CPM schemes for adaptive modulation based coded MC-CDMA systems

Multi-carrier code division multiple access (MC-CDMA)has been considered as a strong candidate for next generation wireless communication system due to its excellent performance in multi-path fading channel and simple receiver structure. However, like all the multi-carrier transmission technologies such as OFDM, the inter-carrier interference (ICI) produced by the frequency offset between the transmitter and receiver local oscillators or by Doppler shift due to high mobility causes significant BER (bit error rate) performance degradation in MC-CDMA system. Many ICI cancellation methods such as windowing and frequency domain coding have been proposed in the literature to cancel ICI and improve the BER performance for multi-carrier transmission technologies. However, existing ICI cancellation methods do not cancel ICI entirely and the BER performance after ICI cancellation is still much worse than the BER performance of original system without ICI. Moreover, popular ICI cancellation methods like ICI self-cancellation reduce ICI at the price of lowering the transmission rate and reducing the bandwidth efficiency. Other frequency-domain coding methods do not reduce the data rate, but produce less reduction in ICI as well. In this paper, we propose a novel ICI cancellation scheme that can eliminate the ICI entirely and offer a MC-CDMA mobile system with the same BER performance of a MC-CDMA system without ICI. More importantly, the proposed ICI cancellation scheme (namely Total ICI Cancellation) does not lower the transmission rate or reduce the bandwidth efficiency. Specifically, by exploiting frequency offset quantization, the proposed scheme takes advantage of the orthogonality of the ICI matrix and offers perfect ICI cancellation and significant BER improvement at linearly growing cost. Simulation results in AWGN channel and multi-path fading channel confirm the excellent performance of the proposed Total ICI Cancellation scheme in the presence of frequency offset or time variations in the channel, outperforming existing ICI cancellation methods.

Continuous phase modulation (CPM) is known for its attractive spectral properties. Furthermore, CPM has the property that the modulation signal maintains a constant envelope, so CPM is effective for satellite communications. Recently, generalized nonlinear CPM (GNCPM), which can achieve a larger minimum Euclidean distance than ordinary CPM, was introduced. In this paper, we propose a convolutional coded GNCPM to improve the bit error rate (BER) performance of uncoded GNCPM without expanding bandwidth. Combination CPM with convolutional coding causes an increased number of phase trajectory patterns which can cause the bandwidth expansion. Therefore, in our proposed model, to control the bandwidth, we pay attention to the phase trajectory pattern which is already owned by uncoded GNCPM and change the modulation index appropriately, depending on the input symbols, to keep the number of phase trajectory patterns the same as in uncoded GNCPM. First, we consider the most suitable assignment of the modulation index which can increase the Euclidean distances without bandwidth expansion. Next, we theoretically derive the upper bound on the error event probability of the proposed model in additive white Gaussian noise (AWGN). The performance of the proposed convolutional GNCPM, obtained by computer simulation, shows that the proposed model can improve BER performance without bandwidth expansion. © 1998 Scripta Technica. Electron Comm Jpn Pt 1, 81(8): 42–50, 1998

Recently, multi-carrier code division multiple access (MC-CDMA) has been attracting much attention as a broadband wireless access technique for the next generation mobile communications systems. Frequency-domain equalization (FDE) based on the minimum mean square error (MMSE) criterion can take advantage of the channel frequency-selectivity and improve the average bit error rate (BER) performance due to frequency-diversity gain. However, conventional MC-CDMA requires the insertion of guard interval (GI) and this reduces the transmission efficiency. Overlap FDE technique has been proposed that requires no GI insertion. Recently, we showed that MC-CDMA using overlap FDE can provide almost the same BER performance as the conventional MC-CDMA downlink using the GI insertion. However, our previous work assumed the ideal channel estimation. In this paper, we propose a pilot assisted channel estimation technique suitable for MC-CDMA downlink using overlap FDE and evaluate the BER performance by computer simulation.

Multicarrier code division multiple access (MC-CDMA) is a transmission scheme that combines the robustness of orthogonal modulation with the flexibility of CDMA schemes. It can lower the symbol rate and is therefore suited to the bandwidth limited optical wireless indoor channel. In this paper, new work is presented for an optical wireless (OW) system that utilizes MC-CDMA to improve the bit error rate (BER) performance. Our results indicate that at a BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> a 7 dB SNR improvement is obtained by a 4 user MC-CDMA OW system compared to a single user OW OOK system. This improvement is obtained while enabling multi-user communication. The performance degrades gradually as the number of users increases due to the increase in multiple access interference (MAI). A consistent BER performance is achieved for all receiver positions in a line strip spot-diffusing transmitter system (LSMS) with an angle diversity receiver, whereas a fluctuating BER performance is obtained in a conventional diffuse optical wireless system (CDS).

The convergence of high data rate multimedia services such as speech, audio, video, image and data will be the main features of future wireless systems. Multi-carrier code division multiple access (MC-CDMA) is one of the powerful techniques to meet the requirements of next generation broadband and wireless mobile communication systems. MC-CDMA system allows many users to get the benefit of both orthogonal frequency division multiplexing (OFDM) technique and code division multiple access scheme (CDMA). It offers high spectral efficiency, high data rate, low transmit power, lower implementation complexity, large system capacity and narrowband interference rejection capability. Multicarrier systems are very sensitive to carrier frequency offset (CFO) caused by Doppler shifts and imperfections of transmitter and receiver oscillators, which leads to loss of orthogonality among the subcarriers resulting in intercarrier interference (ICI). In this paper, a low complexity receiver phase rotated conjugate cancellation (RPRCC) algorithm using an iterative maximum likelihood (ML) based CFO estimation is proposed. The proposed method reduces the ICI which in turn reduces the multiple access interference (MAI) and subsequently increases the system capacity. Simulation results show that the RPRCC method provides better carrier to interference ratio (CIR) and bit error rate (BER) performance compared to transmitter phase rotated conjugate cancellation (PRCC) technique and conventional conjugate cancellation (CC) scheme.

Multi carrier code division multiple access (MC-CDMA), a strong contender for future mobile communication is a combination of orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA). Turbo coding technique can be used for improving the performance of MC-CDMA system in terms of bit error rate (BER). The objective of this work is to form a coding scheme that reduces BER of MC-CDMA system. In this paper a time domain turbo block code (TBC) designed using powerful low density parity check (LDPC) code is used in MC-CDMA system for improving the BER performance and the same is proved through simulation. This code also has error correction capability along with high performance efficiency in terms of BER.

Multi-carrier code division multiple access (MC-CDMA) has been one of the candidates for the next generation wireless communication systems. In an uplink, the MC-CDMA system suffers from the different access timing, the different fading, and the different frequency offset of each active user. In this paper we analyze the effects of the frequency offset compensation with MMSE-MUD (minimum mean square error based multi-user detection) for MC-CDMA in a quasi-synchronous uplink. We consider the MC-CDMA system with two subcarrier mapping schemes, the continuous mapping scheme and the discrete mapping scheme. From our theoretical analysis and computer simulation, we show that the MMSE-MUD can compensate the different frequency offsets among users. We also show that the MMSE-MUD significantly improves the bit error rate (BER) for the MC-CDMA system with the continuous mapping scheme.

Effect of atmospheric turbulence on the BER performance of an optical CDMA FSO link with SIK receiver

This letter presents an analysis of asynchronous multicarrier code-division multiple-access (MC-CDMA) systems with a guard period (GP) in the form of a cyclic prefix over frequency-selective multipath fading channels, which results in closed-form bit-error rate performance. Furthermore, it confirms that the frequency diversity of MC-CDMA systems is identical to the path diversity. The analytical approach proposed here is extended to the case without a GP. The derived analytical results show that a GP is required for MC-CDMA systems in order to mitigate not only the effect of intersymbol interference and intercarrier interference, but also the desired signal power degradation.

We present a coherent detection technique for continuous phase modulation (CPM) operating in the Rayleigh flat fading channel. The technique is based on the idea of inserting periodically data dependent pilot symbols that force the CPM signal to pass through known phase states. This transmission format enables the receiver to extract from the received signal the channel fading gains at regularly spaced instants. When coupled with proper channel estimation filters, very accurate channel state information (CSI) can be estimated at the receiver for fading compensation. Moreover, the accuracy of the CSI can be further refined by adopting a multiple-pass decoding approach. The paper discusses (a) the pilot symbol encoding technique required to force a M-level CPM scheme with a modulation index of p/M, p is an integer, to return periodically to a set of known phase states, (b) the optimal channel estimation filters, (c) a trellis-based precoding technique that can reduce the bit error rate in M-level CPM systems by close to 50%, and (d) a multiple-pass channel estimator/demodulator. Analytical and simulation results are presented for minimum shift keying (MSK), Gaussian MSK, and four-level continuous phase frequency shift keying with a modulation index of 1/4. It is observed that our pilot symbol-assisted CPM schemes exhibit no irreducible error floor even at a channel fade rate of three percent the symbol rate. The implicit phase coding in CPM and the accurate CSI provided by the pilot symbols lead to a diversity effect in the bit error rate curves of these modulation schemes.

Continuous phase modulation (CPM) with noncoherent detection over a one-symbol interval is discussed. In order to obtain good performance with this simple demodulation strategy, the phase function of the modulation scheme has to be designed properly. An approach to shaping the purchase function to achieve noncoherent orthogonal performance for various values of the modulation index (h) is presented. Using the Gram-Schmidt orthogonality procedure, an entire family of orthogonal phase functions for h>or=0.68 is derived. All CPM schemes with these phase functions produce the same performance as noncoherent orthogonal signaling. For h=1 the phase function reduces to that of the FSK phase function. It is shown that there are CPM schemes that give the same SNR performance as noncoherent orthogonal FSK but have some spectral advantages over the FSK modulation format. >

Throughput improvement of STS-based MC DS-CDMA system with adaptive modulation

In this letter, motivated by the superiority of continuous phase modulation (CPM) signals, a binary CPM with modulation index 0.5 is considered into downlink symbol- asynchronous non-orthogonal multiple access (ANOMA) to enhance power and spectral efficiency, especially for resource-limited wireless communications. According to the maximum likelihood (ML) criterion, a Viterbi-based block-by-block detector (VB-BBD) is first proposed. Considering the asymptotics of Toeplitz matrices, a DFT matrix diagonalization-based symbol-by-symbol detector (DFTD-SSD) is also given. Finally, the detection complexity analysis is provided. Numerical results reveal the optimal time offsets with different modulation formats for a two-user system, and show that time asynchrony can bring about 3dB and 6dB performance gains at a bit error rate (BER) of 10−4. The relationship between BER performance and different power levels, frequency pulses, response lengths is also investigated.

A Novel Analytical OFDM Modulation Framework using Wavelet Transform with Window Function in the Hilbert space

Multi-carrier code division multiple access (MC-CDMA) technique is one of the strong candidates for next generation wireless mobile communication systems. Multi-carrier systems are very much sensitive to carrier frequency offset (CFO) results in intercarrier interference (ICI). To mitigate ICI without any spectral loss, a second order duobinary coded phase rotated conjugate cancellation algorithm is proposed in this paper. In the conventional phase rotated conjugate cancellation (PRCC) technique, one path carries the MC-CDMA signal with a phase spin of ϕ and the other path carries the conjugate of the first path signal with -ϕ phase spin. This artificial phase rotation allows the transmitter to tune the transmitted signals so that the ICI effects could be mutually cancelled at the receiver. Although the PRCC technique reduces the spectral efficiency, the limitation can be overcome by the joint second order duobinary coding scheme with PRCC technique. In the proposed method, the correlative coding between the binary symbols modulated on adjacent subcarriers is used to reduce the ICI without any spectral loss. Simulation results show that the proposed PRCC method provides better carrier to interference ratio (CIR) and bit error rate (BER) performances compared to the conventional conjugate cancellation (CC) technique.