Markku Renfors

Optimization of flexible filter banks based on fast-convolution

Multirate filter banks can be implemented very efficiently using fast-convolution (FC) processing. The main advantage of the FC filter banks compared with the conventional polyphase implementations is their increased flexibility, that is, the number of channels, their bandwidths, and the center frequencies can be independently selected. In this paper, an approach for optimizing the FC filter banks is proposed. First, a subband representation of the adjustable FC filter banks is derived. Then, the optimization problems are formulated with the aid of the subband model. Finally, these problems are conveniently solved with the aid of a nonlinear optimization algorithm. Two examples are included to illustrate the performance of the proposed overall scheme.

Efficient Fast-Convolution-Based Waveform Processing for 5G Physical Layer

This paper investigates the application of fast-convolution (FC) filtering schemes for flexible and effective waveform generation and processing in the fifth generation (5G) systems. FC-based filtering is presented as a generic multimode waveform processing engine while, following the progress of 5G new radio standardization in the Third-Generation Partnership Project, the main focus is on efficient generation and processing of subband-filtered cyclic prefix orthogonal frequency-division multiplexing (CP-OFDM) signals. First, a matrix model for analyzing FC filter processing responses is presented and used for designing optimized multiplexing of filtered groups of CP-OFDM physical resource blocks (PRBs) in a spectrally well-localized manner, i.e., with narrow guardbands. Subband filtering is able to suppress interference leakage between adjacent subbands, thus supporting independent waveform parametrization and different numerologies for different groups of PRBs, as well as asynchronous multiuser operation in uplink. These are central ingredients in the 5G waveform developments, particularly at sub-6-GHz bands. The FC filter optimization criterion is passband error vector magnitude minimization subject to a given subband band-limitation constraint. Optimized designs with different guardband widths, PRB group sizes, and essential design parameters are compared in terms of interference levels and implementation complexity. Finally, extensive coded 5G radio link simulation results are presented to compare the proposed approach with other subband-filtered CP-OFDM schemes and time-domain windowing methods, considering cases with different numerologies or asynchronous transmissions in adjacent subbands. Also the feasibility of using independent transmitter and receiver processing for CP-OFDM spectrum control is demonstrated

Optimization of Flexible Filter Banks Based on Fast Convolution

Multirate filter banks can be implemented efficiently using fast-convolution (FC) processing. The main advantage of the FC filter banks (FC-FB) compared with the conventional polyphase implementations is their increased flexibility, that is, the number of channels, their bandwidths, and the center frequencies can be independently selected. In this paper, an approach to optimize the FC-FBs is proposed. First, a subband representation of the FC-FB is derived. Then, the optimization problems are formulated with the aid of the subband model. Finally, these problems are conveniently solved with the aid of a general nonlinear optimization algorithm. Several examples are included to demonstrate the proposed overall design scheme as well as to illustrate the efficiency and the flexibility of the resulting FC-FB.

Fast-convolution filtered OFDM waveforms with adjustable CP length

This paper investigates the application of flexible and effective fast-convolution (FC) filtering scheme for multiplexing OFDM physical resource blocks (PRBs) in a spectrally well-localized manner. The scheme is able to suppress interference leakage between adjacent PRBs, thus supporting independent waveform parametrization for different PRBs, as well as asynchronous multiuser operation. These are considered as important features in the 5G waveform development. This contribution focuses on the parametrization alternatives and constraints, which are mostly due to the forward and inverse discrete Fourier transform (DFT/IDFT) lengths feasible for practical implementation. Special attention is on cyclic prefix (CP) OFDM system dimensioning with adjustable CP length while the overall symbol duration is fixed. It is demonstrated that a wide range of alternative CP-lengths are usually available through the FC-filtered OFDM scheme.

Parallel processing intensive digital front-end for IEEE 802.11ac receiver

Modern computing platforms offer increasing levels of parallelism for fast execution of different signal processing tasks. In this paper, we develop and elaborate on a digital front-end concept for an IEEE 802.11ac receiver with 80 MHz bandwidth where parallel processing is adopted in multiple ways. First, the inherent structure of the 802.11ac waveform is utilized such that it is divided, through time-domain digital filtering and decimation, to two parallel 40 MHz signals that can be processed further in parallel using smaller-size FFTs and, e.g, legacy 802.11n digital receiver chains. This filtering task is very challenging, as the latency and the cyclic prefix budget of the receiver cannot be compromised, and because the number of unused subcarriers in the middle of the 80 MHz signal is only three, thus necessitating very narrow transition bandwidth in the deployed filters. Both linear and circular filtering based multirate channelization architectures are developed and reported, together with the corresponding filter coefficient optimization. Also, full radio link performance simulations with commonly adopted indoor WiFi channel profiles are provided, verifying that the channelization does not degrade the overall link performance. Then, both C and OpenCL software implementations of the processing are developed and simulated for comparison purposes on an Intel CPU, to demonstrate that the parallelism provided by the OpenCL will result in substantially faster realization. Furthermore, we provide complete software implementation results in terms of time, number of clock cycles, power, and energy consumption on the ARM Mali GPU with half precision floating-point arithmetic along with the ARM Cortex A7 CPU.

Enhanced multicarrier techniques for narrowband and broadband PMR coexistence

The coexistence of broadband systems with currently deployed professional mobile radio (PMR) communication systems like public protection and disaster relief has gained much deserved attention. Recently, it has been shown that a conventional orthogonal frequency division multiplexing (OFDM)-based long-term evolution (LTE) system needs sufficient guard-band or additional filtering in order to coexist in a PMR band, even without considering the radio frequency (RF) impairments of the transmitter. In practice, it is imperative to take into account the RF impairments in radio communication systems, such as the non-linear (NL) nature of a high power amplifier (PA). This paper aims at investigating some enhanced multicarrier techniques allowing the coexistence of narrowband and broadband PMR systems that can support asynchronism, have low latency, and be able to be operated with NL devices. The simulation results show that a LTE-like post-OFDM broadband system working with NL PA devices can still coexist with legacy PMR systems with high spectrum and energy efficiencies when proper PA linearisation techniques are applied in combination with effective peak-to-average power ratio reduction methods. Copyright

Optimized fast convolution based filtered-OFDM processing for 5G

This paper investigates the application of flexible fast-convolution (FC) filtering scheme for multiplexing orthogonal frequency-division multiplexing (OFDM) physical resource blocks (PRBs) in a spectrally well-localized manner. This scheme is able to suppress interference leakage between adjacent PRBs, thus supporting independent waveform parametrization and numerologies for different PRBs, as well as asynchronous multiuser operation. These are considered as important features in the 5G waveform development. This contribution focuses on optimizing FC based OFDM transmultiplexers such that the in-band interference is minimized subject to the given out-of-band emission constraint. The performance of the optimized designs is demonstrated using resource block groups (RBGs) of different sizes and with various design parameters. The proposed scheme has great flexibility in tuning the filtering bandwidths dynamically according the resource allocation to different users with different requirements regarding the OFDM waveform numerology. Also the computational complexity is competitive with existing time-domain windowing approaches and becomes superior when the number of filtering bands is increased.

Efficient fast-convolution based implementation of 5G waveform processing using circular convolution decomposition

Multirate fast convolution (FC) has recently been introduced as an effective tool for communication waveform processing, especially for advanced multicarrier systems targeting at well-contained spectrum. These include filter bank based multicarrier waveforms and filtered OFDM schemes which are receiving increasing attention in the 5G radio access development. Recalling that the key idea of FC is effective implementation of high-order linear filtering through frequency-domain processing, this paper investigates possibilities to reduce the complexity of FC based waveforms. Special focus is on scenarios where a relatively small part of the bandwidth is in active use, which could be the case, e.g., in low-rate machine-type communication devices. A new variant of fast-convolution filter bank (FC-FB) is developed which uses circular convolution decomposition. The narrowband variant of decomposed structure, called D-FC-FB, achieves significantly reduced complexity, which is proportional to the active bandwidth, while maintaining filtering performance equivalent to FC-FB. Therefore, this variant is considered as a low-complexity solution for low-rate devices. D-FC-FB can be used in any multicarrier scheme that utilizes filtering at subcarrier or resource block level. This paper develops closed-form complexity expressions for the case of filter bank multi-carrier with offset-QAM subcarrier modulation (FBMC/OQAM) demonstrating significant complexity reduction in a case study.

Filtered multitone multicarrier modulation with partially overlapping sub-channels

Future wireless networks demand multicarrier modulation schemes with improved spectrum efficiency and superior spectrum containment. Orthogonal frequency division multiplexing (OFDM) has been the favorite technique in recent developments, but due to its limited spectrum containment, various alternative schemes are under consideration for future systems. Theoretically, it is not possible to reach maximum spectrum efficiency, high spectral containment, and orthogonality of subcarriers simultaneously, when using quadrature amplitude modulation (QAM) for subcarriers. This has motivated the study of non-orthogonal multicarrier modulation schemes. This paper focuses on the filtered multitone (FMT) scheme, one of the classical configurations of filter bank multicarrier (FBMC) modulation utilizing QAM subcarrier symbols. Our main aim is to improve the spectral efficiency of FMT by introducing controlled overlap of adjacent subchannels. An analytical model is developed for evaluating the tradeoffs between spectrum efficiency and intercarrier interference (ICI) introduced by the overlap. An efficient fast convolution waveform processing scheme is adopted for the generation of the proposed waveform. It allows effective adjustment of the roll-off and subcarrier spacing to facilitate waveform adaptation in real time. Analytical studies, confirmed by simulation results, indicate that the proposed FMT system can obtain significant spectral density improvement without requiring additional ICI cancellation techniques.

Multirate Signal Processing and Filterbanks

This chapter introduces certain important theories and signal processing tools as background for later developments in this book. After a brief introduction to real and complex linear systems, spectral models for discrete-time systems are formulated in a generalized way and conditions for alias-free sampling are formulated for real and complex baseband and passband systems. Next, multirate filtering concepts are introduced, again with emphasis on complex and bandpass signal models. Also polyphase filters and filterbanks are introduced in this context. Then the Nyquist pulse shaping principle is explained as a central element of both classical communication theory and filterbank based waveforms. Finally, after a brief introduction to the Discrete Fourier Transform (DFT), the basic form of effective uniform filterbanks, the DFT FilterBank (DFT-FB), is introduced, and general filterbank concepts and classifications are summarized.