Femtocells, despite their name, pose a potentially large disruption to the carefully planned cellular networks that now connect a majority of the planet's citizens to the Internet and with each other. Femtocells - which by the end of 2010 already outnumbered traditional base stations and at the time of publication are being deployed at a rate of about five million a year - both enhance and interfere with this network in ways that are not yet well understood. Will femtocells be crucial for offloading data and video from the creaking traditional network? Or will femtocells prove more trouble than they are worth, undermining decades of careful base station deployment with unpredictable interference while delivering only limited gains? Or possibly neither: are femtocells just a "flash in the pan"; an exciting but short-lived stage of network evolution that will be rendered obsolete by improved WiFi offloading, new backhaul regulations and/or pricing, or other unforeseen technological developments? This tutorial article overviews the history of femtocells, demystifies their key aspects, and provides a preview of the next few years, which the authors believe will see a rapid acceleration towards small cell technology. In the course of the article, we also position and introduce the articles that headline this special issue.

/pdf/femtocells-past-present-and-future-di96bi1e7j.pdf

Femtocells: Past, Present, and Future

Embedding pico/femto base-stations and relay nodes in a macro-cellular network is a promising method for achieving substantial gains in coverage and capacity compared to macro-only networks. These new types of base-stations can operate on the same wireless channel as the macro-cellular network, providing higher spatial reuse via cell splitting. However, these base-stations are deployed in an unplanned manner, can have very different transmit powers, and may not have traffic aggregation among many users. This could potentially result in much higher interference magnitude and variability. Hence, such deployments require the use of innovative cell association and inter-cell interference coordination techniques in order to realize the promised capacity and coverage gains. In this paper, we describe new paradigms for design and operation of such heterogeneous cellular networks. Specifically, we focus on cell splitting, range expansion, semi-static resource negotiation on third-party backhaul connections, and fast dynamic interference management for QoS via over-the-air signaling. Notably, our methodologies and algorithms are simple, lightweight, and incur extremely low overhead. Numerical studies show that they provide large gains over currently used methods for cellular networks.

Cell Association and Interference Coordination in Heterogeneous LTE-A Cellular Networks

Orthogonal frequency division multiplexing (OFDM) has been widely adopted in modern wireless communication systems due to its robustness against the frequency selectivity of wireless channels. For coherent detection, channel estimation is essential for receiver design. Channel estimation is also necessary for diversity combining or interference suppression where there are multiple receive antennas. In this paper, we will present a survey on channel estimation for OFDM. This survey will first review traditional channel estimation approaches based on channel frequency response (CFR). Parametric model (PM)-based channel estimation, which is particularly suitable for sparse channels, will be also investigated in this survey. Following the success of turbo codes and low-density parity check (LDPC) codes, iterative processing has been widely adopted in the design of receivers, and iterative channel estimation has received a lot of attention since that time. Iterative channel estimation will be emphasized in this survey as the emerging iterative receiver improves system performance significantly. The combination of multiple-input multiple-output (MIMO) and OFDM has been widely accepted in modern communication systems, and channel estimation in MIMO-OFDM systems will also be addressed in this survey. Open issues and future work are discussed at the end of this paper.

Channel Estimation for OFDM

Communicating between stations over a shared medium comprises: receiving, at a destination station, a first waveform that includes one or more segments of a payload that originated from an origin station with a sequence of multiple segments, the one or more segments included in the first waveform having been transmitted over the shared medium by the origin station and by each of one or more repeater stations, and the first waveform indicating which of the sequence of multiple segments were not correctly decoded by at least one of the repeater stations; generating, based on the first waveform, acknowledgement information that specifies which of the sequence of multiple segments have been correctly decoded by the destination station; and transmitting a second waveform from the destination station over the shared medium, the second waveform including the acknowledgement information.

Repeating for low-overhead communication in a network

Systems and methods for decoding block and concatenated codes are provided. These include advanced iterative decoding techniques based on belief propagation algorithms, with particular advantages when applied to codes having higher density parity check matrices. Improvements are also provided for performing channel state information estimation including the use of optimum filter lengths based on channel selectivity and adaptive decision-directed channel estimation. These improvements enhance the performance of various communication systems and consumer electronics. Particular improvements are also provided for decoding HD Radio signals, including enhanced decoding of reference subcarriers based on soft-diversity combining, joint enhanced channel state information estimation, as well as iterative soft-input soft-output and list decoding of convolutional codes and Reed-Solomon codes. These and other improvements enhance the decoding of different logical channels in HD Radio systems.

Systems and methods for advanced iterative decoding and channel estimation of concatenated coding systems

Coherent OFDM detection requires accurate channel state information (CSI). Mobile radio channels are both time and frequency dispersive, especially at high vehicular speeds, which makes channel estimation a challenging problem in system design. Conventional preamble-based and pilot-aided channel estimation require numerous reference signals, which significantly compromises the system throughput. This paper proposes a novel low complexity iterative turbo channel estimation technique which makes use of preamble, pilots and soft decoded data information in an iterative fashion to improve the system performance over the time and frequency selective fading channel while maintaining the system throughput. The numerical and analytical results show that the proposed technique can approach the performance of systems with perfect CSI with much fewer preamble and pilots symbols compared to existing channel estimation methods.

/pdf/iterative-turbo-channel-estimation-for-ofdm-system-over-4ksu3l7zrd.pdf

Iterative Turbo Channel Estimation for OFDM System over Rapid Dispersive Fading Channel

This invention addresses the problem of channel estimation in fast fading communications channels, particularly for OFDM systems. It finds wide application in existing and future systems such as WLAN and WiMax. In particular, the invention involves a method of channel estimation and data detection for rapid dispersive fading channels due to high mobility. The invention involves decoding a symbol of the received transmission by retrieving pilot tones from it and using these to estimate variations in the channel frequency response using an iterative maximum likelihood channel estimation process, in which the estimation process comprises the following steps: In a first iteration, deriving soft decoded data information, that is information having a confidence value or reliability associated with it, from the estimates of the channel frequency response for the symbol obtained from pilot tones. And, in at least a second iteration using the soft decoded data information as virtual pilot tones together with the pilot tones to re-estimate the channel frequency response for the symbol. In other aspects the invention concerns a receiver and software designed to perform the method.

Channel estimation for rapid dispersive fading channels

Current OFDM systems assume the channel is not time varying within one OFDM frame, and use channel estimates obtained from preamble or pilots for data symbols within the same frame. This performs poorly in rapid dispersive fading channel with high mobility. This paper proposes a novel iterative turbo channel estimation to improve the performance of OFDM system over fading channel with both time and frequency selectivity. Unlike the conventional channel estimation techniques only using preamble or pilots, the proposed channel estimation technique makes use of preamble, pilots and soft coded data information in a turbo iterative approach. Mean square error analysis and simulation results show that the proposed channel estimation technique can approach the performance of perfect channel state information (CSI) with fewer pilots insertion.

/pdf/iterative-turbo-channel-estimation-for-ofdm-system-over-3k1eftoyty.pdf

A two-tier UMTS network is considered where a large number of randomly deployed Wideband Code Division Multiple Access (WCDMA) femtocells are laid under macrocells where the spectrum is shared. The cochannel interference between the cells may be a potential limiting factor for the system. We study the uplink of this hybrid network and identify the critical scenarios that give rise to substantial interference. The mechanism for generating the interference is analyzed and guidelines for interference mitigation are provided. The impacts of the cross-tier interference especially caused by increased numbers of users and higher data rates are evaluated in the multicell simulation environment in terms of the noise rise at the base stations, the cell throughput, and the user transmit power consumption.

/pdf/on-uplink-interference-scenarios-in-two-tier-macro-and-femto-2wysr9cze3.pdf

On uplink interference scenarios in two-tier macro and femto co-existing UMTS networks

A joint iterative channel estimation and data detection receiver is proposed in this paper for multiple-input- multiple-output orthogonal frequency deivision multiplexing (MIMO-OFDM) system, which employs two mandatory MIMO profiles as in IEEE 802.16e MIMO mobile WiMAX system, namely Alamouti space time coding (STC) and spatial multiplexing (SM), over time and frequency dispersive fading channel. A novel channel estimator with time-domain and frequency-domain combining of channel estimates from preamble/pilot/soft decoded data information is proposed and integrated with maximum ratio combining (MRC) data detector and co-antenna interference canceler (IC) for STC system and SM system respectively. Numerical results show that the proposed receiver has 2dB gain compared to conventional non-iterative receivers in pedestrian low mobility condition and more than 6dB gain in vehicular high Doppler environment.

Ming Zhao

Papers

Iterative Turbo Channel Estimation for OFDM System over Rapid Dispersive Fading Channel

Channel estimation for rapid dispersive fading channels

Iterative Turbo Channel Estimation for OFDM System over Rapid Dispersive Fading Channel

On uplink interference scenarios in two-tier macro and femto co-existing UMTS networks

An Iterative Receiver with Channel Estimation for MIMO-OFDM Over a Time and Frequency Dispersive Fading Channel