Analysis on the efficiencies of CDMA and TDMA technologies in a wireless communication network
By agajo
Analysis on the efficiencies of CDMA and TDMA technologies in a wireless communication network
1James Agajo, 2 Awolo C. I.3Sylvester Ezewele
Department of Electrical/Electronics
Federal Polytechnic, Auchi
Edo State ,Nigeria
Email: agajojul@yahoo.com
Phone: +2348053312732
ABSTRACT
A comparative review of the two competing digital wireless technologies used today in the communication industry is the subject of this research project. The wireless technologies in question are; Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). TDMA which uses time-sharing protocol technique is closely compared with CDMA which uses spread spectrum technique. The different method approaches adopted for the analysis are: The Shannon-Hartley Model, which specifically analyzes the system capacity and spectral efficiency of the system considering the background White Gaussian Additive Noise. A simple Analytical or Mathematical Model for the inter-cell interference which modifies the capacity for the two systems considering their uplink and downlink power system controls and an overview in the spectral efficiencies, The Channel Model, which provides the basic relationship between the bit error rate probability and the signal to noise ratio. Some data used for the analysis were primarily got from the two network providers: Visaphone (CDMA Technology) and Zain (TDMA Technology). The result obtained from the simulation analysis showed that CDMA technology are about 5 to 10 times more superior in capacity and more suitable than the TDMA Technology for the sub-Saharan Africa in particular and for the world in general.
Keyword: wireless, antenna, channel capacity , CDMA, TDMA.
1.0 Introduction
In today’s world cellular phone has become the single greatest tool in day-to-day life. It has become a necessity that business associates should be able to communicate on the go. That is why it has become so important to make choices in choosing which handheld device one should go for. A handheld device is selective according to its features and benefits, like; does it provide capacity and a better spectral efficiency? Does it access better quality of services, reliability, flexibility, higher data rate, and better packet and voice services? Does it provide access to internet and e-mail? In summary, does it provide better performance than the existing ones?
An important question when designing and standardizing cellular systems is the selection of the multiple access schemes. There are three basic principles in multiple access, FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), and CDMA (Code Division Multiple Access).[1,2] All the three principles allow multiple users to share the same physical channel. But the two competing technologies (TDMA & CDMA) differ in the way users share their common resource.
TDMA allows the user to share the same frequency channel by dividing the signal into different time slots. Each user takes turn in a round robin fashion for transmitting and receiving over the channel. CDMA uses a spread spectrum technology that is; it spreads the information contained in a particular signal of interest over a much greater bandwidth than the original signal. In TDMA, users can only transmit on their respective time slots. In CDMA, several users can transmit over the channel at the same time.
According to International Engineering Consortium [3], defined Time Division Multiple Access (TDMA) as a digital transmission technology that allow 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, even up to eight slots ( in GSM) with each 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.
The TDMA concept was developed in the late 1960s for use in digital satellite communication system and first became operational commercially in the mid 1970s. However, the TDMA is the most popular access method used in second generation digital cellular systems.
The wireless industry began to explore converting the existing analog network to digital as a means of improving capacity back in the late 1980s. In 1989, the Cellular Telecommunications Industry Association (CTIA) chooses TDMA over Motorola’s Frequency Division Multiple Access (FDMA) (today known as Narrowband and Analog Mobile-Phone Service (NAMPS). Narrowband standard is the technology of choice for existing 900MHz cellular markets and for emerging 1.8 GHz markets.[3]
TDMA systems supports a variety of services from the end user, such as voice,
data, fax, short message services (SMS), and broadcast messages.
2.0 Shannon Hartley Model
Shannon Hartley law states that If a channel has bandwidth and if the signal to noise ratio S/N (expressed in watts not in db), then the capacity C measured in bits per second of this channel is given by
C = Blog2 (1+S/N)……………………………..2.0
C = Capacity
B Bandwidth. [17,18]
The purpose of SNR stage is to compute the current average power SNR result for the arriving packet. This calculation is usually based on values obtained during the stages these include the received power, background noise, and interference noise. The SNR of the packet is an important performance measure that supports determination of the receiver’s ability to correctly receive the packet’s content.[6,7]
Also from the mathematical model, a relationship between signal to noise ratio and distance between base station and the mobile station were derived.
SNR = 2 / Log10 D ------------------------------------2.0.1
Where D is the distance between base stations and mobile stations expressed in Km
Technology Modulation type Channel Quality indicator
Bandwidth
AMPS Analog FM 30kHz C/I = 17Db
NAMPS Analog FM 10kHz C/I = 17dB
TDMA DPQSK 10kHz C/I = 17dB
GSM GMSK 200kHz C/I = 17dB
CDMA QPSK/OQPSK 1,250kHz Eb/No = 6dB
Table 2.0 TECHNOLOGY AND ITS MODULATION TYPE
2.1 SYSTEM DESIGN
The purpose of system design is to ascertain whether or not a given model of the system will
Meet some predetermined requirements. For a wireless system, the requirements are often that a user should perceive his or her radio connection to be one of high quality (i.e. the user is well within the coverage area of a cell or sector).
Computation of Radio Capacity for CDMA and TDMA
The cellular radio capacity, m, of TDMA can be determined by the relationship
M= (Bt/Bc)/k -------------------------------2.1 [21,22]
where Bt is the total allocated spectrum for the system, Bc is the channel bandwidth, and L is the number of cells in a frequency reuse pattern and can be obtained by
K = q2/3 ----------------------------------------------2.2
Where q is the Co-Channel Interference Reduction Factor (CIRF)
In mobile radio environment, we may assume a fourth power rule i.e.
m = Bt / Bc√(2/3)(C/I) …………………….. 2.3
m = M/K Number of channels/cell --------------- 2.4
Where M is the total number of equivalent channels and (C/I) is the minimum received carrier –to-interference ratio per channel or per time slot [8,9]
The C/I ratio of CDMA and TDMA system is related to Eb/No through equation 4 [24,30]
C/I = Eb/No – Rb/Bc ---------------------------------------------- 2.5
Where Rb is the transmission data rate, Bc is the transmission bandwidth, Eb is the energy per bit and No is the interference power per hertz.[26,28]
2.2 THE COMPARISON
The following comparative facts between the two competing technologies can be obtained from the graphs of multipath effect, frequency reuse, Bit Error rate.
2.3 Capacity Comparison
TDMA capacity
The capacity equation of a TDMA system is given as
N = I/B / K ------------------------------------------------------2.6
Where B is the bandwidth per channel in megahertz and K is the cluster size. For the purpose of clarity, assume that the bandwidth is 8Kbps and that a cluster size of 3 is achieved through the use of directional antennas. The number of users who can be accommodated, then, is around 40 users/cell/MHz.[4,9]
2.4 CDMA Capacity
Consider mobiles communicating with a base station.[25,27] The signal from a particular mobile arrives with a power S. the other mobiles have perfect power control, so their total signal strength is (N-1) S. the SIR experienced by the first mobile is[10,11]
SNR = 1/ (N-1) ---------------------------------------2.7
The SNR is given by the processing gain, G, multiplied by the SIR. Substituting for the SNR and rearranging to determine the number of users, it can be see that
N = G/ SNR + 1 -------------------------------------2.8
Which is the basic CDMA capacity equation?
From the practical point of view it has been observed that allowing the same factors for both the TDMA and CDMA capacity equations, what has been transpired from proof is that for an equivalent system, cellular CDMA has a capacity of approximately 30% more than cellular TDMA.[13,14]
3.0 SYSTEM IMPLEMENTATION
3.1 SIMULATION RESULT
From the analysis made, results were obtained using simulation technique to actualize the proof on the comparative analysis of TDMA and CDMA Multiple Access Technique.
The values in the tables were the raw data obtained from the reference network providers, these were based on measurements made within a geographic environment (Broad streetLagosNigeria as a reference point) affected by reflective surfaces, shadows and tall buildings. This entails the deep effect of multipath on the received signal strengths for CDMA and TDMA, which network is affected more than the other. [15,16]
Table 3.0 THE MULTIPATH EFFECT ON THE RECIEVED SIGNALS FOR TDMA AND CDMA
Distance in km 0.0 1.5 2 1.5 2.0 2.5 3 3.5 4
Received signals -52.4 -58.4 -64.4 -67.9 -70.4 -72.4 -75.9 -78.4 - 79
from CDMA
signal, dBm
Received signals in -69.0 -79.2 -89.5 -95.4 -99.7 103.0 109.0 -113.2 -114
TDMA Signals dBm
Source: Visaphone and Zain measurement chart
distance reduces the receiver signal level by the square of the ratio of change in distance. More sophisticated propagation modeling, which is widely used in the industry, will reveal that the relationship is far more volatile than the inverse of the square of the distance. With the use of CDMA and TDMA performance measurements exhibit that the degradation can be to the inverse of third or fourth order, making the proper selection of a desired receiver threshold even more important than with a conventional FM two-way radio modeling[17,18] From the simulation result above, the effect of multipath on both multiple access techniques were shown. The time of arrival of the signals, mostly affected the TDMA signals, CDMA signals are inherently resistant to interference and jamming, Rake receivers are used to combat multipath effects on the CDMA signals.
5.0 CONCLUSION
In this work, the researchers considered a model for capacity for inter-cell interference in a cellular system with frequency reuse factors for CDMA and TDMA systems, the signal strength effect in a multipath environment, the spectral efficiency with respect to the number of users, the channel capacity using the same factors for both CDMA and TDMA. We found out that from the analysis that CDMA had an edge over TDMA systems, from the conclusive point, it is opined that CDMA provide around 8 to 10 times more capacity than the TDMA providers. Also has a better spectral efficiency, in the sense that it accommodates more users within a geographic environment with a better quality of service QOS. Also CDMA bring more benefits to carriers and consumers, including better voice quality more traffic support, soft handoff, broader coverage and stronger security.
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SYSTEM IMPLEMENTATION (SIMULATION TABLE AND RESULTS)
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