Detailed presentation of telecom technologies, GSM, CDMA 3G and 4G networks design and implementation
Detailed presentation of telecom technologies, GSM, CDMA 3G and 4G networks design and implementation
CDMA & GSM -Jnanesh M S
Overview Electromagnetic radiation and spectrum Overview of radio communication CDMA overview GSM overview
EM Radiation and Spectrum Electromagnetic radiation (often abbreviated E-M radiation or EMR) is an ubiquitous phenomenon that takes the form of self-propagating waves in a vacuum or in matter It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation. Electromagnetic radiation is classified into several types according to the frequency of its wave; these types include (in order of increasing frequency and decreasing wavelength): radio waves, microwaves, terahertz radiation, infrared radiation visible light, ultraviolet radiation, X-rays and gamma rays EM radiation carries energy and momentum that may be imparted to matter with which it interacts.
Radio Communication Radio waves" transmit music, conversations, pictures and data invisibly through the air, often over millions of miles -- it happens every day in thousands of different ways All radios today, however, use continuous sine waves to transmit information (audio, video, data). The reason that we use continuous sine waves today is because there are so many different people and devices that want to use radio waves at the same time Any radio setup has two parts: The transmitter The receiver The transmitter takes some sort of message (it could be the sound of someone's voice, pictures for a TV set data for a radio modem or whatever), encodes it onto a sine wave and transmits it with radio waves. The receiver receives the radio waves and decodes the message from the sine wave it receives. Both the transmitter and receiver use antennas to radiate and capture the radio signal
Radio communication(contd) A cell phone is also a radio and is a much more sophisticated device A cell phone contains both a transmitter and a receiver, can use both of them simultaneously, can understand hundreds of different frequencies, and can automatically switch between frequencies. Here are some of the important characteristics of a typical analog cell phone: Modulation: Frequency Modulation (FM) Frequency range: 800 MHz Number of frequencies: 1,664 (832 per provider, two providers per area) Transmitter power: 3 watts A typical cell phone contains both a transmitter and a receiver, and both operate simultaneously on different frequencies. A cell phone communicates with a cell phone tower and can transmit 2 or 3 miles (3-5 km).
Radio communication(contd) You can get an idea for how a radio transmitter works by starting with a battery and a piece of wire. In How Electromagnets Work, you can see that a battery sends electricity (a stream of electrons) through a wire if you connect the wire between the two terminals of the battery. The moving electrons create a magnetic field surrounding the wire, and that field is strong enough to affect a compass.
Radio communication(contd) Let's say that you take another wire and place it parallel to the battery's wire but several inches (5 cm) away from it
Radio communication(contd) To create a simple radio transmitter, what you want to do is create a rapidly changing electric current in a wire. You can do that by rapidly connecting and disconnecting a battery, like this: By creating a sine wave and running it through a wire, you create a simple radio transmitter.By sending that signal to an antenna, you can transmit the sine wave into space.
Radio communication (contd) Transmitting Information Pulse Modulation Amplitude Modulation Frequency Modulation
CDMA Code division multiple access (CDMA) is a channel access method utilized by various radio communication technologies One of the basic concepts in data communication is the idea of allowing several transmitters to send information simultaneously over a single communication channel This allows several users to share a bandwidth of different frequencies. This concept is called multiplexing. CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel An analogy to the problem of multiple access is a room (channel) in which people wish to communicate with each other. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different languages (code division). CDMA is analogous to the last example where people speaking the same language can understand each other, but not other people. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can understand each other.
CDMA (contd) CDMA Modulation Data for transmission is simply logically XOR (exclusive OR) added with the faster code. The figure shows how spread spectrum signal is generated The data signal with pulse duration of Tb is XOR added with the code signal with pulse duration of Tc. (Note:bandwidth is proportional to 1 / T where T = bit time) Therefore, the bandwidth of the data signal is 1 / Tb and the bandwidth of the spread spectrum signal is 1 / Tc. Since Tc is much smaller than Tb, the bandwidth of the spread spectrum signal is much larger than the bandwidth of the original signal. The ratio Tb / Tc is called spreading factor or processing gain and determines to certain extent the upper limit of total number of users supported simultaneously by a base station
CDMA (contd) Each user in a CDMA system uses a different code to modulate their signal. Choosing the codes used to modulate the signal is very important in the performance of CDMA systems. The best performance will occur when there is good separation between the signal of a desired user and the signals of other users. The separation of the signals is made by correlating the received signal with the locally generated code of the desired user The best performance will occur when there is good separation between the signal of a desired user and the signals of other users. The separation of the signals is made by correlating the received signal with the locally generated code of the desired user If the desired user's code has nothing in common with the signal the correlation should be as close to zero as possible (thus eliminating the signal); this is referred to as cross correlation
Basic categories in CDMA Code Division Multiplexing (Synchronous CDMA) Uses the concept of orthogonalitybetweenvectors binary string "1011" is represented by the vector (1, 0, 1, 1). Vectors can be multiplied by taking their dot product by summing the products of their respective components. If the dot product is zero, the two vectors are said to be orthogonal to each other (note: if u=(a,b) and v=(c,d), the dot product u·v = a*c + b*d) Each user in synchronous CDMA uses a code orthogonal to the others' codes to modulate their signal. An example of four mutually orthogonal digital signals shown below Some properties of the dot product aid understanding of how W-CDMA works. If vectors a and b are orthogonal, then
CDMA (contd) Each user is associated with a different code, say v. If the data to be transmitted is a digital zero, then the actual bits transmitted will be –v, and if the data to be transmitted is a digital one, then the actual bits transmitted will be v. For example, if v=(1,–1), and the data that the user wishes to transmit is (1, 0, 1, 1) this would correspond to (v, –v, v, v) which is then constructed in binary as ((1,–1),(–1,1),(1,–1),(1,–1)). For the purposes of this article, we call this constructed vector the transmitted vector. Each sender has a different, unique vector v chosen from that set, but the construction method of the transmitted vector is identical. If sender0 has code (1,–1) and data (1,0,1,1), and sender1 has code (1,1) and data (0,0,1,1), and both senders transmit simultaneously, then this table describes the coding steps:
CDMA (contd) Encoding: Because signal0 and signal1 are transmitted at the same time into the air, they add to produce the raw signal:(1,–1,–1,1,1,–1,1,–1) + (–1,–1,–1,–1,1,1,1,1) = (0,–2,–2,0,2,0,2,0) Decoding: Further, after decoding, all values greater than 0 are interpreted as 1 while all values less than zero are interpreted as 0. For example, after decoding, data0 is (2,–2,2,2), but the receiver interprets this as (1,0,1,1).
Advantages of CDMA A frequency (channel) can be used again within an FDMA or TDMA network, but cells using the same frequency must be separated by an appropriate distance.Adjacent cells must be assigned a different set of frequencies. For example, a cell using frequency A must not be adjacent to another cell using frequency A. As a result, each cell site in the site is able to use only 1/7 of the possible frequencies.
Advantages CDMA CDMA employs the spread spectrum techniques to transmit the signal. Spread-spectrum techniques are methods by which electromagnetic energy generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference and jamming, to prevent detection Frequency reuse is the ability to reuse the same radio channel frequency at other cell sites within a cellular system In the FDMA and TDMA systems frequency planning is an important consideration. The frequencies used in different cells need to be planned carefully in order to ensure that the signals from different cells do not interfere with each other In a CDMA system the same frequency can be used in every cell because channelization is done using the pseudorandom codes. Reusing the same frequency in every cell eliminates the need for frequency planning in a CDMA system; however, planning of the different pseudorandom sequences must be done to ensure that the received signal from one cell does not correlate with the signal from a nearby cell.
Advantages of CDMA Since adjacent cells use the same frequencies, CDMA systems have the ability to perform soft handoffs. Soft handoffs allow the mobile telephone to communicate simultaneously with two or more cells. The best signal quality is selected until the handoff is complete. In a hard handoff situation, as the mobile telephone approaches a handoff, signal strength may vary abruptly. In contrast, CDMA systems use the soft handoff, which is undetectable and provides a more reliable and higher quality signal.
GSM GSM- Global System for mobile communication. In 1982, the European Conference of Postal and Telecommunications Administrations (CEPT) created the GroupeSpécial Mobile (GSM) to develop a standard for a mobile telephone system that could be used across Europe GSM makes use of two principles: -Time division Multiplexing -Frequency Division Multiplexing Time division Multiplexing - radio frequency say 890 Mhz is shared by different users in time - This means if user A, B, C and D all talk at the same time. You assign the 890 Mhz frequency to A for some time and allow him to talk, then you assign 890 band to B for sometime to speak, then to C , and finally to D, before coming back to A - This way many users talk at same time on the same frequency. This has to be done, because as we now frequency or Bandwidth is a scarce resource and is not available in plentiful, so it must be shared.`
GSM(Contd) Frequency division multiplexing (FDMA) - In Frequency Division Multiplex, users A, B, C and D, all use different frequency say 890, 900, 910 , 920 for their respective communications - A very good example of this is Radio broadcasting. Because all the radio operators like Rad FM, Go FM, Radio Mirchi want to operate in the same area, they use different frequencies for communication 91.0FM, 93.5FM, 94.6 FM, 108FM. So to listen to different communications, you have to tune in the receiver set to different frequencies
GSM(Contd) GSM uses a combination of TDMA and FDMA This means that users A and B are not only sharing the channel in time but also frequency This means that user A is on the channel 890Mhz for 2 seconds, then jumps to 900Mhz channel for the next to seconds, then jumps to 910Mhz for the next 2 seconds and so on Thus, each user is uses a different frequency at different time slots. This is called Frequency Hopping CDMA in comparison with GSM
Extended reach - beneficial to rural users situated far from cells.
Disadvantages of CDMA include: - Due to its proprietary nature, all of CDMA's flaws are not known to the engineering community. - CDMA is relatively new, and the network is not as mature as GSM. - CDMA cannot offer international roaming, a large GSM advantage.
GSM is already used worldwide with over 450 million subscribers.
International roaming permits subscribers to use one phone throughout Western Europe. CDMA will work in Asia, but not France, Germany, the U.K. and other popular European destinations.
GSM is mature, having started in the mid-80s. This maturity means a more stable network with robust features. CDMA is still building its network.
GSM's maturity means engineers cut their teeth on the technology, creating an unconscious preference.
- The availability of Subscriber Identity Modules, which are smart cards that provide secure data encryption give GSM m-commerce advantages
Conclusion Today, the battle between CDMA and GSM is muddled. Where at one point Europe clearly favoured GSM and North America, CDMA, the distinct advantage of one over the other has blurred as major carriers like AT&T Wireless begin to support GSM, and recent trials even showed compatibility between the two technologies. GSM still holds the upper hand however. There's the numerical advantage for one thing: 456 million GSM users versus CDMA's 82 million.