Abstract: Orthogonal Wavelet Spread Spectrum (OWSS), a recently developed high speed WLAN signaling scheme, is a more bandwidth efficient system compared to OFDM. OWSS pulses are derived from the OWDM (Orthogonal Wavelet Division Multiplexing) pulses by spreading them in the wavelet domain, using a suitable family of PN codes. A synthesis tree for generating the OWDM pulses is shown in Fig. 1. OWSS appears to have other significant advantages over current signaling systems like OFDM and TDMA like wide time support, wide frequency support and efficient multipath equalization. This paper discusses the effect of PA non-linearity on the passband spectral characteristics of the OWSS signal, especially the spectral regrowth with varying levels of output backoff based on the Rapp Model, given by , where A is the input signal amplitude and p=2.2. Fig. 2 illustrates the effect of PA non-linearity on the passband spectra of 108 Mbps (18 Msps sampling rate, 64 active users, 64QAM modulation and smoothing window W=200) OWSS. The figure also shows the spectral regrowth for backoff values of -10dB, -7dB and -5dB on the 108 Mbps OWSS spectrum. It is observed that for -6 dB backoff, any significant distortion occurs only at -28 dBr. To compensate for this spectral regrowth, a pre-distortion scheme is employed based on the inverse function of the Rapp PA Model which is illustrated in Fig. 3. The analog OWSS signal is normalized and appropriate backoff is applied (to shift the operating point of the non linear amplifier). The magnitude and phase are extracted from the signal and the magnitude is clipped (if required) and passed through the inverse Rapp function model given by . The predistorted magnitude is then combined with the phase to get the predistorted OWSS signal. Fig. 4 shows the effect of pre-distortion on the OWSS signal for 6dB backoff. As can be seen, with compensation any significant distortion occurs only at -38 dBr as compared to -28dBr without compensation. This represents a significant improvement of 10 dB in spectral regrowth distortion levels in the passband for the 108 Mbps OWSS signal.
Abstract: This paper presents a novel MIMO STC-OFDM technique to achieve both high spectral efficiency and high performance over frequency selective channels. This is accomplished by a combination of MIMO OFDM, group coded transmit signals and antennas, array processing at the receiver for interference suppression on a per carrier basis, and a new decoding scheme. The group coding reduces the complexity of receivers in space time coded MIMO systems. It partitions transmitter antennas into small groups. The received signals are then processed by a technique called group interference suppression. By using interference suppression on a per carrier basis after the FFT at the receiver, the scheme lends itself to use on frequency selective multipath channels. Finally, a novel decoding scheme is employed. It uses samples from frequencies K/2 apart, where K is the total number of frequencies, to introduce another element of diversity, and then uses a least squares step to yield reliable statistics for symbol detection. At 22 dB, this new scheme achieves a BER of 4x10(-5), without coding or interleaving. The data rate achieved, over a bandwidth of 20 MHz, is 144 Mbps with a corresponding spectral efficiency of 9.6 bits/s/Hz. In addition, the scheme is a low complexity scheme.
Notes: IEEE Radio and Wireless Symposium, Orlando, FL, JAN 22-24, 2008
Abstract: A novel MIMO STC-OFDM technique has recently been developed that achieves both high spectral efficiency and high performance over frequency selective channels. This is accomplished by a combination, or layering, of MIMO OFDM, group coded transmit signals and antennas, array processing at the receiver for interference suppression on a per carrier basis, and a new decoding scheme. It also incorporates frequency diversity by pairing, for STC purposes, subcarriers that are K/2 apart, where K is the total number of FFT frequencies. This paper discusses a channel estimation scheme for this new 4x4 MIMO WLAN system. Time domain matrix-vector formulation leads to estimates that have an SNR on the order of 48 dB when just one block of high power QPSK symbols per transmitter is used, while the receiver AWGN SNR is 19 dB or higher. The final 4x2 equivalent channel matrix, for each of the two component subsystems, is shown to have a corresponding SNR of 37 dB. Most importantly, the loss in system BER performance due to the channel estimation process (compared to the known channel case) is found to be negligible. At 19 dB signal SNR, this new scheme achieves a BER of 3x10(-4), without the need for coding or interleaving. The data rate achieved, over a bandwidth of 20 MHz, is 144 Mbps with a corresponding spectral efficiency of 7.2 bits/s/Hz.
Notes: IEEE International Symposium on Circuits and Systems (ISCAS), Kobe, JAPAN, MAY 23-26, 2005
Abstract: A recently reported signaling system called OWSS (Orthogonal Wavelet Spread Spectrum) appears to have significant bandwidth advantage over the familiar OFDM. Moreover, this new technique has been shown to readily extend to high rates, such as 108 Mbps, in a bandwidth efficient manner. This paper discusses a CSMA/CA protocol for the MAC layer of packetized 108 Mbps OWSS, including the selection of key parameters, in order to provide full rate shared access to bursty users. Presented also is a theoretical model for performance analysis which indicates a saturation throughput of 66% and an average packet delay of 5 ms using RTS-CTS - for a moderate number of stations.
Notes: 37th Asilomar Conference on Signals, Systems and Computers, Pacific Grove, CA, NOV 09-12, 2003
Abstract: A recently reported signaling system called Orthogonal Wavelet Spread Spectrum (OWSS) is aimed at high speed WLANs. This letter discusses the generation and spectral properties of its basis pulses and the spectral characteristics of the transmit signal. If all virtual channels are active, the theoretical baseband spectrum of the OWSS systems is shown to be perfectly flat. Further, the simulated passband spectrum is found to be relatively flat and 30-40% more compact than that of the familiar 802.11a OFDM, due largely to the avoidance of the prefix, elimination of channel coding, and a sharp rolloff.
Abstract: Orthogonal wavelet spread spectrum (OWSS) is a new wireless LAN signaling scheme aimed at bit rates of 100 Mbps or higher. Starting with power complementary full-tree wavelet pulses, it spreads them using a suitable set of PN codes. The resulting system pulses preserve the power complementary property and satisfy the generalized Nyquist criterion, while possessing both a wide time and a wide frequency support. Benefits from these properties include successful equalization in spite of deep local fades caused by multipath channels, and high spectral efficiency-30-40% higher than OFDM for comparable net bit rates. This paper presents an overview of OWSS and explores the fundamental limits to its performance. In the overview, it discusses the properties of the OWSS pulses, the transmit signal spectrum, and the structure of the transmitter/receiver. Then, the investigation of fundamental limits to the performance is carried out byformulating the system as a multi-rate signal processing system, using hierarchical matrices, and thereupon minimizing the total mean-square error (TMSE). The TMSE governs the BER performance of the system, and is defined as the sum of the MSE of the unequalized residual error and the MSE due to the channel noise amplified by the forward equalizer. We formulate the problem at the chip level so as to truly discern the fundamental limits to the performance of the equalizer. This approach enables estimation of the optimum equalizer for mitigating the effect of the multipath channel, prior to correlation and detection blocks embedded in the FE-DFE loop, and thereby the system performance. Simulation results demonstrate its effectiveness. For a 108 Mbps system with a 50 ns delayspread channel, a low BER of 10(-5) and spectral efficiency up to 5.2 bits/s/Hz can be achieved at an Eb/No of 19 dB. (c) 2005 Elsevier Inc. All rights reserved.
Abstract: The current popularity of WLANs is a testament primarily to their convenience, cost efficiency and ease of integration. Even now the demand for high data rate wireless communications has increased fourfold as consumers demand better multimedia communications over the wireless medium. The next generation of high speed WLANs is expected to meet this increased demand for capacity coupled with high performance and spectral efficiency. The current generation of WLANs utilizes Orthogonal Frequency Division Multiplexing (OFDM) modulation. The next generation of WLAN standards can be made possible either by developing a different modulation technique or combining legacy OFDM with Multiple Input Multiple Output (MIMO) systems to create MIMO-OFDM systems. This dissertation presents two different basis technologies for the next generation of high speed WLANs: OWSS and MIMO-STC-OFDM.
OWSS, or Orthogonal Wavelet Division Multiplexed - Spread Spectrum is a new class of wavelet pulses and a corresponding signaling system which has significant advantages over current signaling schemes like OFDM. In this dissertation, CSMA/CA is proposed as the protocol for full data rate multiplexing at the MAC layer for OWSS. The excellent spectral characteristics of the OWSS signal is also studied and simulations show that passband spectrum enjoys a 30-40% bandwidth advantage over OFDM. A novel pre-distortion scheme was developed to compensate for the passband PA non-linearity. Finally for OWSS, the fundamental limits of its system performance were also explored using a multi-level matrix formulation. Simulation results on a 108 Mbps OWSS WLAN system demonstrate the excellent effectiveness of this theory and prove that OWSS is capable of excellent performance and high spectral efficiency in multipath channels.
This dissertation also presents a novel MIMO-STC-OFDM system which targets data rates in excess of 100 Mbps and at the same time achieve both high spectral efficiency and high performance. Simulation results validate the superior performance of the new system over multipath channels. Finally as channel equalization is critical in MIMO systems, a highly efficient time domain channel estimation formulation for this new system is also presented. In summary, both OWSS and MIMO-STC-OFDM appear to be excellent candidate technologies for next generation of high speed WLANs.
Abstract: An email system, configured by creating a user controlled email domain for each user, rather than a dedicated email address. The user is then allowed to create unlimited email addresses within the user controlled email domain. The system and method of the present invention provides improved policing of unwanted email communications. When unwanted communications on a compromised email address are detected, they can be identified and effortlessly subverted while known communicators may continue use of the compromised email address.
