Figure 1. Model of CDMA Systems
With the emergence of Code Division Multiple Access (CDMA), the communication world has been bestowed with stateof- the-art multiple access schemes that are used to allow many simultaneous subscribers to the same fixed bandwidth radio spectrum. The problem, however, is that there is a limit to the capacity and performance of this technique as Multiple Access Interference (MAI) stems along with this technology. Therefore, an analysis which basically focuses on the effect of this Multiple Access Interference is essential. The main goal of this work is to analyze the multiple access interference in Direct-Sequence Code Division Multiple Access (DS-CDMA). It mainly focuses on the effects on the Bit Error Rate (BER) as a result of a change in Signal-to-Noise (SNR), which varies with the number of users assigned to a channel. The result of this study demonstrated that the probability of occurrence of an error in the received signals increase as the number of users sharing the same channel increases, maintaining other parameters constant. This is directly attributed to the fact that SNR decreases as the number of subscribers to the same channel increases. However, further analysis requires the due consideration of other parameters like external effects to decide on the optimum number of users to share a given channel.
Multiple access schemes are used to allow many simultaneous users to use the same fixed bandwidth radio spectrum. In any radio system, the bandwidth which is allocated to it is always limited(Dinan and Jabbari, 1998). For mobile phone systems, the total bandwidth is typically 50 MHz, which is split in half to provide the forward and reverse links of the system(Agarwal, 2006). Sharing of the spectrum is required in order to increase the user capacity of any wireless network. FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), and CDMA (Code Division Multiple Access) are the three major methods of sharing the available bandwidth to multiple users in wireless system. There are many extensions, and hybrid techniques for these methods, such as OFDMA (Orthogonal Frequency Division Multiple Access), and hybrid TDMA and FDMA systems(Sarkar, 2003). From these techniques, CDMA is widely employed for mobile communication systems, attributed to its characteristics nature.
Code Division Multiple Access (CDMA) is a spread spectrum technique that uses either frequency channels or time slots. In CDMA, the narrow band message (typically digitized voice data) is multiplied by a large bandwidth signal which is a pseudo random noise code (PN code). All users in a CDMA system use the same frequency band and transmit simultaneously. The transmitted signal is recovered by correlating the received signal with the PN code used by the transmitter(Bhat and Sudha, 2012). Figure 1 shows the general use of the spectrum using CDMA.
Figure 1. Model of CDMA Systems
CDMA technology was originally developed by the military during World War II Røpke, et al., 2010). Researches were spurred into looking at ways of communicating that would be secure and work in the presence of jamming. Among the many properties that have made CDMA useful are its capability in signal hiding and non-interference with existing systems, anti-jam and interference rejection and multiple user access.
For many years, spread spectrum technology was considered solely for military applications. However, with rapid developments in LSI and VLSI designs, commercial systems are starting to be used.
This work, in general, intends to analyze the interferences in a multi-user DS-CDMA technique. It specifically aims to:
Code Division Multiple Access (CDMA) is a method of multiplexing (wireless) users by distinct (orthogonal) codes. All users can transmit at the same time, and each is allocated the entire available frequency spectrum for transmission. CDMA is also known as Spread-Spectrum Multiple Access (SSMA).
CDMA does not require the bandwidth allocation of FDMA, nor the time synchronization of the individual users needed in TDMA. A CDMA user has full time and full bandwidth available. Moreover, in CDMA each user has its own PN code, uses the same RF bandwidth and transmits simultaneously (asynchronous or synchronous). The problem, however, is this sharing mechanism of the same bandwidth poses interference among the different messages; whereby one message signal appears as a noise to another, and hence, is the term Multiple Access Interference (MAI).
At the receiving end in the communication system, detector receives a signal composed of the sum of all users' signals, which overlap in time and frequency. Multiple Access Interference (MAI) refers to the interference between direct-sequence users and is a factor which limits the capacity and performance of DS-CDMA systems(Ma, et al., 2015) . In a conventional DS-CDMA system, a particular user's signal is detected by correlating the entire received signal with that user's code waveform(Nagarajan and Dananjayan, 2010) . The conventional detector does not take into account the existence of MAI.
Similar to the common communication models, the overall structure of the system here encompasses the components required to exchange information between end-users in mobile phone context (Figure 2).
As depicted in Figure 2, successful transmission and proper receiving necessitates six sequential processes: such as sampling, spreading the message signal using the PNsequence, modulation, transmission of the composite signal over channel, demodulation and finally, dispreading (recovering the information from the received signal using corresponding PN-sequence).
Figure 2. Transceiver Model for DS-CDMA
The signals to be transmitted are first sampled and quantized for digitalization. This step also makes spreading easier since the spreading PN-sequences are in bipolar form (1's and -1's). The four sample signals (see the waveforms in Figure 3) utilized for this test are:
Figure 3. Transmitted Signals for Four Users Sharing the Same Channel
where sigi is the analog signal to be transmitted by each user.
The next step is spreading each user's signal with different spreading codes. Here, the PN-sequence used is an orthogonal Walsh code of length 16. For this specific analysis, the four users' PN-sequences applied are:
These sequences are orthogonal; in a sense that the cross correlation between the sequences is zero. Spreading is done through bit level multiplication(Simon and Michael, 2006) of each signal with corresponding PN-sequence as follows.
where spread_signalj is the spread sequence of signal bit, signalj, n is the length of the PN-sequence.
This step is essential to take the baseband signal to pass band for transmission over channel. Binary Phase Shift Keying (BPSK) is used for this specific work as shown in Figure 4.
Figure 4. Modulation Results of the Signals
where spreadsignal is the signal spread by a PN-sequence, A is signal the amplitude of the carrier signal and fc is the carrier frequency.
where modulated_signali is individual's modulated signal, and noise is a white Gaussian noise used for simulation purpose (-3dB).
Thus, the graph in Figure 5 indicates the composite signal in the channel when four users are transmitting simultaneously.
Figure 5. Composite Signal of the Four Users
At the receiving end, demodulation of the channel signal precedes dispreading of each user's signal by its PNsequence. This could be performed by multiplying the received noise-like signal with the synchronized carrier signal (Figure 6).
Figure 6. Demodulated Signal to all Receiver
Finally at the receiving end, the corresponding signal sent from the transmitting end can be recovered by multiplying the incoming demodulated signal by the specified PNsequence, synchronized with one at the corresponding transmitting end, of the given receiver. The result of simulation for this particular work is given in Figure 7.
Figure 7. Recovered Signals of the Four Signals at Corresponding Receiving End
The main objective of this paper was to analyse the implementation of Direct Sequence Spread Spectrum Code Division Multiple Access (DS-CDMA) for mobile phone transceiver using simulation code developed in matlab language, and to test the performance of the system by varying the parameters which affect the effectiveness of the design like SNR, increase in the number of users and orthogonality of the PN-sequence. But since Walsh code whose cross correlation is zero was employed, the third parameter (orthogonality) wasn't an issue in this work. Hence, the analysis was made with respect to a changing number of subscribers and its effect on the induced BER. This parameter is essential as it indirectly affects the SNR in the channel.
In order to demonstrate the performance of the transceiver designed, bit error rate was used for different Gaussian noise at various numbers of subscribers using the channel simultaneously.
It is evident that the number of users sharing a channel at the same time is directly proportional to the SNR. This is because of the fact that the signal of one user behaves like a noise to other receivers. Thus, an increase in the number of users implies as an increase in channel noise, as can be seen from the following Bit Error Rate (BER) versus Signal to Noise Ratio (SNR) graphs for different number of users (Figures 8 to 10).
Figure 8. BER vs SNR for a Single Signal in the Channel
Figure 9. BER vs SNR for Three Signals in the Channel
Figure 10. BER vs SNR for Four Signals in the Channel
The simulation results imply that the probability of occurrence of an error in the received signals increase as the number of users sharing the same channel increases. For example, the bit error rate for a single user in the channel, as shown in Figure 8, is almost negligible. On the other hand, BER is becoming more and more significant as the number of users increases as shown in Figures 9 & 10.
Furthermore, from a test with sound signal, it's important to note the fact that speed of proceeding need to be given due attention when such system is to be implemented. However, this won't be a problem in real world application as fast and dedicated processors can be used.
In summary, it is evident that this type of recent technology has resulted in advanced communication system. From the simulation results, one can easily understand that the system is noise and interference resistant and secured, and enables the optimum use of the very limited bandwidth available these days. It is noise and interference resistant in the sense that the receiver only identifies the corresponding signal from the required transmitter by using the synchronized PN-sequence. It is secured because receiver with the same PN-sequence can only access a corresponding message. In terms of economy, it provides exploration of a given bandwidth for multiple users in contrast to that of Frequency Division Multiple Accesses, which assigns separate bandwidth which is a limited resource, for different users. But this should be at optimum number of users because the simulation has demonstrated that as the number of users increases, the received signal becomes more and more corrupted to some extent. And it also presents with the ease of transmission of different users at the same time in contrary to Time Division Multiple Accesses.
In general, spread spectrum allows resource saving in terms of frequency, lets multiple usage of the same band at the same time and hinders jamming of information which is the main concern of today's world.
This work has been successful in terms of bringing the insight to the analysis of DS-CDMA technique except the time consuming nature of the simulation process using Matlab code. However, this won't be a problem in real application since dedicated processors at station and Integrated Circuits (IC) at each mobile cell are employed. So, if applied with this IC's, the code will guarantee a DS spread spectrum application for the number of users it has been designed.
It also ensures a secured communication system, especially for military purpose due to the fact that the signal will be considered as noise at the enemy who do not know the corresponding PN-sequence to receive the message. But it's important to work closely on the PNsequence's combination to avoid the probability of penetration by intruders.
Furthermore, the analysis can be all-rounded if it's allowed to continue until the optimum number of subscribers for transmission is obtained with tolerable interference. In this context, the analysis can be considerate of the probability of multiple users in the channel at the same time and their effect on the efficiency of the overall system.