Erdem Bala

Full duplex cellular systems: will doubling interference prevent doubling capacity?

Recent advances in antenna and RF circuit design have greatly reduced the crosstalk between the transmitter and receiver circuits on a wireless device, which enable radios to transmit and receive on the same frequency at the same time. Such a full duplex radio has the potential to double the spectral efficiency of a point-to-point radio link. However, the application of such a radio in current cellular systems (3GPP LTE) has not been comprehensively analyzed. This article addresses the fundamental challenges in incorporating full duplex radios in a cellular network to unlock the full potential of full duplex communications. We observe that without carefully planning, full duplex transmission might cause much higher interference in both uplink and downlink, which greatly limits the potential gains. Another challenge is that standard scheduling methods which attempt to achieve the maximum capacity gain lead to a severe loss in energy efficiency. In this article, we identify new tradeoffs in designing full duplex enabled radio networks, and discuss favorable conditions to operate in full duplex mode. New scheduling algorithms and advanced interference cancellation techniques are discussed, which are essential to maximize the capacity gain and energy efficiency. Under this new design, most of the gain is achievable with full duplex enabled base stations, while user equipment still operates in half duplex mode.

Improving small cell capacity with common-carrier full duplex radios

Recent progress in establishing the capability of radios to operate in full duplex mode on a single channel has been attracting growing attention from many researchers. We extend this work by considering the application to small cells, in particular resource-managed cellular systems similar to the TDD variant of LTE. We derive conditions where full duplex operation provides improved throughput compared to half duplex for a single cell scenario. We present a hybrid scheduler that defaults to half duplex operation but can assign full duplex timeslots when it is advantageous to do so. We compare the performance of such a scheduler with a traditional half duplex scheduler in terms of throughput and energy efficiency. Our simulation results show that we achieve as much as 81% of the capacity doubling promised by full duplex, with limitations deriving from interference effects specific to full duplex operation.

Small-Cell Traffic Balancing Over Licensed and Unlicensed Bands

The Third-Generation Partnership Project (3GPP) has recently started standardizing the “licensed-assisted access using LTE” for small cells, which is referred to as dual-band femtocell (DBF) in this paper, which uses the Long-Term Evolution (LTE) air interface in both the licensed and unlicensed bands based on the LTE carrier aggregation feature. Alternatively, the Small Cell Forum introduced the integrated femto-WiFi (IFW) small cell, which simultaneously accesses both the licensed band (via cellular interface) and the unlicensed band (via WiFi interface). In this paper, a practical algorithm for IFW and DBF to automatically balance their traffic in licensed and unlicensed bands, based on the real-time channel, interference, and traffic conditions of both bands, is described. The algorithm considers the fact that some “smart” devices (sDevices) have both cellular and WiFi radios, while some WiFi-only devices (wDevices) may only have WiFi radio. In addition, the algorithm considers a realistic scenario where a single small-cell user may simultaneously use multiple sDevices and wDevices via either the IFW or the DBF in conjunction with a wireless local area network. The goal is to maximize the total user satisfaction/utility of the small-cell user, while keeping the interference from small cells to macrocells below predefined thresholds. The algorithm can be implemented at the radio link control or the network layer of the IFW and DBF small-cell base stations. Results demonstrate that the proposed traffic-balancing algorithm applied to either IFW or DBF significantly increases the sum utility of all macrocell and small-cell users, compared with the current practices. Finally, various implementation issues of IFW and DBF are addressed.

A framework for femtocells to access both licensed and unlicensed bands

Cellular operators have been offloading data traffic from their licensed bands to unlicensed bands through a large number of WiFi hotspots over the past years. Although this approach improves the cellular network capacity to some extent, it falls short of getting significant throughput gains. In this paper, it is argued that femtocells, covering a short range, can be a perfect platform to jointly exploit the merits of both licensed and unlicensed frequency bands. In particular, a framework is proposed for a femtocell to simultaneously access both licensed and unlicensed bands. The performance of coexisting femtocell and WiFi networks operating over a fully-utilized unlicensed band are analytically modeled and are verified via simulations. Impact of femtocell channel access parameters on the performance of WiFi and cellular networks is also investigated, shedding light on how a femtocell can best adjust its channel access parameters to coexist with incumbent unlicensed spectrum users like WiFi networks.

Mechanisms for LTE coexistence in TV white space

This paper presents a high level description of an LTE system operating in license exempt bands. Since wireless networks potentially using different air interfaces may operate in these bands, coexistence is a challenge that needs to be addressed. This paper focuses on non-coordinated mechanisms for secondary users coexistence, and introduces a coexistence gap based method for LTE to dynamically share the spectrum with other secondary users. A simulation based analysis of the coexistence gap method is presented, and the results are compared with an energy based sensing channel access method.

Dual-band femtocell traffic balancing over licensed and unlicensed bands

Many cellular user equipments (UEs) today are able to access both the unlicensed band (e.g., via WiFi) and the cellular licensed band. However, in most regions of the world, they are not allowed to access both bands simultaneously for data communications - users can only choose either WiFi or cellular. In order to allow UEs to enjoy both bands at the same time, in our earlier work a framework for femtocells to simultaneously access both licensed and unlicensed bands was proposed. In this paper, the framework is extended to include strategies for femtocells to balance their traffic in licensed and unlicensed bands. The goal is to maximize the sum utility (i.e., user satisfaction) of femto and WiFi users while keeping the femto-to-macro interference below predefined thresholds. The optimal femto traffic balancing scheme is obtained and implemented in a network simulator which considers the activities and interactions of macro, femto and WiFi networks. Simulation results demonstrate that the proposed scheme significantly increases sum utility of all macro, femto and WiFi users, compared with the current practices where users can choose only one band (licensed or unlicensed).

Resource block Filtered-OFDM for future spectrally agile and power efficient systems☆

Abstract Spectrum sharing is a common paradigm in future communication systems and a spectrally agile baseband waveform with minimal out-of-band emissions is a critical component. In this paper, we propose a new multicarrier modulation technique, called resource block Filtered-OFDM (RB-F-OFDM) and present the transceiver design. This waveform can be used over channels with non-contiguous spectrum fragments and exhibits very low adjacent channel interference, which is required for cognitive radio systems with multi channel carrier aggregation capabilities. As such, regulatory based very stringent adjacent channel leakage ratio (ACLR) and adjacent channel selectivity (ACS) requirements can be met. We show that the transceiver complexity may be reduced by utilizing an efficient polyphase implementation that is commonly used in the filter bank multicarrier (FBMC) modulation. In addition, some efficient peak-to-average power ratio (PAPR) reduction techniques can be naturally applied. The new design is backwards compatible with legacy OFDM based systems. Simulation results to evaluate the performance, including measured bit error rate (BER) in multipath channels, are provided.

Flexible DFT-S-OFDM: Solutions and Challenges

Discrete Fourier transform spread orthogonal DFT-S-OFDM, adopted in 3GPP LTE uplink, enables the synthesis of block-based single carrier waveforms with various bandwidths by changing the size of the DFT-spread block. Conceptually, it also allows a transition between block-based multicarrier and single-carrier schemes when multiple DFT-spread blocks are employed in the structure. Recently, it has been shown that DFT-S-OFDM can also accommodate an internal guard period that offers flexibility on the duration of the guard periods without affecting the symbol duration. In this article, we present further modifications of DFT-S-OFDM that offer improvements in flexibility and discuss the latest enabling techniques on this topic. Considering the flexibility introduced by DFTs-OFDM and its variations, the DFT-S-OFDM family also offers a set of promising waveforms for 5G networks, which will require a flexible physical layer.

Antenna Selection Training in MIMO-OFDM/OFDMA Cellular Systems

Antenna selection allows multiple-antenna systems to achieve most of their promised diversity gain, while keeping the number of RF chains and, thus, cost/complexity low. In this paper we investigate antenna selection for fourth-generation OFDMA- based cellular communications systems, in particular, 3GPP LTE (long-term evolution) systems. We propose a training method for antenna selection that is especially suitable for OFDMA. By means of simulation, we evaluate the SNR-gain that can be achieved with our design. We find that the performance depends on the bandwidth assigned to each user, the scheduling method (round-robin or frequency-domain scheduling), and the Doppler spread. Furthermore, the signal-to-noise ratio of the training sequence plays a critical role. Typical SNR gains are around 2 dB, with larger values obtainable in certain circumstances.

A resource block based filtered OFDM scheme and performance comparison

Dynamic spectrum sharing is a key technique to meet the high data rate demands of future wireless systems evolving towards heterogeneous networks. To make the best use of available frequency resources without creating interference to neighbor nodes, spectrally contained and scalable waveforms are needed. While OFDM has become the most popular multicarrier modulation (MCM) scheme, it suffers from high side lobes. Filtering is a popular technique used to reduce the out-of-band emission of OFDM. However, to utilize non-contiguous spectrum fragments, filters need to be dynamically designed for each fragment, making the use of filtered OFDM challenging. On the other hand, OFDM-OQAM, a filter bank based multicarrier (FBMC) modulation, has emerged as an alternative to OFDM to offer better spectral containment and more flexibility for dynamic spectrum allocations. In this paper, we first present a novel technique, called resource block (RB) filtered OFDM (RB-F-OFDM), which divides the available spectrum fragments into chunks of contiguous subcarriers, referred to RBs, and generates and filters the signal transmitted on each RB individually. This approach has the advantage of being modular and scalable since the same transmit and receive modules are used for all RBs. Then, we compare the performance of OFDM, filtered OFDM, RB F-OFDM, and OFDM-OQAM. The simulations are performed under various adjacent channel interference (ACI) conditions. Simulation results show that OFDM-OQAM offers the best performance under channels with moderate delay spread and ACI; however, it has the undesired high latency and its signal is not backwards compatible to legacy OFDM receivers. The performance of RB F-OFDM is similar to filtered OFDM while adding support for non-contiguous spectrum, making it a viable solution for dynamic spectrum sharing systems.