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Two most important variations of the multi carrier spread spectrum systems are the MC-CDMA frequency domain spreading and MC direct sequence CDMA (MC-DS-CDMA) (time domain spreading). One way of looking at MC-CDMA is as a combination of CDMA and OFDM, resulting in better frequency diversity and higher data rates. In MC-CDMA, every symbol is spread utilizing code chips and transmitted on numerous subcarriers. There is no necessity for the number of carriers to be equal to the code length; thus offering a degree of flexibility in our construction. MC-DS-CDMA differs in the fact that the data is spread in time domain rather than in frequency; with every sub channel representing a regular DS-CDMA system. The basic principle outlining these is illustrated in Fig-1. It illustrates the case of a single-user scenario where the data is spread utilizing a code of length 4 and number of subcarriers is the same.
The eminent advantage of MC-CDMA is the increase in bandwidth efficiency; the reason being the multiple accesses made possible through proper systems construction utilizing orthogonal codes. The additional flexibility offered by the possibility to utilize variable length codes offers greater resilience to error. It is also possible to interleave the data in both frequency and time to exploit both time and frequency diversity and employ time-frequency (TF) spreading. MC-CDMA and OFDM also have issues of high peak to average power ratios (PAPR), the challenges of synchronization in both the time and frequency domains and dealing with carrier frequency offset and multiple access interference (MAI).
Reference: Multi-carrier and spread spectrum systems
By Khaled Fazel, Stefan Kaiser
CDMA concept with Multi-Carrier modulation.
Multi-Carrier modulation is very successful in the broadcast applications. This makes the researchers investigate the stability of MC modulation in wireless mobile communications. There are mainly two different concepts were introduce when DS-CDMA combined with MC modulation. One concept is MC-CDMA is also called combination of CDMA and OFDM. Second concept is referred as MC-DS-CDMA.
MC-CDMA is based on a serial concatenation of DS spreading, and MC modulation, cf CF (characteristic function). The high rate DS spread data stream is MC transformed in such a way, that the L chips of a spread data symbol are transmitted in parallel on subcarriers. Thus, the assigned data symbol is concurrently transmitted on L subcarriers. If the number of subcarriers Nc is equal to the spreading code length L, MC-CDMA requires the total bandwidth for the transmission of a single data symbol comparable to DS-CDMA. When choosing L smaller than Nc and introducing an appropriate frequency interleaving, the flexibility of an MC-CDMA system can be increased and the complexity of the data detector can be reduced fig shows the principle of an equivalent realization of the serial concatenation of DS spreading, cf., and MC modulation, cf. . Every data symbol is copied onto L sub-streams before multiplication with one chip of the spreading code per sub stream. A chip of the MC-CDMA spreading code grouped in the frequency domain has the duration Tc, cf. Fig. which is by a factor of Nc greater than the duration Tc of a chip of the DS-CDMA spreading code.
State of the Art in the Field of MC-CDMA:
The employment of MC-CDMA for mobile multi-user communications has become an active held of research. Up to now, the majority of publications in the held of MC-CDMA was devoted to the investigation of data detection methods suitable for mobile radio systems in the downlink. Due to the novelty of the subject, investigations including the potential of channel coding with code bit interleaving or the influence of non-perfect channel estimation were scarcely carried out. This section gives an overview on the state of the art concerning the topics data detection, channel coding, and channel estimation applied in MC-CDMA mobile radio systems. Contributions to these topics made by the author of the thesis are not included in this overview; Adaptive modulation for a multi-user downlink MC-CDMA system that employs frequency domain spreading. By assembling the subcarriers into groups, the spreading codes align themselves in a synchronous manner, and thus with the appropriate equalization pair wise orthogonality is preserved and adaptive modulation can be performed. Such a configuration leads to the origin of an analytical expression for the instantaneous SNR of a group of sub-carriers utilized for adaptive modulation and resource allocation purposes.
An effective channel function might be evaluated for every group of sub-carriers for every user operating in that said group, and every group might then be interpreted as an equivalent sub-carrier of a conventional OFDM modem, thereby enabling any existing adaptive schemes originally intended for OFDM to be deployed to MC-CDMA. Based on the equivalent sub-carrier concept, we introduce and apply two adaptive modulation schemes for multi-user MC-CDMA.
This scheme employs the target BER process and attempts to let every user to transmit as maximum symbols as his channel allows, it has no rate constraint or matching imposed. In general, every user will have various channel conditions than that of other users operating with a particular control/base station. Thus, the data rates that every of these channels can support will naturally vary from user to user, and this scheme allows any particular user to reserve none, or one or more orthogonal codes in one or more sub-carrier groups.
Let K be the number of modulation schemes (MS) available for utilize between the control/base station and every user (predetermined by construction according to some criteria, e.g. complexity). For every group of sub-carriers, provided the channel state information for every user, the transmitter can compute the effective instantaneous SNR for every spreading code utilized for every user. This allocation process determines how maximum data bits are accommodated in an OFDM symbol for the user. The classification and allocation process is done for every spreading code assigned to every user. The chips generated are synchronously added together in every group across the N subcarriers before the IFFT operation.
The MC-CDMA air interface allows high-capacity networks and robustness in the case of frequency-selective channels; taking benefits from CDMA ability offered by the spread spectrum method, and MC modulation as orthogonal frequency division multiplex (OFDM). A possible generic downlink transmission scheme is depicted in Figure.
Each user data can be concurrently processed at the spreading step before MC modulation. In the following, due to their good properties for the downlink , Walsh-
Hadamard (WH) spreading sequences will be considered. The presented MC-CDMA configuration is based on the transmission of multiple data per MC-CDMA symbol for each user. Data di j (n) denotes the ith, 1 ≤ i ≤ Nb, data transmitted by user j, 1 ≤ j ≤ Nu, in the nth MC-CDMA symbol.
The maximum number of available users, which is also equal to the length of the WH spreading sequences, will be denoted Nu. The total number of subcarriers is Nc = Nz + Ncu, where Nz and Ncu are the number of unused and utilized subcarriers, respectively. Therefore, the number of data transmitted by each user in one MC-CDMA symbol is Nb = Ncu/Nu. Frequency interleaving is performed in order to fully exploit the frequency diversity offered by OFDM modulation.
At the receiver part, despreading is done according to the specific user sequence after equalization in the frequency domain. The system synchronisation and intermediate frequency (IF) and baseband (BB) conversions problems are beyond the scope of this paper and will not be addressed. Among various equalisation methods, we especially focus on single-user detection methods. Channel estimation function can efficiently be performed by utilizing pilot subcarriers insertion. The arrangement of these pilots should guarantee an optimum sampling of the channel transfer function in time and in frequency, depending on the bandwidth coherence and on the time coherence of the channel.
Obviously, MC-CDMA system offers high flexibility in resources (spectral efficiency, number of users) allocation which consequently induces large design spaces. As a result, high-level design methods are convenient in order to deal with such complexity and for efficient implementation.
MC-CDMA addresses the issue of how to spread the signal bandwidth without increasing the adverse effect of the delay spread. As a MC-CDMA signal is composed of N narrowband subcarrier signals each of which has a symbol duration much larger than the delay spread, a MC-CDMA signal will not experience an increase in susceptibility to delay increases and ISI as does DS-CDMA. In addition, since the F-parameter can be chosen to determine the spacing between subcarrier frequencies, a smaller spreading factor than one required by DS-CDMA can be utilized to make it unlikely that all of the subcarriers are located in a deep fade in frequency and consequently achieve frequency diversity.
Implementation of MC-CDMA system:
Below block diagram is a possible implementation of transmitter model of MC-CDMA.
Walsh-Hadamard Code Matrix
Parallel to serial converter
Inverse Fast Fourier Transform (I-FFT)
In the implementation of MC-CDMA transmitter model, user signals have been given to Walsh-hadamard code matrix. In this user signals are converted into orthogonal codes. These orthogonal codes are assigned to Inverse Fourier Transform (I-FFT). By doing this we can get a matrix form. Finally this matrix is converted from parallel to serial converter then signals are sending through the transmitter.
For the possible receiver model of MC-CDMA, the same process can be done in the reverse or opposite order.
This is a model of the transmitter for one possible implementation of an MC-CDMA system. The input data symbols, am[k], are assumed to be binary antipodal where k denotes the kth bit interval and m denotes the mth user. In the analysis, it is assumed that am[k] takes on values of -1 and 1 with equal probability. The generation of an MC-CDMA signal can be described as follows. A single data symbol is replicated into N parallel copies. The ith branch (subcarrier) of the parallel stream is multiplied by a chip, cm [i], from a pseudo-random (PN) code or some other orthogonal code of length N and then BPSK transformed to a subcarrier spaced apart from its neighboring subcarriers by F/Tb where F is an integer number. The transmitted signal consists of the sum of the outputs of these branches. This process yields a multicarrier signal with the subcarriers containing the PN-coded data symbol.
Observing the model of the transmitter in Fig, the implementation of an MC-CDMA transmitter appears prohibitive with the bank of oscillators, one for each of the subcarriers. However, it should be noted for the case of F = 1, as mentioned above, the MC-CDMA signal shares the same signal structure as OFDM. The analysis of OFDM has shown that the discrete-time version of the OFDM transmitter is simply a Discrete Fourier Transform (DFT). Thus, the transmitter model of MC-CDMA in Fig.2 may simply be replaced by an FFT operation for F = 1. In the analysis, we will assume a continuous-time receiver model as shown in Fig. 2. This model makes the analysis simpler and more instructive.
When M active users, the received signal is
Where the effects of the channel have been included in rm, i and qm, i and n(t) is additive white Gaussian noise (AGWN) with a one-sided power spectral density of N0. Assuming the transmitter model of Fig. 2, a possible implementation of the receiver is shown in Fig. 3 where it has been assumed that m = 0 corresponds to the desired signal. With this model, N matched filters with one matched filter for each subcarrier. The output of each filter contributes one component to the decision variable, n0. Each matched filter consists of an oscillator with a frequency corresponding to the frequency of the particular BPSK transformed subcarrier that is of interest and an integrator. In addition, a phase offset equal to the phase distortion introduced by the channel, θ0, i is included in the oscillator to synchronize the receiver to the desired signal in time. To extract the desired signal's component, the orthogonality of the codes is utilized. For the ith subcarrier of the desired signal, the corresponding chip, c0[i] from the desired user's code is multiplied with it to undo the code. If the signal is undistorted by the channel, the interference terms will cancel out in the decision variable due to the orthogonality of the codes. This comment will be discussed in detail in the next paragraph. As the channel will distort the subcarrier components, an equalization gain, d0, i may be included for each matched filter branch of the receiver. Applying the receiver model of Fig. to the received signal provided in Eq. yields the following decision variable for the kth data symbol assuming the users are synchronized in time
EVLUATION AND TESTING
We estimated the upper-bound BER of Approach I upon combining paths in the receiver. The BER of hard-detection based on the approach was also plotted as a benchmarked, assuming that the receiver exploited the explicit knowledge of the DS patterns. The parameters related to the computations were shown in the figures. The results demonstrate that the system provides dramatic BER improvements, when the number of combined diversity paths, increases.
The proposed adaptive schemes may be applied to any multi-user frequency domain spreading downlink MC-CDMA system. The sub-carrier grouping structure and spreading confinement ensure synchronization between various users' that is essential to the recovering and division of user symbols at the receiver. There is no limit imposed on the group size, this permits various spreading code lengths to be utilized for various scenarios or for performance tuning etc. The equivalent sub-carrier concept further allows a group of subcarriers to be replaced by an equivalent sub-carrier of a conventional OFDM modem for the purpose of bit/power loading. This enables various powerful bit-loading schemes, originally developed for OFDM, to be directly deployed to MC-CDMA systems.
The proposed MC DS-CDMA system is capable of efficiently amalgamating the methods of slow FH, OFDM and DS-CDMA.