A Novel Multi-h Cpm-sc-fdma Transmission Scheme for Aeronautical Telemetry

In this paper, we propose multi-h CPM-SC-FDMA, a novel transmission scheme which combines many of the key characteristics of continuous phase modulation (CPM) with single carrier Frequency Division Multiple Access (SC-FDMA) to produce a power efficient, robust modulation which is suitable for high-rate multiple-access aeronautical telemetry applications. The basis of our approach is found in the observation that the discrete-time samples from a CPM waveform constitute a constant envelope time domain sequence which can be pre-coded and subsequently mapped to a set of orthogonal subcarriers for an FDMA-style transmission. The resulting waveform exhibits power efficiency and easily supports multiple access. INTRODUCTION Continuous phase modulation (CPM) forms a class of constant envelope, continuous phase signaling formats that are known to be efficient in both power and bandwidth [1]. The constant envelope property results in a peak-to-average power ratio of unity, thus making it ideal in aeronautical telemetry applications, where restrictions on device size and weight warrant efficient power amplification. Advances in modern aeronautical telemetry system complexity have driven operation to larger bands in order to accommodate data rates on the order of 10–20 Mbits/s in the spectrum allocated to aeronautical telemetry in the United States. Consequently, spectral efficiency has become an important criterion for system design and performance. While PCM/FM has been the dominant form of carrier modulation in aeronautical telemetry for over 40 years, the relatively recent identification of a class of constant envelope waveforms with higher spectral efficiency has culminated in the adoption of shaped offset QPSK (SOQPSK) [2] and the Feher patented QPSK (FQPSK) [3] as Tier I waveforms, and the Advanced Range telemetry (ARTM) multi-h CPM Tier II waveform [4]—all of which can be modeled as variants of CPM waveforms. Now, operating over larger bandwidths and having an increased need for spectral efficiency, these communications systems must cope with a plethora of challenges, including the determination of the min1 imumal carrier spacing between distinct telemetry signals, the presence of adjacent channel interference, the development of efficient user separation techniques, and compensati8on for the effects of frequency selective multipath fading. An important criterion for efficient spectrum use is that the subcarriers assigned to distinct telemetry signals be spaced as closely together as possible, and this has been the subject of study [5]. However, as the carrier spacing between telemetry signals decreases, the effect of overlapping spectra is to create adjacent channel interference, which limits receiver performance. Recently, the use of interference cancellation techniques has been investigated as a method to allow denser carrier packing [6, 7]. For communication systems offering high data rates, orthogonal frequency division multiplexing (OFDM) has received a lot of attention in the past few years. OFDM is a popular broadband wireless syste, which is currently in use in wireless LAN [8]–[9], fixed broadband wireless access [10] and in digital video and audio broadcasting [11]–[12]. The spectral efficiency of this system is based on the salient observation that the orthogonality of subcarriers provides a way to pack more subchannels into the same channel spectrum. In OFDM, the subcarriers are generated using the computationally efficient Discrete Fourier Transform (DFT) and can thus exploit the well-known circular convolutional properties of the DFT in order to implement low complexity frequency domain equalization techniques. In addtiion, the orthogonality of the subcarriers allows users to transmit with overlapping spectra. Consequently, interference cancellation techniques are not necessary. Although OFDM has many excellent characteristics, it suffers from a high peak-to-average power ratio, which degrades the transmit power efficiency. Single carrier FDMA (SC-FDMA) is a variant of Orthogonal Frequency Division Multiple Access (OFDMA) which utilizes single carrier modulation and frequency domain equalization. One advantage over OFDM is that SC-FDMA generally exhibits lower peak-to-average power ratio because of its inherent single carrier structure [13]. SC-FDMA continues to draw a great deal of attention as an attractive alternative to OFDMA, particularly for uplink communications where having a lower PAPR greatly benefits the mobile terminal. SC-FDMA has been adopted for the uplink multiple access scheme for the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), or Evolved UTRA [14]–[15]. In this paper, we propose CPM-SC-FDMA, a novel scheme which combines the power efficiency of CPM with the spectral efficiency and low computational complexity of SC-FDMA. This new modulation integrates key signal features from both technologies in order to enhance the ability of current aeronautical telemetry systems to more easily cope with the classical problems of frequency selective fading, carrier packing, adjacent channel interference and multi-user separation while maintaining the power efficiency advantage of CPM. Our approach is based upon the observation that the discrete-time samples from a continuous-time CPM waveform can be used to transmit a CPM-like waveform in an OFDM-style transmission. Consequently, multiple access is as easily enabled as it is in a conventional SC-FDMA transmission. Hence, the major contribution of this paper is to unveil a hybrid transmission scheme which constitutes a major advance in the state of the art for modern telemetry. OVERVIEW OF SC-FDMA Fig. 1 illustrates a block diagram of an SC-FDMA transmission system [13]. In SC-FDMA, a block of time domain data symbols are transformed to the frequency domain by application of the DFT, and then mapped to a subset of the total available subcarriers, which enables OFDMA modulation. As in OFDMA,