Multiuser MIMO-OFDM for Next-Generation Wireless Systems

This overview portrays the 40-year evolution of orthogonal frequency division multiplexing (OFDM) research. The amelioration of powerful multicarrier OFDM arrangements with multiple-input multiple-output (MIMO) systems has numerous benefits, which are detailed in this treatise. We continue by highlighting the limitations of conventional detection and channel estimation techniques designed for multiuser MIMO OFDM systems in the so-called rank-deficient scenarios, where the number of users supported or the number of transmit antennas employed exceeds the number of receiver antennas. This is often encountered in practice, unless we limit the number of users granted access in the base station’s or radio port’s coverage area. Following a historical perspective on the associated design problems and their state-of-the-art solutions, the second half of this treatise details a range of classic multiuser detectors (MUDs) designed for MIMO-OFDM systems and characterizes their achievable performance. A further section aims for identifying novel cutting-edge genetic algorithm (GA)-aided detector solutions, which have found numerous applications in wireless communications in recent years. In an effort to stimulate the cross pollination of ideas across the machine learning, optimization, signal processing, and wireless communications research communities, we will review the broadly applicable principles of various GA-assisted optimization techniques, which were recently proposed also for employment inmultiuser MIMO OFDM. In order to stimulate new research, we demonstrate that the family of GA-aided MUDs is capable of achieving a near-optimum performance at the cost of a significantly lower computational complexity than that imposed by their optimum maximum-likelihood (ML) MUD aided counterparts. The paper is concluded by outlining a range of future research options that may find their way into next-generation wireless systems

Near-Instantaneously Adaptive HSDPA-Style OFDM Versus MC-CDMA Transceivers for WIFI, WIMAX, and Next-Generation Cellular Systems

Burts-by-burst (BbB) adaptive high-speed downlink packet access (HSDPA) style multicarrier systems are reviewed, identifying their most critical design aspects. These systems exhibit numerous attractive features, rendering them eminently eligible for employment in next-generation wireless systems. It is argued that BbB-adaptive or symbol-by-symbol adaptive orthogonal frequency division multiplex (OFDM) modems counteract the near instantaneous channel quality variations and hence attain an increased throughput or robustness in comparison to their fixed-mode counterparts. Although they act quite differently, various diversity techniques, such as Rake receivers and space-time block coding (STBC) are also capable of mitigating the channel quality variations in their effort to reduce the bit error ratio (BER), provided that the individual antenna elements experience independent fading. By contrast, in the presence of correlated fading imposed by shadowing or time-variant multiuser interference, the benefits of space-time coding erode and it is unrealistic to expect that a fixed-mode space-time coded system remains capable of maintaining a near-constant BER

Performance Analysis of Multicarrier Code Division Multiple Access (MC-CDMA) Systems

A thesis presented to the faculty of the College of Science and Technology at Morehead State University in partial fulfillment of the requirements for the Degree of Master of Science by Pravinkumar Patil on August 11, 2008

Performance evaluation of fixed WiMax physical layer under high fading channels

This thesis was submitted for the degree of Master of Philosophy and awarded by Brunel University.A radio channel characteristic modelling is essential in every network planning. This project deals with the performance of WiMax networks in an outdoor environment while using fading channel models. The radio channels characteristics are analyzed by simulations have been done using Matlab programming. Stanford University Interim(SUI) Channels set was proposed to simulate the fixed broadband wireless access channel environments where IEEE 802.16d is to be deployed. It has six channel models that are grouped into three categories according to three typical different outdoor Terrains, in order to give a comprehensive study of fading channels on the overall performance of the system, WiMax system has been tested under SUI channels that modified into account for 30o directional antennas, with 90% cell coverage and with 99.9% reliability in its geographical covered area. Furthermore, in order to combat the fading which occurs in urban areas and improve the capacity and the throughput of the system, multiples antennas at both ends of communication link are used, the transmission gain obtained when using multiple antennas instead of only a single antenna. Space-time coding and maximum ratio combining for more than one transmit and receive antenna is implemented to allow performance investigations in various MIMO scenarios. It has been concluded that uses multiple antennas at the receiver offers a significant improvement of 3 dB of gain in the channel SNR. This thesis also contain implementation of all compulsory features of the WiMax OFDM physical layer specified in IEEE 802.16-2004 using Matlab coding. In order to combat the temporal variations in quality on a multipath fading channel, an adaptive modulation technique is used. This technique employs multiple modulation schemes to instantaneously adapt to the variations in the channel SNR, thus maximizing the system throughput and improving BER performance. WiMax transceiver has been tested with and without encoding and studied the effect of encoding on multipath channel. Testing the system with flexible channel bandwidth has been part of this thesis. Finally it has been explained in this thesis the affect of increasing the size of cyclic prefix on overall performance of WiMax system

Evaluation of Overlay/underlay Waveform via SD-SMSE Framework for Enhancing Spectrum Efficiency

Recent studies have suggested that spectrum congestion is mainly due to the inefficient use of spectrum rather than its unavailability. Dynamic Spectrum Access (DSA) and Cognitive Radio (CR) are two terminologies which are used in the context of improved spectrum efficiency and usage. The DSA concept has been around for quite some time while the advent of CR has created a paradigm shift in wireless communications and instigated a change in FCC policy towards spectrum regulations. DSA can be broadly categorized as using a 1) Dynamic Exclusive Use Model, 2) Spectrum Commons or Open sharing model or 3) Hierarchical Access model. The hierarchical access model envisions primary licensed bands, to be opened up for secondary users, while inducing a minimum acceptable interference to primary users. Spectrum overlay and spectrum underlay technologies fall within the hierarchical model, and allow primary and secondary users to coexist while improving spectrum efficiency. Spectrum overlay in conjunction with the present CR model considers only the unused (white) spectral regions while in spectrum underlay the underused (gray) spectral regions are utilized. The underlay approach is similar to ultra wide band (UWB) and spread spectrum (SS) techniques utilize much wider spectrum and operate below the noise floor of primary users. Software defined radio (SDR) is considered a key CR enabling technology. Spectrally modulated, Spectrally encoded (SMSE) multi-carrier signals such as Orthogonal Frequency Domain Multiplexing (OFDM) and Multi-carrier Code Division Multiple Access (MCCDMA) are hailed as candidate CR waveforms. The SMSE structure supports and is well-suited for SDR based CR applications. This work began by developing a general soft decision (SD) CR framework, based on a previously developed SMSE framework that combines benefits of both the overlay and underlay techniques to improve spectrum efficiency and maximizing the channel capacity. The resultant SD-SMSE framework provides a user with considerable flexibility to choose overlay, underlay or hybrid overlay/underlay waveform depending on the scenario, situation or need. Overlay/Underlay SD-SMSE framework flexibility is demonstrated by applying it to a family of SMSE modulated signals such as OFDM, MCCDMA, Carrier Interferometry (CI) MCCDMA and Transform Domain Communication System (TDCS). Based on simulation results, a performance analysis of Overlay, Underlay and hybrid Overlay/Underlay waveforms are presented. Finally, the benefits of combining overlay/underlay techniques to improve spectrum efficiency and maximize channel capacity are addressed

Investigating the relation between optimum guard interval and channel delay spread for a MC-CDMA system

This paper demonstrates a novel approach to determining the optimum guard interval for a multicarrier code division multiple access (MC-CDMA) system. Analytical expressions for useful and interference power are derived as a basis for comparison. From these, an expression for the signal-to-noise ratio of a detected bit is derived and used to determine the optimum guard interval for a given channel profile and system parameters. In contrast to other works, we use channel models based on actual measurements and we highlight important differences from theoretical models to support our approach. From our results, we propose an empirical rule for optimum guard intervals given prevailing channel parameters. We show that the optimum guard interval can be selected as the delay window that includes 95% and 99% multipath power for Es /N0 = 10 dB and Es /N0 = 20 dB, respectively. In our case, the optimum guard interval was between 2 τrms and 4 τrms for Es /N0 = 10 dB and between 3 τrms and 6.4 τrms for Es/N0 = 20 dB

On feasibility of the UE power saving signal for the 5G new radio

Abstract. The objective of this thesis is to study and evaluate physical layer signals and channels to achieve the user equipment (UE) power saving in the 3rd generation partnership project (3GPP) new radio (NR). The fifth generation (5G) mobile network has strict objectives regarding power consumption and performance. The UE power consumption also has a big impact on the end user’s quality of experience (QoE) and future deployment of NR devices. Therefore, it is very important to study ways to reduce UE power consumption. One feasible power saving technique is the usage of so-called power saving signal or channel, which triggers the UE to transition to the active mode from the power saving mode. The first part of this work provides an overview of general properties of the NR and its physical downlink signals and channels, as well as the UE operation and power consumption in the connected mode. Then, examples of existing power saving techniques are discussed and a new scheme of the wake-up mechanism and the UE power saving signal/wake-up signal (WUS) is described. Lastly, different design options for the power saving signal are described and their detection performance is studied. The power saving signal options of this thesis can be divided into physical downlink control channel (PDCCH) based and sequence-based signals/channels. In the PDCCH based option, the power saving indication is carried as a payload of the PDCCH. Studied sequence-based options are the secondary synchronization signal (SSS), the PDCCH demodulation reference signal (DMRS), the channel state information reference signal (CSI-RS) and a UE-specific sequence that is mapped to all radio resources allocated for the PDCCH. The detection of the latter is done in time domain, and the detection of the other sequences is done in frequency domain. The detection performance of these signals/channels is compared based on link-level simulation results. Simulations were done with a Matlab-based simulator. They show the impact of the frequency- and time-selectivity and implementation impairments. Based on the numerical results, the impact of the UE speed up to 120 km/h and the carrier frequency offset (CFO) up to 400 Hz can be neglected with all the options except CSI-RS. It was shown that the sequence-based WUS options tend to suffer from the frequency-selective radio channel. By making decisions within the channel’s coherence bandwidth and using precoder cycling, the negative impact of the channel can be reduced. With these techniques, PDCCH DMRS outperforms all the other sequence-based options. However, in terms of detection performance, the PDCCH based power saving signal/channel is the most robust option of this set of candidates.Päätelaitteen virransäästösignaalin soveltuvuus 5G:n uuteen radiorajapintaan. Tiivistelmä. Tämän diplomityön tavoitteena on tutkia ja verrata fyysisen kerroksen signaaleja, päätelaitteen (user equipment, UE) virransäästön toteuttamiseksi 3GPP:n uudessa radiorajapinnassa (New Radio, NR). Viidennen sukupolven (5th generation, 5G) mobiiliverkolla on tiukat tavoitteet virransäästön ja suorituskyvyn osalta. Päätelaitteen virrankulutuksella on myös suuri vaikutus loppukäyttäjän kokemukseen ja tulevien NR-laitteiden käyttöönottoon. Siksi onkin erittäin tärkeää tutkia mahdollisia tapoja vähentää päätelaitteen virrankulutusta. Yksi mahdollinen virransäästötekniikka on niin sanottu virransäästösignaali, joka herättää päätelaitteen virransäästötilasta verkkoyhteyteen. Työn ensimmäinen osa käsittelee NR:n yleisiä ominaisuuksia, alalinkin fyysisiä signaaleja ja kanavia, sekä päätelaitteen virrankulutusta verkkoyhteydessä. Seuraavaksi käsitellään olemassa olevia virransäästötekniikoita, sekä käydään läpi uutta herätys-tyyppistä mekanismia ja päätelaitteen virransäästösignaalin/herätyssignaalin (wake-up signal, WUS) toimintaa. Lopuksi kuvataan erilaisia virransäästösignaalivaihtoehtoja ja tutkitaan niiden havaitsemisen suorituskykyä. Työn virransäästösignaalivaihtoehdot voidaan jakaa alalinkin kontrollikanava- (physical downlink control channel, PDCCH) ja sekvenssipohjaisiin signaaleihin/kanaviin. PDCCH-pohjaisessa vaihtoehdossa virransäästösignaali siirretään PDCCH:n hyötykuormana. Tutkitut sekvenssipohjaiset vaihtoehdot ovat toissijainen synkronointisignaali (secondary synchronization signal, SSS), PDCCH-demodulaatio-referenssisignaali (demodulation reference signal, DMRS), kanavan tilatieto-referenssisignaali (channel-state information reference signal, CSI-RS), sekä UE-spesifinen sekvenssi, joka asetetaan PDCCH:n jokaiseen alikantoaaltoon. Jälkimmäisen havaitseminen tehdään aikatasossa ja muiden sekvenssien havaitseminen tehdään taajuustasossa. Näiden signaalien/kanavien havaitsemisen suorituskykyä vertaillaan linkkitason simulointitulosten perusteella. Simulaatiot tehtiin Matlab-pohjaisella simulaattorilla. Ne esittävät aika- ja taajuusselektiivisyyden, sekä toteutuksen epäideaalisuuksien vaikutusta. Numeeristen tulosten perusteella, UE:n nopeus arvoon 120 km/h ja kantoaaltotaajuussiirto (carrier frequency offset, CFO) 400 Hz:iin asti voidaan jättää huomioimatta, kaikkien muiden paitsi CSI-RS:n tapauksessa. Työssä osoitettiin, että sekvenssipohjaiset WUS-vaihtoehdot kärsivät taajuusselektiivisestä radiokanavasta. Kanavan negatiivista vaikutusta voidaan pienentää tekemällä päätöksiä kanavan koherenssikaistanleveyttä pienemmissä osissa, sekä käyttämällä syklistä esikooderia. Näillä tekniikoilla PDCCH DMRS suoriutuu kaikkia muita sekvenssipohjaisia vaihtoehtoja paremmin. Kuitenkin, havaitsemisen suorituskyvyn perusteella PDCCH-pohjainen virransäästösignaali/kanava on vahvin ehdokas näistä vaihtoehdoista

Channel Estimation in Multicarrier Communication Systems

The data rate and spectrum efficiency of wireless mobile communications have been significantly improved over the last decade or so. Recently, the advanced systems such as 3GPP LTE and terrestrial digital TV broadcasting have been sophisticatedly developed using OFDM and CDMA technology. In general, most mobile communication systems transmit bits of information in the radio space to the receiver. The radio channels in mobile radio systems are usually multipath fading channels, which cause inter-symbol interference (ISI) in the received signal. To remove ISI from the signal, there is a need of strong equalizer which requires knowledge on the channel impulse response (CIR).This is primarily provided by a separate channel estimator. Usually the channel estimation is based on the known sequence of bits, which is unique for a certain transmitter and which is repeated in every transmission burst. Thus, the channel estimator is able to estimate CIR for each burst separately by exploiting the known transmitted bits and the corresponding received samples. In this thesis we investigate and compare various efficient channel estimation schemes for OFDM systems which can also be extended to MC DS-CDMA systems.The channel estimation can be performed by either inserting pilot tones into all subcarriers of OFDM symbols with a specific period or inserting pilot tones into each OFDM symbol. Two major types of pilot arrangement such as block type and comb type pilot have been focused employing Least Square Error (LSE) and Minimum Mean Square Error (MMSE) channel estimators. Block type pilot sub-carriers is especially suitable for slow-fading radio channels whereas comb type pilots provide better resistance to fast fading channels. Also comb type pilot arrangement is sensitive to frequency selectivity when comparing to block type arrangement. However, there is another supervised technique called Implicit Training (IT) based channel estimation which exploits the first order statistics in the received data, induced by superimposing periodic training sequences with good correlation properties, along with the information symbols. Hence, the need for additional time slots for training the equalizer is avoided. The performance of the estimators is presented in terms of the mean square estimation error (MSEE) and bit error rate (BER)