Wolfgang Zirwas

Relay-based deployment concepts for wireless and mobile broadband radio

In recent years, there has been an upsurge of interest in multihop-augmented infrastructure-based networks in both the industry and academia, such as the seed concept in 3GPP, mesh networks in IEEE 802.16, and converge extension of HiperLAN/2 through relays or user-cooperative diversity mesh networks. This article, a synopsis of numerous contributions to the working group 4 of the wireless world research forum and other research work, presents an overview of important topics and applications in the context of relaying. It covers different approaches to exploiting the benefits of multihop communications via relays, such as solutions for radio range extension in mobile and wireless broadband cellular networks (trading range for capacity), and solutions to combat shadowing at high radio frequencies. Furthermore, relaying is presented as a means to reduce infrastructure deployment costs. It is also shown that through the exploitation of spatial diversity, multihop relaying can enhance capacity in cellular networks. We wish to emphasize that while this article focuses on fixed relays, many of the concepts presented can also be applied to systems with moving relays.

The role of small cells, coordinated multipoint, and massive MIMO in 5G

5G will have to support a multitude of new applications with a wide variety of requirements, including higher peak and user data rates, reduced latency, enhanced indoor coverage, increased number of devices, and so on. The expected traffic growth in 10 or more years from now can be satisfied by the combined use of more spectrum, higher spectral efficiency, and densification of cells. The focus of the present article is on advanced techniques for higher spectral efficiency and improved coverage for cell edge users. We propose a smart combination of small cells, joint transmission coordinated multipoint (JT CoMP), and massive MIMO to enhance the spectral efficiency with affordable complexity. We review recent achievements in the transition from theoretical to practical concepts and note future research directions. We show in measurements with macro-plus-smallcell scenarios that spectral efficiency can be improved by flexible clustering and efficient user selection, and that adaptive feedback compression is beneficial to reduce the overhead significantly. Moreover, we show in measurements that fast feedback reporting combined with advanced channel prediction are able to mitigate the impairment effects of JT CoMP.

Synchronization of cooperative base stations

We consider synchronization techniques required to enhance the cellular network capacity using base station cooperation. In the physical layer, local oscillators are disciplined by the global positioning system (GPS) and over the backbone network for outdoor and indoor base stations, respectively. In the medium access control (MAC) layer, the data flow can be synchronized by two approaches. The first approach uses so-called time stamps. The data flow through the user plane and through copies of it in each cooperative base station is synchronized using a timing protocol on the interconnects between the base stations. The second approach adds mapping information to the data after the user plane processing is almost finalized. Each forward-error encoded transport block, its modulation and coding scheme and the resources where it will be transmitted are multicast over the interconnect network. Interconnect latency is reduced below 1 ms to enable coherent interference reduction for mobile radio channels.

Capacity and Coverage Analysis of a 3GPP-LTE Multihop Deployment Scenario

Broadband wireless access will be deployed in a cellular way with 3GPP-LTE [1]. For the first rollout the main demand is a huge area coverage. With only few available base station sites that are connected to an access fiber, multihop (relaying) techniques can be used well to fill the coverage gaps. Later with increasing offered traffic, the demand shifts to higher capacity over the area. Even for this purpose relays are beneficial. There is an area around relays where they provide better overall capacity to the user terminal, taking into account all resources used for the first and second hop (the relaying overhead). Relaying or Multihop operation therefore massively improves the coverage as well as the capacity goals at low cost, without the need of a cable or fiber access. This paper analyzes a realistic urban scenario on the island of Jersey. We study the coverage and capacity over the area in three cases. One base station (BS) only, one BS with four relay nodes (RNs), and the latter plus another ring of nine RNs. The BS has fiber access for rates beyond 100 Mbit/s, while the first hop of Relays (HI) is fed over the air from BS using shared resources in the same LTE band. The second hop H2 is fed by the relays of group HI. In this paper we provide the results from numeric analysis based on models we explain here. It is shown that huge gains in coverage and capacity are obtained by relaying.

Interference-aware scheduling in the multiuser MIMO-OFDM downlink

With the introduction of orthogonal frequency- division multiplexing and multiple antennas in cellular networks, there are new opportunities to adapt the transmission to propagation and interference conditions. In this article we describe a practical approach using space-frequency-selective multiuser MIMO scheduling. Frequency-selective feedback is provided on achievable data rates for preferred single- and multistream transmission modes. The base station selects the best mode while providing instantaneous fairness. We observe that multiuser transmission increases the probability of using multistream transmission. Besides the benefits from optimal combining at the physical layer, there is an additional gain at the MAC layer since the estimation of achievable rates becomes more precise. Altogether, 93 percent of the theoretical throughput can be realized by synchronizing the base stations and providing cell-specific reference signals. We have implemented essential functions of the approach in real time on an experimental 3GPP LTE prototype in 20 MHz bandwidth. Feasibility of the key features is proven in laboratory and field trials.

Vision for Beyond 4G broadband radio systems

Mobile communication systems have evolved over the past decades and each new generation brought new experience to the users enabled by technology innovations, while keeping some well established principles from previous generations. This trend continued up to LTE (Long Term Evolution) Advanced, the predominant 4th generation system which has just been standardized in 3GPP and is being rolled out soon. How will this trend continue to future systems which will be deployed in some 10 years from now which will be advanced enough to be called “Beyond 4G” (B4G)? This article presents how such B4G systems will look like and some key technologies they will rely on including versatile numerology, massive virtual MIMO from many base stations, both centralized and distributed architectures using fiber optics as backbone, advanced interference mitigation, cognitive self organization, and wideband RF radios.

Multi-Cell Channel Estimation using Virtual Pilots

Multicellular radio systems are often limited due to the presence of cochannel interference. Proposed physical layer concepts, e.g. coordinated joint transmission and interference rejection combining, try to strengthen the signal while combating the interference. However, the performance may be limited by the available channel knowledge. We provide a concept for multi-cell channel estimation in the downlink applicable for for both physical layer concepts. This concept uses virtual pilots based on block-orthogonal sequences, e.g. Hadamard.

Efficient feedback via subspace-based channel quantization for distributed cooperative antenna systems with temporally correlated channels

It is one of the biggest challenges of distributed cooperative antenna (COOPA) systems to provide base stations (BSs) with down-link channel information for transmit filtering (precoding). In this paper, we propose a novel feedback scheme via a subspace-based channel quantization method. The proposed scheme adopts the chordal distance as a channel quantizer criterion so as to capture channel characteristics represented by subspaces spanned by the channel matrix. We also propose a combined feedback scheme which is based on the hierarchical codebook construction method in an effort to reduce the feedback overhead by exploiting the temporal correlation of the channel. The proposed methods are tested for distributed COOPA systems in terms of simulations. Simulation results show that the proposed subspace-based channel quantization method outperforms the analog pilot retransmission method, and the combined feedback scheme performs as well as the permanent full-feedback scheme with a much smaller amount of uplink resources.

Deployment and implementation strategies for massive MIMO in 5G

Massive MIMO has emerged as one technology enabler for the next generation mobile communications 5G. The gains promised by massive MIMO are augured to overcome the capacity crunch in todays mobile networks and to pave the way for the ambitious targets of 5G. The challenge to realize massive MIMO for 5G is a successful and cost-efficient integration in the overall network concept. This work highlights deployment and implementation strategies for massive MIMO in the context of 5G indoor small cell scenarios. Different massive MIMO deployment scenarios are analyzed for a standard 3GPP indoor office scenario. In particular stand-alone MIMO at a single location, distributed MIMO without cooperation and network MIMO with full cooperation are investigated for varying array configurations. For the performance analysis of the different MIMO configurations the ratio of total transmit antennas to spatial streams is varied stepwise from equality to a factor of ten. For implementation of massive MIMO in 5G networks trends in beamforming techniques, mutually coupled subarrays, over the calibration procedure and estimated ADC performance in 2020 time-frame are discussed. Based on the debate the paper indicates how to integrate large-scale arrays in future 5G networks.

Implementation concepts for distributed cooperative transmission

Information theory predicts huge performance gains in terms of spectrum efficiency by using cooperative transmission from multiple base stations. Cooperation eliminates inter-cell interference and it enables a higher channel rank. The isolated cell capacity is an upper bound for unlimited backbone capacity, infinite signal processing power and infinite channel feedback.We discuss a combined physical, medium access control (MAC) and network layer approach targeting minimal implementation loss while relaxing assumptions. It is based on frequency-selective channel quality identifier (CQI) reports to the serving station selecting the right user(s) on a given resource with best signal to interference and noise ratio (SINR). Such scheduling is performed independently in each cell. In adjacent cells it forms a group of users whose mutual interference is smaller than on average. The task of cooperative transmission is to further reduce the mutual interference within this group. In a multi-user system, the group shares only part of the spectrum. Feedback can hence be limited to the granted resource blocks. Based on virtual pilot sequences we reduce the pilot overhead and use a suitable mirror feedback scheme in addition. Altogether the feedback overhead is reduced by 2 orders of magnitude.