RESEARCH Wireless Systems Laboratory provides research support and education in wireless transmission systems and networks

• Turbo Coding for CPM Signals and ISI Channels

Iterative decoding of interleaved concatenated codes has proven to be a remarkable development in error control coding since the introduction of turbo codes in 1993. Parallel and serial concatenation of convolutional codes have been shown to provide near Shannon-limit error performance. More recently, bandwidth-efficient modulation schemes have been used in conjunction with concatenated coding to provide large coding gains without bandwidth expansion. Many practical systems like mobile satellite communications and land mobile communications employ non-linear amplifiers at the receiver and, therefore, require a very small peak-to-average power ratio for the modulated signal. Continuous phase modulation (CPM) is an attractive technique for such systems because of its constant envelope and compact power density spectrum. When concatenated codes are combined with trellis coded modulation (TCM) and CPM, we can achieve large coding gains without bandwidth expansion and a constant envelope for the modulated signal, making it ideal for use in power and bandwidth limited communication systems. Many physical channels are inter-symbol interference (ISI) channels. They are either static (telephone) ISI channels or fading (cellular and PCS) ISI channels. It is important to devise techniques to either combat the effects of ISI through efficient equalizer structures, or, mitigate the effects of ISI through the use of techniques like orthogonal frequency division multiplexing (OFDM).
We are undertaking research in the following areas as pertaining to turbo coding: 

• Novel approaches to the design and performance analysis of Turbo TCM schemes with CPM for AWGN, flat Rayleigh/Rician fading channels,
• Receiver design for coherent and non-coherent reception of Turbo Trellis coded CPM,
• Performance analysis of Turbo coded systems for ISI channels and receiver structures for joint equalization, decoding and channel estimation,
• Practical receiver structures for Turbo-coded OFDM systems with adaptive parameter estimation.

　

• Turbo Coding for DS/FH CPM

  This project investigates direct sequence (DS)/frequency hopping (FH) CPM waveforms using serial concatenation of interleaved convolutional codes with CPM. The CPM techniques are known to have a compact signal spectrum consisting of a narrow flat main lobe and fast rolloff of side lobes. The combination of DS and FH spread spectrum with CPM has significant potential for LPI/LPD waveform design. The receiver performs iterative demodulation and decoding similar to the decoding of SCCC. Coding techniques can dramatically improve the receiver sensitivity and, thus, reduce detectability of signals to an unintended receiver. Evaluation of the DS/FH CPM receivers  under various jamming threats and optimization of system parameters for various requirements on bandwidth efficiency will be performed through analysis and simulation.

  　

• Channel Estimation for CPM

  CPM is a power and bandwidth efficient signaling scheme. When we consider the CPM modulator as a finite state machine followed by a memoryless mapper, the demodulation of CPM signals can be implemented by aiding to the principle of Maximum Likelihood Sequence Detection (MLSD) or Maximum A-Posteriori Sequence (or Symbol) Detection (MAPSD), in which ideal knowledge of channel state information is typically assumed. However, in a practical system, the channel is a priori unknown and usually time-varying, and thus must be estimated before CPM demodulation.  Previous techniques  for channel estimation entail the inserting of pilot symbols or training sequences into transmitting data packets. The main disadvantages of these methods are that 1) they reduce the effective signal-to-noise ratio and 2) their tracking ability is very limited for rapidly changing channels. Therefore, there is need to develop joint channel estimation and detection techniques, in which the pilot assistance or training is not required and the good tracking and acquiring performance can be achieved. Recently, the so-called Per-Survivor Processing (PSP) was proposed for the hard-output MLSD at the presence of unknown parameters. In our research, we are making a move to the soft-input soft-output (SISO) MAP detection under uncertain environments. A family of adaptive SISO algorithms are being suggested for joint channel estimation and CPM demodulation.

• Software Defined Radio

Software defined radio is one of the fields of wireless communications that are receiving an increased amount of interest due to the high flexibility with which they provide wireless communications systems. Research in this field is mainly directed towards improving the architecture and increasing the computational efficiency of software defined radio systems. In the Wireless Systems Laboratory, different methods for improving the computational efficiency and reducing the cost of implementing and operating the computationally demanding parts of software defined radio systems such as the ADC/DAC, sample rate conversion, and channelization, are considered.

　

• Bluetooth and 802.11b Interference Mitigation testing using Software Radio testbed

Bluetooth and IEEE 802.11b operate in the unlicensed 2.4 GHz Industrial, Scientific and Medical (ISM) band, posing a strong interference problem in the dense WLAN/ WPAN (Wireless Local Area Networks / Wireless Personal Area Networks) environments. The objective of the proposed research work is to cancel the Bluetooth interference in the presence of IEEE 802.11b. 'Adaptive Notch Filter' is considered for the interference mitigation.

• Wireless Access to Next Generation Internet

The Yamacraw Mission is a strategic new initiative combining the efforts of the academia, the state government of Georgia and private enterprise to secure Georgia's leadership in microchip design and high-bandwidth communications. Because of research already being done in Georgia, particularly at the Georgia Institute of Technology, three key areas of the telecom industry are targeted: optical and wireless networks, high-speed access devices and content processing. We, the telecom group, along with signal processing, microelectronics and the RF groups of Georgia Tech are working hard in order to design and demonstrate a unique indoor wireless system combining state of the art techniques of the recent times which include the use of space-time processing of signals, orthogonal frequency division multiplexing (OFDM), low density parity check (LDPC) codes etc. to increase the throughput of the wireless channel. In particular, WSL is concerned with the issues related to the receiver implementation of the next generation wireless system. A high speed coherent wireless system has to perform efficient channel estimation and robust time synchronization. Besides guarding against channel estimation and synchronization errors, the receiver has to deal with the drift in sampling clock frequency, non-linearities of the devices and insufficient guard interval. All these algorithms make a wireless receiver work as desired. As part of the Yamacraw project, WSL is also closely involved with the progress of the IEEE 802.16 working group for Broadband Fixed Wireless Access. We have made several key contributions in the area of preamble design for the IEEE 802.16 OFDM systems.

　

• Highly Selective Interference-Resistant Wireless Receivers

  This project is composed of two main research areas - Synchronization and Interference Cancellation. The synchronization is a critical function in any practical receiver. This project addresses synchronization issues for GSM and EDGE under heavy co-channel interference. The goals of synchronization is to devise efficient algorithms for estimating symbol timing, carrier frequency and phase offsets between the transmitter and receiver. It also involves frequency offset and phase tracking as well as estimation of the channel parameters under this stringent environment. Efficient estimation of these parameters will assist the receiver to function properly and make it robust against interference. The outline of interference cancellation is to design a co-channel interference (CCI) resilient TDMA receiver for IS-136/EDGE system in heavy CCI. This includes development of CCI cancellation techniques with MIMO system model and estimation of channel impulse response, carrier phase in decision feedback or iterative approach associated with the CCI cancellation techniques developed.

  　

• Channel Estimation for OFDM

  To combat the effect of intersymbol interference (ISI) while transmitting data over the time-dispersive channels and to support the higher data rate applications in a multimedia wireless services, block-oriented digital communication systems, such as OFDM have the merits over single carrier systems. In the block transmission systems, it is usually assumed that the channel is constant over the duration of a data block, even if the channel fades. Thus, channel estimation plays an important role of the performance of the wireless communication systems of fast time-varying channels. Moreover, an OFDM can reduce frequency-selective multipath channel into frequency-nonselective channel, which is easier to estimate the channels. The conventional techniques to estimate channels is to insert pilot symbols periodically and to interpolate between the pilot symbols using the channel characteristics. In our research, we will implement the linear predictive receiver to estimate the channels based on the Viterbi algorithm. However, since Viterbi algorithm has a drawback in the presence of the unknown channel quantities, the PSP decoding algorithm should be jointly implemented with estimation and detection instead of the Viterbi algorithm. Furthermore, this algorithm also will be extended into the MIMO OFDM channel estimations.

• PAPR Reduction in OFDM

  In wireless mobile radio communications, there is an endless quest for increased capacity and improved quality. OFDM techniques are quickly becoming a popular method for advanced communications networks. One of the major drawbacks of OFDM is its output waveform with a very high peak-to-average power ratio (PAPR).
  Higher capacities in wireless communication systems can be achieved by using MIMO systems. MIMO refers to radio links with multiple antennas at the transmitter and the receiver side. Given multiple antennas, the spatial dimension can be exploited to improve the performance of the wireless link. MIMO in conjunction with OFDM can improve communication quality and capacity. My current research is emphasized on reducing PAPR in MIMO OFDM systems.

• PAPR Reduction and Anti-Jamming in Multicarrier Systems

  Multicarrier modulation techniques such as OFDM have high spectral efficiency since the sub-carriers overlap in frequency and adaptive bit loading techniques can be employed. The intersymbol interference (ISI) is easy to mitigate by employing a cyclic guard interval. The major disadvantage of OFDM is its characteristically high PAPR. Also, spread spectrum (SS) techniques are well known for their ability to counter either intentional jamming or non-intentional co-channel interference from other communication links. Extensions of OFDM have lead to several multicarrier spread spectrum techniques, such as multicarrier frequency-diversity spread spectrum (MC-SS), multicarrier direct-sequence spread spectrum (MC-DS), and multi-tone spread spectrum (MT-SS). However, when compared conventional DS/SS, the complex envelope of a multicarrier spread spectrum signal is not constant, even with BPSK or QPSK signaling. This property is due to the IDFT transform, and will cause distortion when the signal is passed through a nonlinear power amplifier. We will investigate different modulation schemes to address the high PAPR problem of multicarrier modulations for both OFDM and MC-SS. The anti-jamming performance, together with multiuesr access techniques are also investigated for the MC-SS modulation.

• Adaptive Antennas for Mobile-to-Mobile Communications

Information theory has shown that the wireless channel can support enormous theoretical capacities if the multipath is properly exploited using a MIMO system. The capacities increase linearly with the number of antennas without requiring extra bandwidth and power. Considering the demand for high speed wireless services, MIMO systems seem to be the choice for future communication systems. Double mobility also has been shown to improve the performance in some wireless environment. A MIMO system in a mobile-to-mobile environment is a prime interest of the research. Current specific areas of research are:

• Channel Characteristics of MIMO in mobile-to-mobile environment
• MIMO Capacity Analysis

　

• Mobile-to-Mobile Communication Radio Channel Modeling

The past decade or so has witnessed tremendous growth in mobile communications due to its ability to provide communications to people “on the move”. Such mobile links communicate between a fixed i.e. stationary base station and a mobile user. In this sense, the mobility is limited at user end only for all present day mobile radio links. However, future applications and trends portend the need for direct communication between mobile users without need for central base stations in order to provide greater mobility, flexibility and convenience of usage. Such systems are termed mobile-to-mobile or doubly mobile systems. They find applications in military as well as commercial arena in form of ad-hoc mobile wireless networks, intelligent vehicle systems and broadband highway communications. To enable design of such systems, our research focuses on study and characterization of radio propagation channels for mobile-to-mobile links. The main issues addressed are:

• Development of wideband mobile-to-mobile channel models that characterize power delay profiles and power-delay-angle spectra based on data available through field trials
• Development of methods to simulate wideband as well as narrowband mobile-to-mobile channels in software

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