Performance analysis of turbo decoding algorithms in wireless OFDM systems

This thesis presents two original ontributions on orthogonal frequency division multiplexing (OFDM). The first is a thorough investigation of the error performance and channel capacity of asymmetrically clipped optical OFDM (ACO-OFDM). ACO-OFDM is a modulation scheme designed specifically for optical communications. The second concerns the use of multiple-input multiple-output (MIMO) OFDM in radio frequency (RF) systems. A new scheme which uses polynomial cancellation coding (PCC) OFDM with time offsets is proposed. This new scheme is robust to both carrier frequency offset (CFO) and channel estimation errors. OFDM has been used extensively to reduce the effect of dispersion in wired and wireless RF systems. Although dispersion is also a problem in optical wireless systems, OFDM has not until recently been considered for use in these. This is because optical wireless systems use intensity modulation (IM) and so the transmitted signals must be nonnegative and as a result conventional OFDM signals which are bipolar cannot be used directly. ACO-OFDM and DC biased optical OFDM (DCO-OFDM) are two non-negative forms of OFDM developed for optical systems. Both are robust to ISI but ACO-OFDM is in most cases much more power efficient. In this thesis results are presented for the performance of ACO-OFDM, DCO-OFDM and conventional optical modulation schemes such as on-off keying (OOK) and pulse position modulation (PPM) for both dispersive and non-dispersive channels. These show that ACO-OFDM is more tolerant to dispersion and requires less optical power than the other modulation schemes. To investigate the theoretical limits of an ACO-OFDM system, closed form analytical expressions for the channel capacity of ACO-OFDM are derived both for a flat channel and for representative dispersive optical wireless channels. The capacity of ACO-OFDM cannot be compared with the general case of an intensity modulated signal, because no general solution for this has yet been found. Instead the mutual information of an ACO-OFDM signal is calculated and compared with two other signal distributions: an exponential distribution and a clipped-Gaussian distribution. The combination of OFDM and MIMO has been used in several radio standards. However, MIMO OFDM systems like single-input single-output (SISO) OFDM systems are very sensitive to CFO. An improved MIMO PCC OFDM system is proposed in this thesis. In this system, the signals transmitted from different antennae have different time offsets. At the receiver, OFDM symbols are demodulated by using each of these time offsets in a matched detection window. This system shows better performance than a MIMO PCC OFDM system without time offsets in two cases: when there are channel estimation errors and in MIMO systems with more transmit antennae than receive antennae.

Light emitting diode (LED) nonlinearities have a significant impact on the performance of optical wireless (OW) communication systems based on orthogonal frequency division multiplexing (OFDM) modulation. In this paper, different techniques to compensate for the induced nonlinearity distortions are presented and analyzed. First, single-carrier frequency division multiple access (SC-FDMA) is proposed instead of OFDM for OW systems. In SC-FDMA, some of the major OFDM problems such as high peak-to-average power ratio are inherently resolved. It is shown that the performance of OW OFDM systems and their counterpart SC-FDMA systems are traded off for certain input signal powers and modulation orders. Second, OW OFDM systems with hard, soft, and turbo channel coding techniques are considered. It is reported that significant performance gains can be achieved through channel coding. Finally, a novel method to reduce signal clipping distortion through the use of iterative signal clipping (ISC) and multiple transmit LEDs is proposed. The ISC technique is based on iterative clipping of the time domain OFDM signal and transmission from a multiple LED transmitter. Transmit LEDs are synchronized, located close to each other, and placed to emit light in the same direction. Hence, the channel path gains from each LED to the receiver photodiode are similar. The received signals from the different LEDs add coherently at the receiver. Reported results demonstrate that the effect of distortion is eliminated or significantly reduced based on the considered number of transmit LEDs.

Localization networks based on orthogonal frequency-division multiplexing (OFDM) ranging systems provide desirable location-awareness with commodity WiFi chipsets in harsh propagation conditions. Understanding the fundamental limits of OFDM ranging can guide the design and operation of these systems. Existing studies on these fundamental limits generally do not account for the processing impairments such as the estimation error of the phase offset and packet detection delay, yet ignoring processing impairments may lead to inaccurate evaluation of OFDM ranging accuracy. To address such inconsistency, we develop a framework to determine the fundamental limit of OFDM ranging, accounting for both channel parameters and processing impairments. In particular, we derive the equivalent Fisher information (EFI) for the propagation delay of the first path and quantify the effects of the number of multipaths, the number of channels, and processing impairments on the ranging accuracy through case studies. Numerical results verify the insights gained from the theoretical analysis.

The IEEE 802.16 standards are based on Orthogonal Frequency Division Multiplexing (OFDM) and multi-user OFDM (also known as OFDMA) for their desirable characteristics and potentials. Although well established, these standards only provide a framework of necessary mechanisms for deployable systems. Majority of the resource allocation issues are left unspecified. The conventional one-dimension scheduling cannot be readily applied in an 802.16 system. In this paper, a proportional fairness scheduling algorithm for 802.16 systems is proposed that utilizes the flexibility of frame division. Monte Carlo simulations are performed to evaluate these new methods in comparison to previously proposed techniques. The results indicate that the proposed methods lead to satisfactory performances, thanks to their improved two-dimensional feature.

This paper proposed a Guard Interval Less (GI-less) transmission on orthogonal frequency division multiplexing (OFDM) for Long Term Evolution (LTE). The system will utilise the application of parallel packet transmission and the useful operation of the characteristics of IFFT. In addition with perfect frequency equalizer FEQ, the intersymbol and interchannel interference can be eliminated. Therefore, the proposed system offered the similar performance of QOS at a reasonable complexity enhancement. In this paper, an in-depth overview of OFDM with focus on physical layer will be examined.

This paper applies wavelet transform (WT) on Orthogonal Frequency Division Multiplexing (OFDM) and single carrier (SC) digital signals twice (WT-T) to extract the transient characteristics of them, and then by using the transient characteristics in WT-T domain, it is easy to identify the two types of signals. The simulations show that the proposed identification algorithms can obtain good performances in low signal to noise ratio (SNR) and multipath Rayleigh fading channel. In addition, the effects of sample rate and symbol rate on the identification algorithms are analyzed and simulated. To obtain the best performances of the identification algorithms, sample rate is set to be equal to 2.4 times of carrier frequency approximately, therefor, the difference of characteristics between OFDM and SC signals reaches maximum, while OFDM and SC signals can be well separated if only the symbol rate is greater than 20 kHz.

14 - LTE radio access: An overview

A novel ranging technique based on orthogonal frequency division multiplexing (OFDM) is proposed for the ranging systems in space by using the properties of OFDM in time and frequency domains. The coarse estimation of the time delay is obtained by the correlation of the waveforms of OFDM signals in the time domain, and the fine estimation of the time delay is obtained by the phase differences between the subcarriers in the frequency domain by the proposed method. The simulation results show that the performance of the proposed technique is better than that of the pseudonoise ranging method and that of the phase ranging method.

Rapid developments and emergence of diverse wireless systems call for radio frontends with agile and multistandard properties. Within this publication the performance of a direct RF digitizing ΣΔ-modulator based analog-to-digital converter (ADC) is evaluated based on orthogonal frequency division multiplex (OFDM) system simulation on the bitstream layer. The necessary transmit and receive building blocks are introduced as well as the previously developed reconfigurable ADC under test. The proposed approach allows flexible testing of different ADCs by analyzing the influence of the ADC's noise shaping on performance indicators like error vector magnitude (EVM) and symbol error rate (SER). The necessary steps for the combination of the OFDM signal generation and the system level simulation of the ΣΔ-modulator-ADC into a multi-domain simulation are introduced and discussed.

Based on spread spectrum technique, two wireless video transmission schemes are designed and implemented in this paper, namely direct sequence spread spectrum (DSSS) scheme and multi-code spread spectrum (MCSS) scheme. We use software defined radio (SDR) method for hardware implementation of two schemes, where a XILINX Virtex-4 field programmable gate array (FPGA) for the transmitter and another Virtex-5 FPGA for the receiver. In order to evaluate their performance, we compared the two designed schemes with our previous work on orthogonal frequency division multiplexing (OFDM) scheme. The experimental results show that two schemes of DSSS and MCSS have better anti-jamming performance than OFDM scheme at the cost of spectrum efficiency. Also, DSSS scheme possesses higher power efficiency than MCSS scheme and OFDM scheme due to its lower peak-to-average power ratio.

In this paper, we proposed a new PAPR reduction technique using selective mapping (SLM) cascade with adaptive peak power reduction (APPR) methods. This technique is used in a system based on Orthogonal Frequency Division Multiplexing (OFDM). OFDM has many orthogonally modulated sub-carriers which unexpectedly produce high peak to average power ratio (PAPR). The high PAPR will occasionally reach the amplifier saturation region and therefore result in signal distortion, which causes bit error rate (BER) degradation. The proposed method, a sequence of input data is rearranged by SLM for the reduction of PAPR and then fed into the APPR process in the proposed system. The APPR method controls the peak levels of the modulation signal by an adaptive algorithm. It reduces modulation signals over a predefined range. The parameter for simulation used standard WLAN in IEEE 802.11a system. In simulation results, the proposed method shows the improvement on PAPR, power spectrum density (PSD) and high performance BER compared with conventional APPR.

We propose a novel Simultaneous Wireless Information and Power Transmission (SWIPT) scheme to enable a compatible multiplexing of energy and information based on the frequency that can be used with the state-of-art transceivers. In the proposed scheme, the transmitter superimposes a Direct-Current (DC) signal on Orthogonal Frequency-Division Multiplexing (OFDM) Radio Frequency (RF) signal to generate OFDM-DC signal for SWIPT where the OFDM part is used for information transmission and the DC signal is used for energy transfer. The power level of the DC signal is configured by a controller at the transmitter side. The corresponding receiver is constructed with direct-conversion-receiver front-end such that the DC signal is extracted using a low-pass filter whose bandwidth is configured based on the subcarrier spacing of the OFDM waveform and then fed to a voltage-to-current converter to generate current to charge a battery for energy storage. The OFDM signal without DC power signal is then fed to Analog-to-Digital (A/D) converter for information decoding. The numerical and simulation evaluations provide encouraging evidence in the favor of the proposed OFDM-DC scheme.

This paper addresses cyclic delay diversity (CDD) on an ultrawideband communication system based on orthogonal frequency division multiplexing (OFDM) technique. Symbol error rate (SER) and outage probability have been derived. Results are obtained using a simplified version of IEEE802.15.3 standard channel model which takes into account multipath clustering and Poisson arrival properties. It is shown that with only two transmit antennas, CDD effectively improves SER performance and reduces outage probability significantly especially on the channel with short delay spread. Both simulation and analytical results agree well in all considered cases.

In this paper, a downlink wireless transmission scheme is proposed for roadside deployment. The scheme is based on orthogonal frequency-division multiplexing (OFDM) technology, and it also employs virtual multiple input multiple output (MIMO) to further improve system capacity. In the "virtual" MIMO scheme proposed in this paper, signals can be sent from two base stations (BS) to one user, which creates a virtual MIMO system that may offer additional capacity gains in fading channels. This scheme is especially fit for the bi-sectional BSs with zonal coverage along roadside. Simulation results show that the proposed scheme can support both high and low speed users on road, and achieve a high system capacity gain especially in the cell edge. Within this scheme, the OFDM symbols are characterized into two groups: localized resource group (LRG), which is continuous in time-frequency (TF) domain; and distributed resource group (DRG) which is TF discrete. As LRG is fit for low-speed users to utilize the slowly varying channel while DRG is fit for high-speed users with rapid varying channels, the combination of them makes the allocation of OFDM subchannel more elastic to support users with different speed

Compensating signal clipping distortions due to light-emitting diode (LED) operational constrains in optical wireless communication (OWC) systems based on orthogonal frequency division multiplexing (OFDM) modulation is achieved through the use of iterative signal clipping (ISC) and multiple LED transmitters.