Voice capacity in CDMA xRTT systems Term project (COE-543) By S.M.Rehman #

Outline Introduction Advantages of CDMA systems Propagation Loss Models Parameters considered Plan of action Simulated results Future work Conclusions

Introduction. Code Division Multiple Access (CDMA) is a viable cellular system alternative to both frequency-division multiple access (FDMA) and time-division multiple access (TDMA) technologies. CDMA falls under the category of spread spectrums. Code Division Multiple Access (CDMA) is receiving considerable attention as a major contender in the upcoming 3G wireless standard development.

Introduction (Contd) CDMA xRTT is a 3G wireless technology based on CDMA platform. The 1x in 1xRTT refers to 1x the number of 1.25MHz channels. The RTT stands for Radio Transmission Technology. 1xRTT is forward link synchronous with MHz chipping, and is an evolution of IS-95.

Contd. 3G systems were also designed to increase voice capacity over 2G by various forward and reverse link improvements. These include fast forward link power control, bandwidth efficient modulation, lower rate coding and reverse link pilot assisted coherent demodulation.

Advantages of CDMA systems: Universal one-cell frequency reuse Narrow band interference rejection Inherent multipath diversity Soft hand-off capability Soft capacity limit Improved Channel Capacity.

Propagation models: For any wireless communication system, the first important step is to design the radio link.This is required to determine the base station density in different environments as well as the corresponding radio coverage. One of the most important considerations in the radio coverage planning process is the propagation model. The accuracy of the prediction by a particular model depends on its ability to account for the detailed terrain, vegetation, and buildings.

Contd. Propagation models are used to determine how many base stations are required to provide the coverage requirements needed for the network. The performance of the network is affected by the propagation model chosen because it is used for interference predictions. The propagation model is also used in other system performance aspects including handoff optimization, power level adjustments, and antenna placements.

Most important radio propagation characteristics: Achievable Signal Coverage, The maximum data rate that can be supported by channel, and The rate of fluctuations in the channel.

Definition: Propagation models that predict the mean signal strength for an arbitrary transmitter-receiver separation distance, useful in estimating the radio coverage area of a a transmitter are called large scale propagation models. Propagation models that characterize the rapid fluctuations of the RSS over very short travel distances or short time durations are called small scale or fading models.

Path Loss Radio transmission in a mobile communication system often takes place over irregular terrain.The terrain profile of a particular area needs to be take into account for estimating the path loss. Path Loss which represents signal attenuation is a positive quantity measured in decibels (dB),is defined as the difference between the effective transmitted power and the received power.

Path Loss (Contd) Some of the commonly used propagation models are: Longley Rice Model Durkin’s Model Okumura Model, Hata Model PCS extension of Hata model Walfisch & Bertoni Model Wideband PCS micro cell Model.

Hata model For signal strength prediction and simulation in macro cellular environments, the Hata Model is widely used. The Hata model is an empirical formulation of the graphical path loss data provided by Okumura, and is valid from 150 MHz to 1500 MHz. Hata presented the urban area propagation loss as a standard formula and supplied correction equations for application to other situations.

Hata formula: The standard formula for median path loss in urban areas is given by L50 (urban)dB = logf c log(hte) – a(hre) log(hte) log(d). where, f c is the frequency in MHz from 150MHz to 1500MHz, h te is the effective transmitter antenna height(meters) ranging from 30m to 200m, h re is the effective receiver antenna height(meters) ranging from 1m to 10m, d is the T-R separation distance(km) and a(h re ) is the correction factor for effective mobile antenna height which is a function of the size of the coverage area.

Contd.. For a small or medium sized city the correction factor is given by a(hre) = [(1.1log fc – 0.7)hre – (1.56log fc-0.8)]dB The Free Space Path loss equation is given by L (free) = logfc + 20log (d). The average received power at the mobile station is calculated as Pr(d) = EIRP(dBm) - L(urban)(dB) + Gr(dB) Where EIRP is the Effective Isotropic Radiated Power (1 kw) and Gr is the gain of the receiving antenna.

This model is well suited for large cell mobile systems, but not personal communication systems which have cells on the order of 1km radius.

Parameters Considered: Bandwidth of 1xRTT systems = 1.25MHz. Orthogonality factor = 0.8 Totoal transmitted Power = 24 w Fraction of overhead = 0.2 Eb/No =7dB Number of users between 1 and 50. Cell Radius = 1km Frequency =1500MHz, Height of base station and Height of mobile station equal to 50m & 3m respectively.

Simulated Results using Matlab: Frequency Number of users Radius (Km) Eb/No (dB) Rate (kbps)

Rate (kbps) Eb/No (dB) Radius (Km) Number of users Frequency (MHz)

Results Contd.. Rate (kbps) Eb/No (dB) Radius (Km) Number of users Frequency (MHz)

Contd.. Rate (kbps) Eb/No (dB) Radius (Km) Number of users Frequency (MHz)

Contd.. Rate (kbps) Eb/No (dB) Radius (Km) Number of users Frequency (MHz)

Scatter plot of path loss and distance for a macro cell when the base antenna height was 25m.

Future Work It is possible to extend the study by comparing other Propagation Loss Prediction Models like the Walfisch-Bertoni Model to investigate microcellular environments,effects of structure of the buildings etc. to get a better understanding on the factors effecting system design. The performance of the variable processing gain system can be measured in terms of network throughput and can be compared to the performance of conventional fixed rate CDMA systems in which perfect power control is assumed.

Acknowledgements: I convey my deep sense of gratitude to Dr.Ashraf who has been helping me through out the project. I convey my thankfulness to my compatriots who gave a hearing ear to my presentation.