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PPT ON Wireless Communication using Code Division Multiple Access

Wireless Communication using Code Division Multiple Access (CDMA) ABSTRACT-Code division multiple access (CDMA) is a form of multiplexing (not a modulation scheme) and a method of multiple access that does not divide up the channel by time (as in TDMA), or frequency (as in FDMA), but instead encodes data with a certain code associated with a channel and uses the constructive interference properties of the signal medium to perform the multiplexing. CDMA also refers to digital cellular
telephony systems that make use of this multiple access scheme, such as those pioneered by Qualcomm, or W-CDMA. CDMA system separates users through the assignment of different code sequences. These sequences should have low cross correlation properties and should allow the multipath effects commonly found in wireless channels. CDMA system uses the idea of tolerating interference by Spread spectrum modulation (spreading the spectrum). This paper discusses the concept of CDMA, its various cellular components and its distinguishing features. Also, CDMA systems operating in multipath environment have been simulated considering the thermal noise and interference. The simulation results are analyzed using Racien K factor (Racien K factor is a factor that measures propagation loss in multipath fading environment). 1. INTRODUCTION Multiple access schemes allow multiple users to access the services. One type of multiple access system is, in which a large number of users share a common communication channel to transmit information to a receiver. This can be done by different methods, viz., Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA). Frequency Division Multiple Access (FDMA), First Generation (1G) cellular communication networks is an analogue transmission technique used for mobile phone communications, in which the frequency band allocated to a network is divided into sub-bands or channels. Each frequency channel can carry either a voice conversation or digital data, and one channel will be assigned to each subscriber for the duration of a call. Using FDMA in this way, multiple users can share the available band without the risk of interference between the simultaneous calls. Time Division Multiple Access (TDMA) is digital transmission technology that allows a number of users to access a single radio frequency (RF) channel without interference by allocating unique time slots to each user within each channel. The TDMA digital transmission scheme multiplexes three signals over a single channel. The current TDMA standard for cellular divides a single channel into six time slots, with each signal using two slots, providing a 3 to 1 gain in capacity over advanced mobile-phone service (AMPS). Each caller is assigned a specific time slot for transmission. This technique is used in second generation (2G) cellular communication networks. In Code Division Multiple Access (CDMA), every station uses a code for data transmission, which is only understood by partner stations with an identical code. Code Division Multiple Access (CDMA) has emerged as the mainstream air interface solution for the third-generation networks, since it will provide a multitude of services, especially multimedia and high-bit-rate packet data. 2. BASICS OF CELLULAR COMMUNICATIONS A cellular radio system consists of mobile units linked via a radio network to switching equipment interconnecting the different parts of the system and allowing access to the fixed PSTN. The technology is hidden from view-incorporated in numerous transceivers called Base Stations (BS) which are located at selected strategic places, and covering a given area or cell - hence the term cellular communications. A number of cells grouped together form the "area" and the corresponding Base Stations are connected to a Mobile Switching Centre (MSC) which stores information and directs calls to phones within its area. The various Mobile Switching Centers communicate with each other. The frequencies used by GSM are: For transmission 890-915 MHz For reception 935-960 MHz 2.1. Cellular System Components The cellular system offers mobile and portable telephone stations the same service provided fixed stations over conventional wired loops. It has the capacity to serve tens of thousands of subscribers in a major metropolitan area. The cellular communications system consists of the following four major components that work together to provide mobile service to subscribers. • Public switched telephone network (PSTN) • Mobile telephone switching office (MTSO) • Cell site with antenna system • Mobile subscriber unit (MSU) 2.1.1 PSTN The PSTN is made up of local networks, the exchange area networks, and the long- haul network that interconnect telephones and other communication devices on a worldwide basis. 2.1.2 Mobile Telephone Switching Office (MTSO) The MTSO is the central office for mobile switching. It houses the mobile switching center (MSC), field monitoring, and relay stations for switching calls from cell sites to wire line central offices (PSTN). In analog cellular networks, the MSC controls the system operation. The MSC controls calls, tracks billing information, and locates cellular subscribers. 2.1.3. The Cell Site The term cell site is used to refer to the physical location of radio equipment that provides coverage within a cell. A list of hardware located at a cell site includes power sources, interface equipment, radio frequency transmitters and receivers, and antenna systems. 2.1.4. Mobile Subscriber Units (MSUs) The mobile subscriber unit consists of a control unit and a transceiver that transmits and receives radio transmissions to and from a cell site. The following three types of MSUs are available: • The mobile telephone (typical transmit power is 4.0 watts) • The portable (typical transmit power is 0.6 watts) • The transportable (typical transmit power is 1.6 watts) The mobile telephone is installed in the trunk of a car, and the handset is installed in a convenient location to the driver. Portable and transportable telephones are hand-held and can be used anywhere. The use of portable and transportable telephones is limited to the charge life of the internal battery. 3. CDMA CONCEPTS In CDMA each user is assigned a unique code sequence it uses to encode its information-bearing signal. The receiver, knowing the code sequences of the user, decodes a received signal after reception and recovers the original data. This is possible since the cross correlations between the code of the desired user and the codes of the other users are small. Since the bandwidth of the code signal is chosen to be much larger than the bandwidth of the information-bearing signal, the encoding process enlarges (spreads) the spectrum of the signal and is therefore also known as spread-spectrum modulation. The resulting signal is also called a spread-spectrum signal, and CDMA is often denoted as spread-spectrum multiple access (SSMA). The spectral spreading of the transmitted signal gives to CDMA its multiple access capability. It is therefore important to know the techniques necessary to generate spread-spectrum signals and the properties of these signals. A spread-spectrum modulation technique must fulfill two criteria: The transmission bandwidth must be much larger than the information bandwidth. The resulting radio-frequency bandwidth is determined by a function other than the information being sent (so the bandwidth is statistically independent of the information signal). This excludes modulation techniques like frequency modulation (FM) and phase modulation (PM). The ratio of transmitted bandwidth to information bandwidth is called the processing gain, Gp, of the spread-spectrum system, ; Where Bt is the transmission bandwidth and Bi is the bandwidth of the information-bearing signal. The receiver correlates the received signal with a synchronously generated replica of the spreading code to recover the original information-bearing signal. This implies that the receiver must know the code used to modulate the data. Because of the coding and the resulting enlarged bandwidth, SS signals have a number of properties that differ In CDMA, each phone's data has a unique code. From the properties of narrowband signals. To have a clear understanding, each property has been briefly explained with the help of illustrations, if necessary, by applying direct sequence spread-spectrum techniques. CDMA takes an entirely different approach from TDMA. CDMA, after digitizing data, spreads it out over the entire available bandwidth. Multiple calls are overlaid on each other on the channel, with each assigned a unique sequence code. CDMA is a spread spectrum which simply means that data is sent in small pieces over a number of the discrete frequencies available for use at any time in the specified range.All of the users transmit in the same wide-band chunk of spectrum. Each user's signal is spread over the entire bandwidth by a unique spreading code. At the receiver, that same unique code issued to recover the signal. Because CDMA systems need to put an accurate time-stamp on each piece of a signal, It references the GPS system for this information. Between eight and 10 separate calls can be carried in the same channel space as one analogAMPS call. CDMA technology is the basis for Interim Standard 95 (IS-95) and operates in both the 800-MHz and 1900-MHz frequency bands. Ideally, TDMA and CDMA are transparent to each other. In practice, high-power CDMA signals raise the noise floor for TDMA receivers, and high-power TDMA signals can cause overloading and jamming of CDMA receivers. 3.1 Multiple Access Capability If multiple users transmit a spread-spectrum signal at the same time, the receiver will still be able to distinguish between the users provided each user has a unique code that has a sufficiently low cross-correlation with the other codes. Correlating the received signal with a code signal from a certain user will then only “de spread” the signal of this user, while the other spread-spectrum signals will remain spread over a large bandwidth. Thus, within the information bandwidth the power of the desired user will be larger than the interfering power provided there are not too many interferers, and the desired signal can be extracted. 3.2 Protection Against multi-path interference In a radio channel there is not just one path between a transmitter and receiver. Due to reflections (and refractions) a signal will be received from a number of different paths. The signals of the different paths are all copies of the same transmitted signal but with different amplitudes, phases, delays, and arrival angles. Adding these signals at the receiver will be constructive at some of the frequencies and destructive at others. In the time domain, this results in a dispersed signal. Spread-spectrum modulation can combat this multi-path interference; however, the way in which this is achieved depends very much on the type of modulation used. 3.3 Privacy The transmitted signal can only be dispread and the data recovered if the receiver knows the code. 3.4 Interference Rejection Cross-correlating the code signal with a narrowband signal will spread the power of the narrowband signal thereby reducing the interfering power in the information bandwidth. The spread-spectrum signal (s) receives narrowband interference (i). At the receiver the SS signal is "despread" while the interference signal is spread, making it appear as background noise compared to the de spread signal. 3.5 Anti-Jamming Capability This is more or less the same as interference rejection except the interference is now willfully inflicted on the system. It is this property, together with the next one, that makes spread-spectrum modulation attractive for military applications. 3.6 Low Probability of Interception (LPI) Because of its low power density, the spread-spectrum signal is difficult to detect and intercept by a hostile listener. There are a number of modulation techniques that generate spread-spectrum signals: 3.7 Direct sequence spread-spectrum The information-bearing signal is multiplied directly by a high chip rate code signal. 3.8 Frequency hopping spread-spectrum The carrier frequency at which the information-bearing signal is transmitted is rapidly changed according to the code signal. 3.9 Time hopping spread-spectrum The information-bearing signal is not transmitted continuously. Instead the signal is transmitted in short bursts where the times of the bursts are decided by the code signal. 4. CDMA Features • Narrowband message signal multiplied by wideband spreading signal or pseudo noise code • Each user has his own pseudonoise (PN) code • Soft capacity limit: system performance degrades for all users as number of users increases • Cell frequency reuse: no frequency planning needed • Soft handoff increases capacity • Near-far problem • Interference limited: power control is required • Wide bandwidth induces diversity: rake receiver is used • It would take all the computers ever made as much time as humans have been on earth to crack or decode a single second of CDMA conversation 5. RESULTS A simulation [fig (1, 2)] has been done of CDMA system operating in a multi path/ fading environment. Thermal noise and interference are also included in the simulation model. The simulation model can generate a number of performance results. These include the Bit Error Rate (BER) as a function of the thermal noise level (PE vs Eb/No) with Ricean factor included. Eb /No is stepped from 0-10 dB in one dB steps. Ricean K factor is a factor that measures propagation loss in multi-path fading environment. This factor is included and results are obtained. Three values of Ricean K factor are used -20 dB, 0 dB and 20 dB. K– Factor of -20 dB (k=0.01) produced results close to Raleigh fading. The K-factor of 0 dB (K=1) curve is less smooth than other curves. This is due to time varying nature of the channel, which is more pronounced for intermediate values of K, where the power in the LOS component and total scattered power are approximately equal. 6. REFERENCES [1] John G. Proakis “Digital communication systems” Mc Graw Hill Publications. [2] William C.Y.Lee “Mobile Cellular Telecommunications”, Pearson Education [3] K.S.Shanmugan, “Simulation and Implementation Tools for Signal Processing And Communication Systems” IEEE Communications Magazine July 1994 [4] Kamilo Feher, “Wireless Digital Communications” Modulation and Spread spectrum applications, Printice Hall PTR, NJ 1994 [5] Http:// [6]
PPT ON Wireless Communication using Code Division Multiple Access



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