3.6 Modern Modulation and Coding Techniques in the V/W Band
The primary objective of this project is to statistically characterize and model propagation phenomena at V and W band frequencies. With this in mind, the chosen method of data transmission—which encompasses forward error correction coding, modulation, and pulse shaping—must first and foremost assure high signal integrity (i.e. low BER) during clear-day transmission. Other performance metrics such as spectral efficiency and power efficiency should also be optimized.
Satellite links have traditionally employed various forms of phase-shift keying (PSK) for modulating data primarily because of the higher C/N ratios required to achieve acceptable bit error rates using other schemes (which modulate amplitude or frequency instead of phase) and the fact that non-linear envelope distortions caused by saturating high-power amplifiers do not affect constant-envelope modulations (like PSK) [4]. However, requirements for an efficient physical layer design at W-band frequencies suggest that other modulation techniques may be more suitable for achieving high data rate, power efficiency, and spectral efficiency. According to [1], the most prominent issues for W-band transmission are the following:
1) heavy path loss due to high carrier frequency
2) presence of non-linear distortions introduced by amplifiers operating at the maximum level of power efficiency
3) presence of time and frequency uncertainties that become increasingly relevant as the data rate grows
4) presence of strong Doppler shift in the signal
5) lack of knowledge of signal propagation modalities in the W-band[1]
The chosen coding and modulation technique for a W-band link must therefore anticipate these issues and compensate accordingly. A few of the most notable W-band modulation and coding techniques which adequately address and compensate for these issues are presented in the following section. They will be thoroughly evaluated in Phase I in order to determine which one is the best fit in terms of cost and other parameters of our system.
Manchester-Coded Split-Phase BPSK Modulation (SP-BPSK)
A specialized type of binary phase shift keying (BPSK) called split-phase Manchester-coded BPSK—implemented in both the DAVID-DCE experiment and WAVE mission—is one such modulation and coding scheme. This scheme is similar to standard BPSK except it introduces a residual carrier in the hole spacing two sidelobes, as shown in Figure 1 [1]. The primary consideration in the decision to use this scheme was the need to ensure signal integrity and easy carrier recovery despite the non-ideal behaviors of devices employed for high-frequency digital transmission. The most prominent of these non-ideal behaviors are phase noise introduced by high frequency oscillators and strong Doppler shift. The residual tone carrier introduced by SP-BPSK greatly simplifies the hardware required for carrier recovery during demodulation in the presence of these non-ideal behaviors (a simple second-order phase-locked loop circuit can be employed provided that the bit rate is much higher than the Doppler shift). Also, since the Manchester-coded BPSK signal is characterized by a constant envelope, it avoids the harmonic and intermodulation distortion usually induced on the signal by the non-linear AM/AM and AM/PM characteristics of the high-powered amplifier installed in the earth station [1]. Results for efficient carrier recovery in the presence of high Doppler shift and high phase noise for a simulated 100 Mb/s W-band data link shown in [1] fully confirm that this scheme can be used to achieve simple, robust, and effective carrier recovery [10].
SP-BPSK with Manchester coding presents advantages for simple carrier recovery, but it is accompanied by disadvantages in spectral efficiency (less than 0.5 bit/sec/Hz), power efficiency (the residual carrier introduces about 3 dB of power waste), and potential symbol duration imbalance. The total degradation in performance at the demodulator side introduced by these factors totals about 4.25 dB with respect to the ideal case [10]. The channel BER achieved by simulations for data at 100 Mb/s without any forward error correction coding is shown in Figure 2 below. At a data rate of 100 Mb/s, the Eb/N0 value of 15 dB corresponds to the clear sky working point of the link budget at a C/N0 equal to 95 dBHz [10]. As shown in the figure, simulations reveal a BER of approximately 8*10-4 in the absence of symbol imbalance and 3*10-3 in the presence of 52% symbol imbalance at this C/N0. In compliance with the Consultative Committee for Space Data Systems standard for telemetry, an RS(255,223) Reed-Solomon coding scheme is used to code uplink data, and an RS(255,223) code concatenated with a ½-rate convolutional code is used to code downlink data. These coding schemes ensure a channel BER of 10-11 at threshold BERs of 10-3 and 1.3 x 10-1 for the uplink and downlink respectively [1]. Ultimately, ASI engineers chose this scheme for the DAVID-DCE experiment and WAVE mission because they believed that despite its drawbacks in power and spectral efficiency, the reduced complexity of carrier recovery hardware that SP-BPSK affords was a more important design consideration because it maximized the possibility of acquiring good data using a channel (the W-band) whose properties were largely unknown.
Figure 1. Amplitude spectrum of a SP Manchester-coded BPSK signal [1].
The mathematical expression for a signal transmitted by Manchester-coded BPSK is Eq. 1:
where ωc is the IF radian frequency, φm is a parameter called the modulation index (equal to 60 in this case), bk is the binary Manchester symbol level, and P is the IF signal power [1].
Figure 2. Simulated BER performances achieved by SP-BPSK Manchester-coded modulation for an uplink channel rate of 100 Mb/s.
16-QAM Modulation with Efficient Carrier Recovery
Also of note is the work done in [3]. Here, engineers propose and test a novel carrier recovery algorithm for 16-QAM modulation which accounts for phase noise and other kinds of non-ideal hardware behavior typical of W-band communication devices. This scheme represents an improvement to the approach taken in DAVID in that it achieves higher spectral efficiency and BER. As mentioned before, satellite links usually employ phase-shift keying modulation instead of a mixed-phase amplitude modulation scheme like 16-QAM, because non-linear envelope distortions caused by saturating high-power amplifiers do not affect constant-envelope modulations (like PSK). However, PSK modulations are vulnerable to phase distortions introduced by amplifiers and filters, and they do not perform nearly as well as M-ary QAM modulations in the presence of additive Gaussian noise [3]. In fact, [12] measured an SNR degradation caused by the use of 16-PSK instead of 16-QAM of about 4.20 dB. This exceeds the approximately 2 dB of loss introduced by channel distortions, making the 16-QAM implementation favorable in terms of SNR. This robustness to Gaussian noise is the primary motivation for exploring 16-QAM as a modulation scheme for a satellite link. Using standard Reed-Solomon forward error correction coding and a novel recovery algorithm based on existing schemes which use the 4th power elevation of the received 16-QAM signals, the authors of [3] proved efficient frequency recovery and tracking in the presence of significant phase noise and high Doppler shift. Simulations also showed that 16-QAM implemented this way can achieve spectral efficiencies much higher than those of SP-BPSK [3].
While 16-QAM does offer some potential system improvements over Manchester-coded BPSK, its implementation requires a coherent demodulator using a sophisticated carrier recovery algorithm. This increases both design complexity and cost. Also, high phase noise was found to have a significant negative influence on data BER in simulations. It should also be noted that very high data transmission rate was a critical design consideration in this paper [3]. (The ultimate goal for W-band links is gigabit connectivity.) The squared 16-QAM signal constellation is shown below in Figure 3.
Figure 3. Squared 16-QAM signal constellation.
Trellis-Coded Modulation
Another viable option is the use of a Trellis-coded modulation (TCM) scheme, as presented in [11]. TCM is a bandwidth-efficient modulation scheme based on convolutional coding which exhibits significant improvements over traditional coding schemes in terms of both spectral efficiency and data error rates. A unique characteristic of TCM is that whereas coding, a digital function, and modulation, an analog function, are performed separately in a typical system, they are combined into one function in TCM. A generic TCM system consists of a Trellis code and constellation mapper, as shown in Figure 4 below. These combine the functions of a convolutional coder of rate R = k/k+1 with an M-ary signal mapper that maps M = 2k input points into a larger constellation of M=2k+1 constellation points [13]. With this system, redundancy is introduced without expanding the bandwidth by using a constellation with more points than would be required for the uncoded data. Coding gains of 3 to 6 dB are typical of systems employing TCM [13].
In [11], they consider the use of M-QAM modulation with TCM for the same reasons as in the previous section and rectangular Non-Return-to-Zero waveforms (therefore supposing an absence of linear filtering and the availability of infinite bandwidth). The system is also designed to meet the following constraints: (1) high data transmission rate (approaching 1 Gb/s) (2) bandwidth of less than 500 MHz (3) realistically modeled presence of non-linear distortions and phase noise (4) use of a robust carrier frequency recovery loop without phase recovery. The important task of carrier recovery in the presence of high levels of phase noise and Doppler shift is performed in the same way as in the previous section (by employing the robust scheme proposed [3]), as this method does not introduce a residual carrier in the spectrum. Simulation results for BER vs. phase noise in the presence of all RF impairments are shown in Figure 5. It is clear from this plot that the impact of phase noise on TCM performance is strong. Therefore, for TCM to be implemented practically for W-band transmission, efficient phase-drift compensation is crucial. Without such compensation, TCM robustness can only be exploited in the case of low phase noise at the input of the demodulator [11].
Spectrally Efficient Pulse Shaping
The solution based on trellis-coded modulation presented in the previous section considers the use of a rectangular pulse as a digital waveform. Such a solution achieves resilience against non-linear distortions but also carries high-power sidelobes. These sidelobes make adjacent channel interference a potentially major issue and increase inter-symbol interference (ISI), severely limiting the spectral efficiency and therefore the available capacity of the channel [10]. These considerations lead us to consider pulse shaping techniques in order maximize spectral efficiency and minimize ISI.
Pulse shaping is the process of changing the waveform of a transmitted signal in order to limit its effective bandwidth and minimize ISI. It is typically implemented in satellite systems by applying an appropriate low-pass filter to baseband data before the data is modulated. Theoretically, zero ISI can be achieved by using a low-pass filter with a raised cosine transfer function, as originally proposed by Nyquist. Although a perfect raised cosine transfer function is not realizable with any physical circuit, the basis for the design of all digital links is the ideal zero ISI Nyquist criterion, because a function that approximates the ideal raised cosine transfer function will minimize ISI and maximize symbol rate [9]. Raised cosine pulses can be generated by digital FIR filters and are much less sensitive to filtering than rectangular pulses [14].
The main drawback of a raised cosine filtering scheme is its susceptibility to non-linear distortions introduced by power amplifiers operating in the saturation region. To mitigate this effect, the working point of the linear amplifier is usually fixed at the border of the linear zone of the AM/AM characteristic, introducing an input back-off of the transmitted power. Therefore, power efficiency is sacrificed in order to remove non-linear signal distortions. This reduced power efficiency translates to a noticeable BER increase at the receiver. Typically, the BER is then raised to acceptable levels by adequate forward error correction coding gain [7]. Other disadvantages of raised cosine filtering include: potential inter-pulse interference caused by unlimited pulse duration in time and envelope fluctuations of the modulated signal resulting in high values of peak-to-average-power-ratio [10].
A novel solution to pulse shaping explored in [14] is the use of prolate spheroidal wave functions (PSWF). These pulses are characterized by a near-optimal tradeoff between spectral compactness and envelope compactness, as the energy of the pulse is concentrated in limited regions both in the time and frequency domains. To achieve this, PSWFs are characterized by a few interesting properties:
1.) pulse waveforms of different orders are mutually orthogonal
2.) pulse width and bandwidth can be simultaneously controlled to match arbitrary spectral masks (adaptive pulse shaping)
3.) pulse width and bandwidth are the same for all orders
Experimental results obtained through realistic simulation trials reveal that although QAM using raised cosine pulses exhibits greater spectral efficiency, QAM using PSWFs greatly reduces the peak-to-average-power ratio (PAPR) (the well-known indicator of envelope compactness) [14]. A table of PAPR of modulated signals using the different pulse shapes in shown in Table 1 below.
Table 1. PAPR of Modulated Signals Using Different Pulse Shaping.
References
[1] Sacchi, C., Gera, G., Regazzoni, C.: W-band Physical Layer Design Issues in the Context of the DAVID-DCE Experiment. Int. Jour. of Satellite Communications and Networking 22(2), 193–215 (2004)
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[4] T. Pratt, C. Bostain, and J. Allnutt, “Modulation and Multiplexing Techniques for Satellite Links,” in Satellite Communications, 2nd ed., Danvers, MA: John Wiley and Sons, Inc., 2003, ch. 5, sec. 5.4, pp. 187.
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[7] Sacchi, C.; Rossi, T.; Ruggieri, M.; Granelli, F.; , "Efficient Waveform Design for High-Bit-Rate W-band Satellite Transmissions," Aerospace and Electronic Systems, IEEE Transactions on , vol.47, no.2, pp.974-995, April 2011 doi: 10.1109/TAES.2011.5751238
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[8] T. Pratt, C. Bostain, and J. Allnutt, “Modulation and Multiplexing Techniques for Satellite Links,” in Satellite Communications, 2nd ed., Danvers, MA: John Wiley and Sons, Inc., 2003, ch. 5, sec. 5.3, pp. 176.
[9] C. Sacchi, T. Rossi, “Overview of PHY-Layer Design Challenges and Viable Solutions in W-band Broadband Satellite Communications,” in Personal Satellite Services, 2010, pp. 3-18.
[10] Sacchi, C., Grigorova, A.: Use of Trellis-Coded Modulation for Gigabit/sec Transmissions over W-Band Satellite Links. In: Proc. of 2006 IEEE Aerospace Conf., Big Sky, MT (2006), vailable on CD-ROM
[11] J.G. Proakis, “Digital Communications” (3rd Edition), McGraw-Hill, New York: 1995.
[12] C. Langton, “Trellis Coded Modulation,” (Intuitive Guide to Principles of Communications), 2004, http://www.complextoreal.com/chapters/tcm.pdf (Accessed: December 1, 2012).
[13] Sacchi, C., Rossi, T., Menapace, M., Granelli, F.: Utilization of UWB Transmission Techniques for Broadband Satellite Connections operating in W-band. In: Proc. of 1st IEEE EHF-AEROCOMM Workshop Conf. (in conjunction with IEEE Globecom 2008), New Orleans, LA (2008)
Figure 4. A general trellis-coded modulation block diagram.
Figure 5. Data BER at the output of TCM decoders versus phase noise [11].
