Mihaela C. Beluri

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.

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).

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.

On a new incentive and market based framework for multi-tier shared spectrum access systems

The popularity of the smartphone and the resulting increase in the volume of wireless data traffic has created a large gap between bandwidth supply and demand. An attractive and less expensive solution to re-farming of spectrum is to maximize the use of under-utilized bands through spectrum sharing. Recent efforts in this area, that propose operation of wireless systems in shared spectrum, address the main challenge of how shared spectrum access systems can efficiently share and utilize spectrum resources. In this paper, we focus on the framework for multi-tier shared spectrum operation in wireless networks, where multiple entities - the Shared Spectrum Managers (SSMs) - dynamically acquire, manage and sell shared spectrum. We derive an auction based spectrum resource assignment algorithm which maximizes the overall system efficiency and provides incentives to the incumbents to make spectrum available for sharing. We demonstrate that the algorithm can be implemented in practice through messaging in which the SSM dynamically negotiates spectrum bidding and asking prices with the shared spectrum users. Simulations are performed under different scenarios and assumptions both to verify that the algorithm achieves the maximum utility value for shared spectrum systems, and to compare the outcome behavior under each scenario. Based on the observations, potential regulations that would need to be put in place in each scenario are discussed, and a set of future development areas are identified based on these observations.

Relay Assisted Cooperative OSTBC Communication with SNR Imbalance and Channel Estimation Errors

In this paper, a two-hop relay assisted cooperative Orthogonal Space-Time Block Codes (OSTBC) transmission scheme is considered for the downlink communication of a cellular system, where the base station (BS) and the relay station (RS) cooperate and transmit data to the user equipment (UE) in a distributed fashion. We analyze the impact of the SNR imbalance between the BS-UE and RS-UE links, as well as the imperfect channel estimation at the UE receiver. The performance is analyzed in the presence of Rayleigh flat fading and our results show that the SNR imbalance does not impact the spatial diversity order. On the other hand, channel estimation errors have a larger impact on the system performance. Simulation results are then provided to confirm the analysis.

Dynamic Performance of a Chip Level Adaptive Equalizer in a UMTS High Speed Downlink Packet Access (HSDPA) Terminal

The HSDPA mode of the 3GPP UMTS FDD standard extends the peak downlink rate of the W-CDMA waveform to 14 Mbps. To achieve high rates across a multipath channel the demodulator must include an advanced receiver rather than a conventional Rake. This paper presents the performance of an NLMS-based adaptive equalizer that supports mobile velocities as high as 250 km/h. The equalizer works with radio and implementation impairments typical of a non-HSDPA receiver, so redesign of the terminal front end is not required. A key aspect of the design is dynamically adjusting the equalizer step size and leakage parameter. There is often concern about the ability of an adaptive algorithm to operate in a highly mobile environment. The transient behavior is discussed in the context of HSDPA operation and examples are provided of initial convergence and operation after a sudden change in propagation conditions.