Multi-carrier CDMA using convolutional coding and interference cancellation

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Data Detection and Channel Estimation of OFDM Systems Using Differential Modulation

Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier modulation technique which is robust against multipath fading and very easy to implement in transmitters and receivers using the inverse fast Fourier transform and the fast Fourier transform. A guard interval using cyclic prefix is inserted in each OFDM symbol to avoid the inter-symbol interference. This guard interval should be at least equal to, or longer than the maximum delay spread of the channel to combat against inter-symbol interference properly. In coherent detection, channel estimation is required for the data detection of OFDM systems to equalize the channel effects. One of the popular techniques is to insert pilot tones (reference signals) in OFDM symbols. In conventional method, pilot tones are inserted into every OFDM symbols. Channel capacity is wasted due to the transmission of a large number of pilot tones. To overcome this transmission loss, incoherent data detection is introduced in OFDM systems, where it is not needed to estimate the channel at first. We use differential modulation based incoherent detection in this thesis for the data detection of OFDM systems. Data can be encoded in the relative phase of consecutive OFDM symbols (inter-frame modulation) or in the relative phase of an OFDM symbol in adjacent subcarriers (in-frame modulation). We use higher order differential modulation for in-frame modulation to compare the improvement of bit error rate. It should be noted that the single differential modulation scheme uses only one pilot tone, whereas the double differential uses two pilot tones and so on. Thus overhead due to the extra pilot tones in conventional methods are minimized and the detection delay is reduced. It has been observed that the single differential scheme works better in low SNRs (Signal to Noise Ratios) with low channel taps and the double differential works better at higher SNRs. Simulation results show that higher order differential modulation schemes don¡¯t have any further advantages. For inter-frame modulation, we use single differential modulation where only one OFDM symbol is used as a reference symbol. Except the reference symbol, no other overhead is required. We also perform channel estimation using differential modulation. Channel estimation using differential modulation is very easy and channel coefficients can be estimated very accurately without increasing any computational complexity. Our simulation results show that the mean square channel estimation error is about ¡¼10¡½^(-2) at an SNR of 30 dB for double differential in-frame modulation scheme, whereas channel estimation error is about ¡¼10¡½^(-4) for single differential inter-frame modulation. Incoherent data detection using classical DPSK (Differential Phase Shift Keying) causes an SNR loss of approximately 3 dB compared to coherent detection. But in our method, differential detection can estimate the channel coefficients very accurately and our estimated channel can be used in simple coherent detection to improve the system performance and minimize the SNR loss that happens in conventional method

Orthogonal multicarrier modulation for high-rates mobile and wireless communications

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NA

The integration of land, sea, and air forces within the littoral environment will require fading resistant, high data rate, non-exploitable communications. The large volumes of video and data information, i.e. Internet access, video teleconferencing and data transfer, required to support the war fighter within a Joint Task Force demands technologies that reduce the interference imposed by poor terrestrial and atmospheric conditions. In order to minimize the effect of frequency selective fading that occurs in these conditions and to provide high data rate communications, this thesis presents the analysis of a broadband cellular system featuring a multicarrier, code division multiple access (CDMA) method. The system designed complies with Federal Communication Commission broadband cellular standards and uses CDMA to reduce the probabilities of detection and interception as well as providing for multiple access, which in conjunction with the multicarrier approach enables on demand access to high data rate communications.http://archive.org/details/annalysisofbroad1094532765NAU.S. Navy (U.S.N.) author.Approved for public release; distribution is unlimited

Joint Detection and Decoding of High-Order Modulation Schemes for CDMA and OFDM Wireless Communications

Wireless communications call for high data rate, power and bandwidth efficient transmissions. High-order modulation schemes are suitable candidates for this purpose as the potential to reduce the symbol period is often limited by the multipath-induced intersymbol interference. In order to reduce the power consumption, and at the same time, to estimate time-variant wireless channels, we propose low-complexity, joint detection and decoding schemes for high-order modulation signals in this dissertation. We start with the iterative demodulation and decoding of high-order CPM signals for mobile communications. A low complexity, pilot symbol-assisted coherent modulation scheme is proposed that can significantly improve the bit error rate performance by efficiently exploiting the inherent memory structure of the CPM modulation. A noncoherent scheme based on multiple symbol differential detection is also proposed and the performances of the two schemes are simulated and compared. Second, two iterative demodulation and decoding schemes are proposed for quadrature amplitude modulated signals in flat fading channels. Both of them make use of the iterative channel estimation based on the data signal reconstructed from decoder output. The difference is that one of them has a threshold controller that only allows the data reconstructed with high reliability values to be used for iterative channel estimation, while the other one directly uses all reconstructed data. As the second scheme has much lower complexity with a performance similar to the best of the first one, we further apply it to the space-time coded CDMA Rake receiver in frequency-selective multipath channels. We will compare it to the pilot-aided demodulation scheme that uses a dedicated pilot signal for channel estimation. In the third part of the dissertation, we design anti-jamming multicarrier communication systems. Two types of jamming signals are considered - the partial-band tone jamming and the partial-time pulse jamming. We propose various iterative schemes to detect, estimate, and cancel the jamming signal in both AWGN and fading channels. Simulation results demonstrate that the proposed systems can provide reliable communications over a wide range of jamming-to-signal power ratios. Last, we study the problem of maximizing the throughput of a cellular multicarrier communication network with transmit or receive diversity. The total throughput of the network is maximized subject to power constraints on each mobile. We first extend the distributed water-pouring power control algorithm from single transmit and receive antenna to multiple transmit and receive antennas. Both equal power diversity and selective diversity are considered. We also propose a centralized power control algorithm based on the active set strategy and the gradient projection method. The performances of the two algorithms are assessed with simulation and compared with the equal power allocation algorithm

Papr analysis and channel estimation techniques for 3GPP LTE system

High data rates and secured data communication has become an unavoidable need of every mobile users. 3G technology provided greater data speed and secured networks compared to its predecessor 2G or 2.5G. The highest bit rates in commercially deployed wireless systems are achieved by means of Orthogonal Frequency Division Multiplexing (OFDM) [1]. The next advance in cellular systems, under investigation by Third Generation Partnership Project (3GPP), also anticipates the adoption of OFDMA to achieve high data rates. But a modified form of OFDMA i.e. SCFDMA (Single Carrier FDMA) having similar throughput performance and essentially the same complexity has been implemented as it has an edge over OFDMA having lower PAPR (peak to average power ratio) [2]. SCFDMA is currently a strong candidate for the uplink multiple access in the Long Term Evolution of cellular systems under consideration by the 3GPP. In our project we have worked on PAPR analysis of OFDMA, SCFDMA and various other SCFDMA (with different subcarrier mapping). Though SCFDMA had larger ISI it has lower PAPR which help in avoiding the need of an efficient linear power amplifier. We have analyzed various modulation techniques and implemented various kinds of pulse shaping filters and compared the PAPR for IFDMA, DFDMA and LFDMA (kinds of SCFDMA). Like other communication systems, in SCFDMA we encounter many trade-offs between design parameters (such as roll-off factor) and performance. The project report also constitutes the channel estimation techniques implemented in OFDM systems. Due to multipath fading the channel impulse response fluctuates for different subcarriers in different time slots. But with channel estimation OFDM systems can use coherent detection instead of differential. For MIMO system like OFDM channel information is vital for diversity combining and interference suppression [3]. So we need to estimate the channel as accurately as possible. As we have taken a slow Rayleigh fading channel in our study we used block type pilot arrangement channel estimation which uses LS (least square), MMSE (minimum mean square error) estimator. Due to higher complexity of the MMSE estimator, modified MMSE is implemented where tradeoff is made with performance. Here we have compared various channel estimation techniques used in OFDM systems. There are various other adaptive estimation techniques like LMS and RLS for estimating blind channels and comb type pilot arrangement estimation techniques for fast fading channels