became and currently still is the most widely deployed cellular system in the United States.
AMPS allocates frequency ranges within the 800 and 900 Megahertz (MHz) spectrum to cellular telephone. Each service provider can use half of the 824-849 MHz range for receiving signals from cellular phones and half the 869-894 MHz range for transmitting to cellular phones. The bands are divided into 30 kHz sub-bands, called channels. The receiving channels are called reverse channels and the sending channels are called forward channels. The division of the spectrum into sub-band channels is achieved by using frequency division multiple access (FDMA).
The signals received from a transmitter cover an area called a cell. As a user moves out of the cell's area into an adjacent cell, the user begins to pick up the new cell's signals without any noticeable transition. The signals in the adjacent cell are sent and received on different channels than the previous cell's signals to so that the signals don't interfere with each other.