Multicodes for improved range resolution in radar

Third generation (3G) wireless systems are required to support a variety of communication services like voice, image, motion picture transmission, etc, each of which requires different transmission rates. Multi-code modulation has been introduced therefore as a means of supporting multi-rate services and operating in multi-cell environments [8, 9, 10]. This multi-rate multi-function capability may be used in Radar related applications, too. For example, a single transmitted waveform consisting of two orthogonal codes can be used to simultaneously track a target and obtain high range resolution. Tracking requires low bandwidth and high resolution needs a high bandwidth signal. Orthogonal codes like Walsh codes can be used to provide multiple rates if the codes are chosen from the same matrix, because certain Walsh codes of the same length have very different bandwidths. Therefore, as an extension to its use in communication, multi-codes can be used to enable multi-function operations in a Radar system. The first criterion for choosing a Radar waveform, whether single or multi-code, is its resolving capability in range and Doppler. A measure of range resolution or sensitivity to delay commonly used in Radar literature is the Peak to Sidelobe Level Ratio (PSLR) of the code\u27s autocorrelation function. The multi-codes proposed in this work are found to have better (lower) PSLRs than existing radar codes when the number of simultaneously transmitted codes is large. In the special case of using an entire set of orthogonal codes of any length, the resulting multi-code consists of just a single pulse of thickness equal to the chip width of the code used. This pulse will have a \u27perfect\u27 autocorrelation function with only a single peak at the main lobe and zero sidelobes. This gives the ideal PSLR for radar purposes. An important aspect of using multi-codes in Radar is the need for multiple transmitters to avoid the high peak factor that would result if only a single antenna 15 used. This requires the Radar system to have multiple transmitters as in phased array radar. The best example is a multi-function digital array radar that transmits a unique orthogonal code from each of its antenna elements as described by Rabideau and Parker in [13]. The system described in this publication makes use of the array mode of operation of the Radar to transmit orthogonal codes from each antenna element which are then phased and combined at the receiver. The phase (or angle) of the signal at each receive antenna element can be used to better resolve targets that are spatially separated. This thesis introduces the concept of multicodes in Radar. Further, the advantages of using multiple coded waveforms over the known Radar polyphase codes are demonstrated by simulations

Multiuser Detection for the Uplink of Prefix-Assisted DS-CDMA Systems Employing Multiple Transmit and Receive Antennas

In this paper we consider the uplink transmission within a DS-CDMA system employing CP-assisted (Cylic Prefix) block transmission techniques combined with spatial multiplexing techniques that require multiple antennas at both the transmitter and the receiver. We present an efficient frequency-domain receiver structure with iterative MUD (MultiUser Detection). The performance of the proposed receiver can be close to the single user matched filter bound, even for fully loaded systems and/or severely time-dispersive channels.POSI/CPS/46701/2002 - MC-CDMA and the FCT/POCI 2010 research grant SFRH / BD / 24520 / 200

Efficient Radio Resource Allocation Schemes and Code Optimizations for High Speed Downlink Packet Access Transmission

An important enhancement on the Wideband Code Division Multiple Access (WCDMA) air interface of the 3G mobile communications, High Speed Downlink Packet Access (HSDPA) standard has been launched to realize higher spectral utilization efficiency. It introduces the features of multicode CDMA transmission and Adaptive Modulation and Coding (AMC) technique, which makes radio resource allocation feasible and essential. This thesis studies channel-aware resource allocation schemes, coupled with fast power adjustment and spreading code optimization techniques, for the HSDPA standard operating over frequency selective channel. A two-group resource allocation scheme is developed in order to achieve a promising balance between performance enhancement and time efficiency. It only requires calculating two parameters to specify the allocations of discrete bit rates and transmitted symbol energies in all channels. The thesis develops the calculation methods of the two parameters for interference-free and interference-present channels, respectively. For the interference-present channels, the performance of two-group allocation can be further enhanced by applying a clustering-based channel removal scheme. In order to make the two-group approach more time-efficient, reduction in matrix inversions in optimum energy calculation is then discussed. When the Minimum Mean Square Error (MMSE) equalizer is applied, optimum energy allocation can be calculated by iterating a set of eigenvalues and eigenvectors. By using the MMSE Successive Interference Cancellation (SIC) receiver, the optimum energies are calculated recursively combined with an optimum channel ordering scheme for enhancement in both system performance and time efficiency. This thesis then studies the signature optimization methods with multipath channel and examines their system performances when combined with different resource allocation methods. Two multipath-aware signature optimization methods are developed by applying iterative optimization techniques, for the system using MMSE equalizer and MMSE precoder respectively. A PAM system using complex signature sequences is also examined for improving resource utilization efficiency, where two receiving schemes are proposed to fully take advantage of PAM features. In addition by applying a short chip sampling window, a Singular Value Decomposition (SVD) based interference-free signature design method is presented

Iterative detection of multicode DS-CDMA signals with strong nonlinear distortion effects

Whenever a direct-sequence code-division multiple-access (DS-CDMA) signal is the sum of several components associated with different spreading codes [e.g., the DS-CDMA signal to be transmitted by the base station (BS) in the downlink or any multicode DS-CDMA signal], it has high envelope fluctuations and a high peak-to-mean envelope power ratio (PMEPR), setting strong linearity requirements for the power amplifiers. For this reason, it is desirable to reduce the envelope fluctuations of the transmitted signals. The use of clipping techniques combined with frequency-domain filtering was shown to be an effective way of reducing the envelope fluctuations (and, inherently, the PMEPR) of DS-CDMA signals, while maintaining the spectral occupation of the corresponding conventional DS-CDMA signals. To avoid PMEPR regrowth effects, the clipping and filtering operations can be repeated several times. However, the performance degradation due to nonlinear distortion effects on the transmitted signals can be relatively high, particularly when a very low PMEPR is intended (e.g., when a low clipping level and several iterations are adopted). This can particularly be serious if different powers are assigned to different spreading codes. To avoid significant performance degradation in these situations, we consider an improved receiver where there is an iterative estimation and cancellation of nonlinear distortion effects. Our performance results show that the proposed receiver allows significant performance improvements after just a few iterations, even when we have strong nonlinear distortion effects

Coding schemes for multicode CDMA systems.

Zhao Fei.Thesis (M.Phil.)--Chinese University of Hong Kong, 2003.Includes bibliographical references (leaves 87-89).Abstracts in English and Chinese.Chapter 1. --- Introduction --- p.1Chapter 1.1 --- Multirate Scheme --- p.2Chapter 1.1.1 --- VSF Scheme --- p.3Chapter 1.1.2 --- Multicode Scheme --- p.5Chapter 1.2 --- Multicode CDMA System --- p.7Chapter 1.2.1 --- System Model --- p.7Chapter 1.2.2 --- Envelope Variation of Multicode Signal --- p.9Chapter 1.2.3 --- Drawback of Multicode Scheme --- p.11Chapter 1.3 --- Organization of the Thesis --- p.13Chapter 2. --- Related Work on Minimization of PAP of Multicode CDMA --- p.15Chapter 2.1 --- Constant Amplitude Coding --- p.16Chapter 2.2 --- Multidimensional Multicode Scheme --- p.22Chapter 2.3 --- Precoding for Multicode Scheme --- p.25Chapter 2.4 --- Summary --- p.26Chapter 3. --- Multicode CDMA System with Constant Amplitude Transmission --- p.27Chapter 3.1 --- System Model --- p.28Chapter 3.2 --- Selection of Hadamard Code Sequences --- p.31Chapter 3.3 --- The Optimal Receiver for the Multicode System --- p.37Chapter 3.3.1 --- The Maximum-Likelihood Sequence Detector --- p.38Chapter 3.3.2 --- Maximum A Posteriori Probability Detector --- p.41Chapter 4. --- Multicode CDMA System Combined with Error-Correcting Codes --- p.45Chapter 4.1 --- Hamming Codes --- p.46Chapter 4.2 --- Gallager's Codes --- p.48Chapter 4.2.1 --- Encoding of Gallager's Codes --- p.48Chapter 4.2.2 --- Multicode Scheme combined with Gallager's Code --- p.52Chapter 4.2.3 --- Iterative Decoding of the Multicode Scheme --- p.55Chapter 4.3 --- Zigzag Codes --- p.59Chapter 4.4 --- Simulation Results and Discussion --- p.62Chapter 5. --- Multicode CDMA System with Bounded PAP Transmission --- p.68Chapter 5.1 --- Quantized Multicode Scheme --- p.69Chapter 5.1.1 --- System Model --- p.69Chapter 5.1.2 --- Interference of Code Channels --- p.71Chapter 5.2 --- Parallel Multicode Scheme --- p.74Chapter 5.2.1 --- System Model --- p.74Chapter 5.2.2 --- Selection of Hadamard Code Sequences --- p.75Chapter 6. --- Conclusions and Future Work --- p.82Chapter 6.1 --- Conclusions --- p.82Chapter 6.2 --- Future Work --- p.84Bibliography --- p.8

Improving PA efficiency by chaos-based spreading in multicarrier DS-CDMA systems

In this paper, we investigate the effect of spreading sequences on the peak-to-average power ratio (PAPR) in order to improve the power amplifier efficiency of multicarrier direct-sequence code-division multiple access systems. Baseband shaping has been identified to have a key role in reducing PAPR by spreading and we have found that chaos-based spreading sequences give good results as compared with Gold and i.i.d. sequences behaving differently depending on the number of subcarriers

An improved code rate search scheme for adaptive multicode CDMA

Master'sMASTER OF ENGINEERIN