Inter-carrier Interference Research Articles

Comparison of SPS and FA methods for Blind Carrier Frequency Offset Estimation in OFDM systems

In orthogonal frequency-division multiplexing (OFDM) systems, synchronization issues are of great importance since synchronization errors might destroy the orthogonality among all subcarriers and, therefore, introduce intercarrier interference (ICI) and intersymbol interference (ISI). Several schemes of frequency offset estimation in OFDM systems have been investigated. This paper compares performance and computational complexity of Smoothing Power Spectrum (SPS) and Frequency Analysis (FA) methods for blind carrier frequency offset (CFO) estimation in OFDM systems.

Cell-Free Massive MIMO-OFDM Over Spatially Correlated Doubly Selective Channels

This paper investigates the impact of doubly selective fading channels on the design and performance analysis of cell-free massive multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) networks. Time and frequency selectivity, as well as the intercarrier interference (ICI) effects, are fully characterized, and the spatial channel cross-covariance matrices associated to each mobile station (MS)/access point (AP) pair and for every OFDM symbol/subcarrier conforming a particular resource block (RB) are derived. Assuming that these cross-covariance matrices are known at the APs, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">optimal</i> minimum mean square error (MMSE)-based and suboptimal projected MMSE (PMMSE)-based double selectivity-aware channel estimators/predictors are designed. Spatial cross-covariance matrices and channel estimates/predictions are then used to design scalable double selectivity-aware partial-MMSE (P-MMSE), averaged P-MMSE (AP-MMSE) and statistically averaged P-MMSE (SAP-MMSE) combiners and precoders to be used during the uplink (UL) and downlink (DL) payload data transmission phases. Extensive numerical simulations are conducted to assess the performance of the proposed double selectivity-aware scalable cell-free massive MIMO systems under scenarios showing different degrees of time and frequency selectivity. Simulation results clearly show the performance advantages of double selectivity-aware channel estimation/prediction schemes and combining/precoding strategies when working in high-mobility multipath scenarios of practical interest. Moreover, it is shown that, even though suboptimal PMMSE/AP-MMSE and PMMSE/SAP-MMSE strategies perform relatively well under poor frequency and time selectivity conditions, they suffer from very significant performance losses with respect to the MMSE/P-MMSE scheme as frequency and/or time selectivity levels increase.

Interference-Aware Accurate Signal Recovery in Sub-1 GHz UHF Band Reuse-1 Cellular OFDMA Downlinks

Reuse-1 systems operating in the sub-1 GHz UHF band are limited by substantial co-channel interference (CCI). In such orthogonal frequency division multiple access (OFDMA) cellular systems, the inter-sector or inter-tower interference (ITI) makes accurate signal recovery quite challenging as sub-1 GHz bands only support single-input single-output (SISO) links. Interference-aware receiver algorithms are essential to mitigate the ITI in such low-frequency bands. Such algorithms enable ubiquitous mobile broadband access over the entire homeland, say with >95% geographical coverage with quality of service guarantees. One element of the interference-aware signal recovery is the least-squares-based joint channel estimation scheme that uses non-orthogonal pilot subcarriers. This estimator is then compared with a variant that uses orthogonal pilot subcarriers to bring out the advantage of this joint estimator. It is shown that the proposed joint estimator requires fewer pilots to be well-determined when compared to its under-determined orthogonal counterpart. Moreover, it is easy to implement and does not require any knowledge of channel statistics. This work also derives a compensation factor needed for the interference-aware detector in the presence of inter-carrier interference (ICI) originating from multiple transmitters. Simulation results show that the proposed joint channel estimator outperforms traditional estimators at moderate to high frequency selectivity. The proposed compensation factor to the joint detector is found to be essential for recovering the transmitted signal in the absence of phase-tracking pilots.

Uniform and Nonuniform Filter Banks Design Based on Fusion Optimization

One of the important schemes for modern communication is Filter Bank Multi-Carrier with Offset Quadrature Amplitude Modulation (FBMC OQAM), as it provides better spectral efficiency with small inter-symbol and inter-carrier interference specially in data fusion platforms. Unfortunately, the design of filter banks in FBMC is difficult and complex to achieve the requirement due to complexity of handling the data fusion issues. This paper presents a proposed method to design a uniform and nonuniform filter banks using a data fusion optimization technique. The design process represented as an objective function describes the amplitude in the stop band, and the goal is to minimize the objective function. Different examples are provided to illustrate the efficiency of the proposed design method.

Analysis of Doppler and Multipath on Orthogonal Chirp Division Multiplexing in Shallow Water Acoustic Channel

In the scenario of underwater acoustic communication, Orthogonal Frequency Division Multiplexing (OFDM) is widely used for its high spectral efficiency and high transmission rate. However, OFDM is sensitive to Doppler shift, which will generate inter-carrier interference to degrade bit error rate performance. Orthogonal Chirp Division Multiplexing (OCDM) consists of multiplexing a number of chirp waveforms that are mutually orthogonal with each other and share the same bandwidth and time slot, achieving good robustness against multipath and Doppler shift. This study reviews the concepts of Orthogonal Chirp Division Multiplexing and combines this multicarrier communication scheme with an underwater acoustic channel. Unlike previous studies analyzing within the signal domain, this study formulates the mathematical model of OCDM in shallow water acoustic channel and illustrates the reason why OCDM can outperform OFDM in both Doppler robustness and multipath robustness, which is a necessary and solid complement for OCDM in the further development of underwater acoustic communication.

GFDM-Based Asynchronous Grant-Free Multiple-Access

For next-generation Internet-of-Things (IoT) networks, asynchronous instant transmission has attracted increasing research interest with the expectation of achieving near-zero latency without excessive initiation procedure. However, in an asynchronous multiple-access scenario, there exist significant inter-carrier interference between sub-carriers allocated to different users. To suppress out-of-band emission (OOBE) of each sub-carrier, a new generalized frequency division multiplexing (GFDM) has been proposed, which has lower OOBE than the conventional orthogonal frequency division multiplexing (OFDM). In this paper, by using GFDM, two types of receivers are proposed with the aim of reducing latency and improving throughput: a GFDM-based minimum mean square error (MMSE) receiver and a GFDM-based MMSE-successive interference cancellation (SIC) receiver. Then, we develop a lightweight scheme using an <inline-formula> <tex-math notation="LaTeX">$\epsilon $ </tex-math></inline-formula>-conservative rate control with GFDM-based MMSE receivers and also invent a performance-focused scheme using an advanced rate control with GFDM-based MMSE-SIC receivers. In particular, the latter scheme provides higher throughput with limited increase in computational load of user equipments. Numerical results show that with a high successful transmission probability higher than 99 &#x0025;, the performance-focused scheme and the lightweight scheme achieve up to 85 &#x0025; and up to 70 &#x0025; higher throughput compared to the conventional OFDM-based asynchronous multiple-access scheme, respectively. Furthermore, since our proposal does not require any centralized user scheduling or initiation procedure, it presents a significant reduction in latency compared to the existing low-latency technologies.

Precoded SLM and PTS Scheme for Reduced PAPR in OFDM for Wireless Communications

Orthogonal Frequency Division Multiplexing (OFDM) contribute high data rates in current as well as future wireless communication systems but it aches from high Peak to Average Power Ratio (PAPR). But this high PAPR makes the High-Power Amplifier (HPA) complex which increases the cost of it and leads to the drawbacks like Inter Carrier Interference (ICI) and rise in out of band radiation. Even though many techniques are there to decrease the complexity of HPA by reducing the PAPR, Selected Mapping (SLM) and the Partial Transmit Sequence (PTS) provides less PAPR with low complexity. In this paper, both PTS and SLM are combined with Discrete Sine Transform (DST) and their PAPR and efficiencies are also compared with OFDM signal. The proposed hybrid DST-PTS and DST-SLM provides less PAPR compared to OFDM, SLM and PTS techniques with low complexity. Also, the bit error rate for DST-SLM and DST-PTS techniques are compared for different values of SNR. In future wireless communication systems, these proposed techniques can be used as they provide less bit error rate and less PAPR with low complexity

PERFORMANCE EVALUATION OF FILTER BANK MULTI-CARRIER MODULATION IN MULTIPATH FADING CHANNELS

Filter Bank Multi-Carrier (FBMC) modulation is one of the most significant enablers for future 5G technologies. It is a modulation technique for resolving inter-carrier and inter-symbol interference using two possible methods: Frequency Spreading (FS) and Poly Phase (PP) implementation. Cyclic prefixes are used in OFDM for signal robustness, but they have some disadvantages in orthogonal frequency division multiplexing. FBMC is used to solve the disadvantages of OFDM and save a bandwidth. In this paper, performance comparisons in terms of symbol error rate between OFDM and FBMC systems in AWGN and multipath fading channels are presented. The obtained results show that FBMC over performs OFDM in multipath fading channels and the improvement margin is increased as the number of subcarriers decreased.

Reduced-state maximum-likelihood detection for OFDM systems with insufficient cyclic prefix

Dispersive channels deteriorate the bit error rate (BER) performance of orthogonal frequency-division multiplexing (OFDM) systems with insufficient cyclic prefix (CP). In this paper, we first analyze the intensity of the intersymbol interference (ISI) and intercarrier interference (ICI). For performance enhancement, an effective maximum-likelihood (ML) detection method, named reduced-state maximum-likelihood (RSML), is proposed to detect all target subcarriers parallelly. For each target subcarrier, the proposed RSML detects a few nearby subcarriers jointly, while the remaining subcarriers are directly determined by using pre-detection values. Simulations results show that the proposed RSML achieves a significant performance improvement in terms of BER compared to the existing detection methods and is robust to various channel models and CP lengths. In particular, for certain channel conditions with insufficient CP, the detection performance of RSML is even better than conventional detection with sufficient CP because additional frequency diversity gain is obtained from joint detection at the expense of complexity.

Iterative Interference Cancellation for Multi-Carrier Modulation in MIMO-DWT Downlink Transmission

The Multiple-Input Multiple-Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) scheme represents the dominant radio interface for broadband multicarrier communication systems. However, with insufficient Cyclic Prefixes (CP), Inter-Symbol Interference (ISI) and Inter-Carrier Interference (ICI) occur due to the time-varying multipath channel. This means that the performance of the system will be degraded. In this paper, we investigate the interference problem for a MIMO Discrete Wavelet Transform (MIMO-DWT) system under the effect of the downlink LTE channel. A Low-Density Parity-Check (LDPC) decoder is used to estimate the decoded signal. The proposed iterative algorithm uses the estimated decoded signal to compute the components required for ICI/ISI interference reduction. In this paper, Iterative Interference Cancellation (IIC) is employed to mitigate the effects of interference that contaminates the received signal due to multiple antenna transmission and a multipath channel. An equalizer with minimum mean square error is considered. We compare the performance of our proposed algorithm with the traditional MIMO-OFDM scheme in terms of bit error probability under insufficient CP. Simulation results verify that significant improvements are achieved by using IIC and MIMO-IIC for both systems.

Pilot-Assisted OFDM for Underwater Acoustic Communication

Multicarrier techniques have made it possible to wirelessly transmit data at higher rates for underwater acoustic (UWA) communication. Several multicarrier techniques have been explored in the past for wireless data transmission. OFDM is known to fight off inter-symbol interference due to the orthogonality of its subcarriers. However, due to time variations, OFDM suffers from intercarrier interference. As the UWA channel is both a time and frequency variant, channel estimation becomes complex. We propose a pilot-based channel estimation technique and explore two equalizers for improving the error performance of an OFDM-based UWA system. Both the equalizers employ pilot subcarriers to estimate the UWA channel. One equalizer is a least squares (LS) equalizer and the other is a zero forcing (ZF) equalizer. Using computer simulations, it is observed that, for an acceptable error performance, the number of pilots should be one-fourth the number of subcarriers. Moreover, if the energy of the pilots is increased without changing the overall symbol energy, the error performance degrades. It is also noted that both the LS and ZF equalizers give an acceptable error performance with the ZF performing marginally better than the LS. Furthermore, the error performance of the proposed system is evaluated as a function of the transmitter-receiver distance and an acceptable error performance is observed even at 1250 m.

Turbo DFE Assisted Time-Frequency Packing for Probabilistically Shaped Terabit Superchannels

We present a probabilistically shaped (PS) time-frequency-packing (TFP) wavelength-division multiplexing superchannel system employing higher order modulation (HoM) formats, with an objective to improve the spectral efficiency (SE). However, TFP systems suffer from inter-symbol interference (ISI) and/or inter-carrier interference (ICI). Additionally, the bandwidth limitations of the wavelength selective switches in the fiber link may cause severe ISI for the edge sub-channels (SCs) in a superchannel. Mitigation of such interferences is particularly challenging for HoM systems, since the implementation of the well-known turbo equalization schemes, such as the Bahl-Cocke-Jelinek-Raviv equalizer, is computationally challenging for larger constellations. In this paper, we investigate an expectation propagation based, computationally efficient, turbo decision feedback equalizer for ISI cancellation, in tandem with a parallel interference cancellation based ICI mitigation. By optimizing the parameters in the shaping and the packing dimensions, we show through our numerical results that the proposed PS-TFP superchannels enabling 1.8 Tbps data rates offer up to 1.05 dB packing gain over an unpacked system using the same modulation format, and a 1.15 dB shaping gain over an unshaped system that uses a lower modulation order to achieve the same SE.

Partial Fractional Fourier Transform (PFrFT)-MIMO-OFDM for Known Underwater Acoustic Communication Channels

Communication over doubly selective channels (both time and frequency selective) suffers from significant intercarrier interference (ICI). This problem is severe in underwater acoustic communications. In this paper, a novel partial fractional (PFrFT)-MIMO-OFDM system is proposed and implemented to further mitigate ICI. A new iterative band minimum mean square error (BMMSE) weight combining based on LDLH factorization is used in a scenario of perfect knowledge of channel information. The proposed method is extended from SISO-OFDM configuration to MIMO-OFDM. Simulation results demonstrate that the proposed PFrFT-LDLH outperforms the other methods in the SISO-OFDM scenario and that its performance can be improved in MIMO-OFDM scenarios.

Comparison of digital interpolation and phase compensation in asynchronous optical DMT transmission systems

The sampling frequency offset (SFO) will induce phase rotation, inter-symbol interference and inter-carrier interference, which may seriously deteriorate the receiver’s performance in asynchronous optical discrete multi-tone (DMT) systems. In this work, a high-accuracy training symbols-based SFO estimation (SFOE) method is studied in both the additive white Gaussian noise channel and short-reach directly-modulated laser-based DMT transmission system. Two general SFO compensation (SFOC) methods, i.e., digital interpolation (DI) and phase compensation (PC), are comparatively investigated through both simulations and experiments. The results indicate that the accuracy of the SFOE method can be within ±0.2 ppm under the SFO range of ±500 ppm and different received optical powers. With the high-accuracy SFOE method, the 4th order DI has the best compensation performance compared to PC, 2nd and 3rd order DI methods. Moreover, the negligible EVM performance penalty compared with the case of SFO-free can also be achieved in an optical DMT transmission system. Besides, the PC-based SFOC method has similar EVM performance to the 4th order DI in the case of small SFO within ±300 ppm.

MIMO-OFDM Joint Radar-Communications: Is ICI Friend or Foe?

Intercarrier interference (ICI) poses a significant challenge for OFDM joint radar-communications (JRC) systems in high-mobility scenarios. In this paper, we propose a novel ICI-aware sensing algorithm for MIMO-OFDM JRC systems to detect the presence of multiple targets and estimate their delay-Doppler-angle parameters. First, leveraging the observation that spatial covariance matrix is independent of target delays and Dopplers, we perform angle estimation via the MUSIC algorithm. For each estimated angle, we next formulate the radar delay-Doppler estimation as a joint carrier frequency offset (CFO) and channel estimation problem via an APES (amplitude and phase estimation) spatial filtering approach by transforming the delay-Doppler parameterized radar channel into an unstructured form. To account for the presence of multiple targets at a given angle, we devise an iterative interference cancellation based orthogonal matching pursuit (OMP) procedure, where at each iteration the generalized likelihood ratio test (GLRT) detector is employed to form decision statistics, providing as by-products the maximum likelihood estimates (MLEs) of radar channels and CFOs. In the final step, target detection is performed in delay-Doppler domain using target-specific, ICI-decontaminated channel estimates over time and frequency, where CFO estimates are utilized to resolve Doppler ambiguities, thereby turning ICI from foe to friend. The proposed algorithm can further exploit the ICI effect to introduce an additional dimension (namely, CFO) for target resolvability, which enables resolving targets located at the same delay-Doppler-angle cell. Simulation results illustrate the ICI exploitation capability of the proposed approach and showcase its superior detection and estimation performance in high-mobility scenarios over conventional methods.

Fast Fading Channel Estimation for OFDM Systems with Complexity Reduction

Channel estimation plays a significant role in the Inter-carrier interference (ISI) mitigation and symbol detection in Orthogonal frequency division multiplexing (OFDM) systems. As the speed of mobile terminal increases, the channel changes rapidly, causing fast fading which will degrade the system performance. In this paper, a low complexity fast fading channel estimation method suited for scattered pilot OFDM systems is proposed. This method is characterized by estimating channel matrix directly in the frequency domain, which can significantly reduce the channel estimation complexity with good system performance. Simulation results verify its superiority over the existing estimation methods.

Decision Feedback Kurtosis Minimum Crosstalk Mitigation in Super-Nyquist Multiband CAP Systems

In multiuser access visible light communication (VLC) systems, many multiplex techniques can be utilized to fully exploit the limited bandwidth. In frequency-division multiplexing systems, one possible solution is to dynamically allocate limited bandwidth resources for users to support multiple-service access. To enable high-speed transmissions, the super-Nyquist multiband concept has been proposed in recent years. Specifically, the center frequency spacing is narrowed, and the subcarriers are nonorthogonally overlapped to enable more services and users in multiband systems. Studies have shown that this strategy can support higher spectral efficiency (SE) while simultaneously, introducing intersymbol interference (ISI) and intercarrier interference (ICI). Therefore, a well-designed ICI mitigation scheme is required. In multiband carrierless amplitude phase ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> -CAP) modulation systems, it is technically challenging to achieve the SE beyond one baud/s/Hz under the pre-forward error correction (pre-FEC) threshold without increasing the modulation order. Inspired by the distinctive pulse shaping filter (PSF) structure in a non-orthogonal <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> -CAP scheme, we propose the iterative decision feedback kurtosis minimum (DFKM) scheme and achieve the super-Nyquist SE of approximately 1.06 baud/s/Hz. In contrast to existing ICI mitigation schemes, the theoretical framework reveals that symbol-level crosstalk filters are indispensable to reap the benefit of super-Nyquist transmission, not limited to VLC.

PAPR Reduction an effective approach for next frontier MIMO-OFDM systems

The ‘Orthogonal frequency division multiplexing (OFDM)’ is a well-accepted and effective technology employed today and in future wireless communication systems. The combinations of OFDM and ‘multiple-input multiple-output (MIMO’) offer high quality of services and better throughput. The multicarrier OFDM system experiences a high ‘peak-to-average power ratio (PAPR’), which is the major issue in the OFDM scheme and must be truncated to achieve trustworthy communication. Due to high PAPR in a signal to be transmitted, the power amplifier in the transmitter section enters into saturation region and amplifies the signal nonlinearly, resulting in loss of orthogonality and ultimately in ‘inter-carrier interference (ICI)’. In this article, the 'iterative clipping and filtering (ICF)' method is proposed to minimize the PAPR in the OFDM system. The simulation is carried out using MATLAB (version 2014b). The result of the proposed ICF method and the ‘selective mapping (SLM)’ scheme is analyzed and compared. From the analysis, it is shown that the proposed ICF technique is more suitable for minimizing the PAPR effectively without affecting ‘bit error rates (BER)’ much and the simplicity of the system. The simulation result of the proposed ICF scheme using ‘Quadrature Phase Shift Keying (QPSK)’, FFT size of 128, and clipping and filtering level up to 6 shows that the proposed ICF scheme for clipping level of 6 reduces PAPR to 5dB. Also, the BER is minimized at the level of 3×10−5 at 12 dB SNR.

Mobile underwater acoustic communication with orthogonal signal division multiplexing under inter-carrier interference larger than a guardband

Underwater acoustic (UWA) communication is a key technology providing wireless communication, but using UWA channels is challenging due to the existence of large Doppler and delay spreads. UWA channels also have long but sparse reverberation tails. To address these issues simultaneously, we have proposed Doppler-resilient orthogonal signal division multiplexing using compressed sensing (D-OSDM-CS). The optimization of compressed sensing (CS) under a large Doppler spread remains difficult since noise induced by inter-carrier interference (ICI) is present in the received signal. Herein, we mathematically clarify the effect of ICI-induced noise on the received signal, and we propose an advanced D-OSDM-CS method that can measure ICI-induced noise and optimize the regularization parameter of CS. We also evaluate the performance of the advanced D-OSDM-CS in simulations and experiments. The obtained results suggest that the advanced D-OSDM-CS achieves better communication quality than the existing D-OSDM-CS under a large-Doppler-spread environment.

QAM-GFDM of Dual-Mode VCSEL Mixed 28-GHz MMW Carrier for Fiber-Wireless Link

An optical heterodyned 28-GHz carrier generation is demonstrated for synthesizer-free fifth-generation (5G) millimeter wave over fiber (MMWoF) link by using orthogonally polarized dual-mode vertical-cavity surface-emitting laser (VCSEL) as the frontend transmitter, and the power envelope detection is employed for self-heterodyne down-conversion of the generalized frequency division multiplexing (GFDM) data stream. The demonstrated MMWoF link exhibits superior immunity to the residual frequency and phase noises as well as the inter-carrier interference induced by the free-running dual-mode carrier. For back-to-back optical downstream wireline transmission, the receiving GFDM data optimizes its bit error rate (BER) to 2.2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> by adjusting the bias current of the dual-mode VCSEL to 8 mA (3 I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> ), and by grouping the GFDM data matrix with total symbols of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N = K×M</i> with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> = 2 subcarriers and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> = 24 timeslots. After a long-reach delivering the optical data stream over 50-km dispersion-managed single-mode fiber (DM-SMF), the qualified receiving power sensitivity is −12 dBm, whereas the decoding GFDM data reveals improved receiving sensitivity with an extremely low power penalty of only 0.3 dB. For the wireless transmission after optical downstream receiving at the remote node, the self-heterodyned MMW carrier exhibits peak power of −62.3 dBm and carrier-to-noise (CNR) of 26.8 dB over long-reach 50-km DM-SMF downstream optical link. In comparison with the mutual heterodyned from two incoherent optical carriers, the self-heterodyned frequency fluctuation can be suppressed from >500 to <330 MHz with significantly suppressed phase noise. After wireless delivering over 2 m in free space, the self-heterodyne down-conversion by power envelope detection guarantees the down-converted quadrature amplitude modulation GFDM (QAM-GFDM) data stream with an average power of -48.4 dBm after optimizing the impedance matching by selecting the appropriate frequency response of the power envelope detector. As a result, the wireless MMW transmission of 8-Gbit/s 4-QAM GFDM data stream is qualified with error vector magnitude (EVM) of 16.9%, signal-to-noise ratio (SNR) of 8.5 dB, and BER of 3.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> . By combining the free-running dual-mode source with two advanced techniques including mode-power equalization and self-heterodyned power envelope detection, the proposed synthesizer-free MMWoF network has demonstrated excellent immunity to the phase/frequency noise, inter-carrier interference which is particularly suitable for building up the compact and simplified optical transmitter and microwave receiver modules with cost-effective MMW self-heterodyne scheme.