A Direct and Generalized Construction of Polyphase Complementary Set with Low PMEPR and High Code-Rate for OFDM System

A major drawback of orthogonal frequency division multiplexing (OFDM) systems is their high peak-to-mean envelope power ratio (PMEPR). The PMEPR problem can be solved by adopting large codebooks consisting of complementary sequences with low PMEPR. In this paper, we present a new construction of polyphase complementary sets (CSs) using generalized Boolean functions (GBFs), which generalizes Schmidt's construction in 2007, Paterson's construction in 2000 and Golay complementary pairs (GCPs) given by Davis and Jedwab in 1999. Compared with Schmidt's approach, our proposed CSs lead to lower PMEPR with higher code-rate for sequences constructed from higher-order ($\geq 3$) GBFs. We obtain polyphase complementary sequences with maximum PMEPR of $2^{k+1}$ and $2^{k+2}-2M$ where $k,M$ are non-negative integers that can be easily derived from the GBF associated with the CS

Quadrature amplitude modulated codes with low peak-to-mean envelope power ratio for orthogonal frequency division multiplexing applications

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2001.Includes bibliographical references (p. 83).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Orthogonal Frequency Division Multiplexing (OFDM) has been adopted as the modulation technique for many of the next generation wireless broadband multimedia communications systems, for example, Digital Audio Broadcasting (DAB), terrestrial Digital Video Broadcasting (DVB), and the wireless local area network (LAN) standards HIPERLAN/2, and IEEE 802.11a. One problem inherent in plain vanilla OFDM is that its signal envelope fluctuates greatly with very high power peaks, necessitating the use of inefficient and complex linear power amplifiers. Solutions to the high peak-to-mean envelope power ratio (PMEPR) problem include signal processing techniques such as clipping, peak windowing, and peak cancellation, as well as coding techniques, i.e. using codes to ensure that only those OFDM signals with low PMEPR are transmitted. It is well known that using codewords generated by mapping binary Golay complementary sequences into BPSK yields OFDM signals with low PMEPR. Frank, Sivaswamy, and others have extended the results of Golay from binary phase shift keying (BPSK) to other PSK constellations. Recently, Davis and Jedwab presented a code structure for these PSK complementary sequences using cosets of first-order Reed-Muller codes in second-order Reed-Muller codes. This yielded OFDM codes using PSK modulation which could be encoded and decoded using well-understood algorithms for Reed-Muller codes. This thesis investigates the properties of quadrature amplitude modulated (QAM) OFDM signals with low PMEPR, focusing in particular on signals based on 4-QAM and 16-QAM constellations. We construct and prove new code structures for sequences in 4-QAM and 16-QAM that result OFDM signals with low PMEPR. Many practical implementations of OFDM use QAM constellations instead of PSK constellations. Thus the codes presented could be used to design pilot symbols for actual OFDM systems, as well as be employed in practical OFDM applications requiring both low PMEPR as well as low computational complexity.by Chan Vee Chong.S.M

Low-PMEPR Preamble Sequence Design for Dynamic Spectrum Allocation in OFDMA Systems

Orthogonal Frequency Division Multiple Access (OFDMA) with Dynamic spectrum allocation (DSA) is able to provide a wide range of data rate requirements. This paper is focused on the design of preamble sequences in OFDMA systems with low peak-to-mean envelope power ratio (PMEPR) property in the context of DSA. We propose a systematic preamble sequence design which gives rise to low PMEPR for possibly non-contiguous spectrum allocations. With the aid of Golay-Davis-Jedwab (GDJ) sequences, two classes of preamble sequences are presented. We prove that their PMEPRs are upper bounded by 4 for any DSA over a chunk of four contiguous resource blocks

Physical Layer Securities in Wireless Communication Systems

Due to the tremendous advancement in the semiconductor and microelectronics technologies, wireless technologies have blossomed in the recent decades. The large scale deployment of wireless networks have revolutionized the way people live. They bring a great deal of convenience and enjoyment to us. Undoubtedly, we have become more and more dependent on these wireless technologies. These include cellular and radio frequency identification (RFID) technologies. However, with great technologies also come great risks and threats. Unlike wired transmissions, the nature of wireless transmissions result in the transmitted signals over the channel can be easily intercepted and eavesdropped by malicious adversaries. Therefore, security and privacy of the employed wireless communication system are easily compromised compared to the wired communication system. Consequently, securing wireless network has attracted a lot of attention in the recent years and it has huge practical implications. Securing wireless networks can be and indeed are performed at all layers of a network protocol stack. These include application, network, data link and physical (PHY) layers. The primary focus of our research is on the PHY layer approaches for securing and attacking wireless networks. In this thesis, we identify three research topics and present our results. They are: 1) PHY layer phase encryption (P-Enc) vs XOR encryption (XOR-Enc); 2) PHY layer signaling scheme to ensure the confidentiality of the transmitted messages from the tag to the reader in RFID systems. 3) Active eavesdropping attack framework under frequency hopping spread spectrum (FHSS) RFID systems. In the first work, we introduce a new OFDM encryption scheme which we call OFDM-Enc, different from convectional XOR-Enc, OFDM-Enc encrypts data by multiplying each of in-phase and quadrature component of the time domain OFDM symbol by a keystream bit. We then perform an initial investigation on the security of OFDM-Enc. We show it is secure against all attacks that are considered in this work. Moreover, depending on the modulation type, OFDM would potentially reduce the keystream size required for encryption, while still achieving the required security level. We also conduct simulations to compare OFDM-Enc with conventional XOR-Enc. We show indeed OFDM-Enc is viable and can achieve good performances. Then we extend OFDM-Enc to general communication systems. Since the encryption is essentially done by changing the phase of the data constellations, we just adopt the term P-Enc. In addition, we form mathematical formulations in order to compare between P-Enc and XOR-Enc in terms of efficiency, security and hardware complexity. Furthermore, we show P-Enc at the PHY layer can prevent traffic analysis attack, which cannot be prevented with the upper layer encryptions. Finally, simulations are conducted again to compare the performance of P-Enc and XOR-Enc. In the second work, we are interested in protecting tag's data from leaking or being compromised to malicious adversaries. As discussed earlier, due to the nature of wireless channels, communications between the tag and the reader is susceptible to eavesdropping. The conventional method uses encryption for confidentiality protection of transmitted messages. However, this requires to pre-share keys between the reader and the tag. As a result, a key management and distribution system needs to be put in place. This introduces heavy system overhead. In this work, we first propose a new PHY layer RFID privacy protection method which requires no pre-shared keys and would achieve the same goal. We also perform theoretical analysis to first validate of our proposed scheme. Finally, we conduct experiments to further verify the feasibility our proposed scheme under the passive eavesdropping attack model. In the third work, we present a new attack on the FHSS RFID system called active eavesdropping attack. In most semi-passive and passive RFID systems, tag to reader communications are accomplished via backscattering modulation. This implies the tag is not required to identify the frequency of the legitimate reader's transmitted signal, it simply responds to a reader's query by setting its impedance in the circuitry to low and high to represent bit 1 and 0. The attacker exploits this design weakness of the tag and broadcasts his own continuous wave (CW) at a different frequency. Consequently, the eavesdropper receives two copies of responses: one from his own broadcasted CW and one from reader's CW. We perform theoretical analysis to show the optimal strategy for the attacker in terms of the decoding error probability. Finally, we conduct simulations and experiments to verify with our theoretical results

Golay Complementary Sequences Over the QAM Constellation

In this paper, we present new constructions for $M^{2}$ -QAM and $2M$ $Q$-$PAM$ Golay complementary sequences of length $2^n$ for integer $n$, where $M=2^{m}$ for integer $m$. New decision conditions are proposed to judge whether an offset pairs can be used to construct the Golay complementary sequences over constellation, and with the new decision conditions, we prove the conjecture 1 proposed by Ying Li~\cite{16}. We describe a new offset pairs and construct new $64$-$QAM$ Golay sequences based on this new offset pairs. We also study the $128$-$QAM$ Golay complementary sequences, and propose a new decision condition to judge whether the sequences are $128$-$QAM$ Golay complementary

New Methods to Construct Golay Complementary Sequences Over the QAMQAM Constellation

In this paper, based on binary Golay complementary sequences, we propose some methods to construct Golay complementary sequences of length $2^n$ for integer n, over the $M^2$-$QAM$ constellation and $2M$-$Q$-$PAM$ constellations, where $M=2^m$ for integer $m$. A method to judge whether a sequence constructed using the new general offset pairs over the $QAM$ constellation is Golay complementary sequence is proposed. Base on this judging rule, we can construct many new Golay complementary sequences. In particular, we study Golay complementary sequences over $16$-$QAM$ constellation and $64$-$QAM$ constellation,many new Golay complementary sequences over these constellations have been found