Licensed-Assisted Access (LAA) is a new feature for 3rd Generation Partnership Project (3GPP) Long-Term Evolution (LTE) system to operate in the unlicensed spectrum. LAA utilizes the carrier aggregation technology to aggregate both licensed and unlicensed carriers in order to opportunistically boost user throughput, while maintaining seamless mobility, quality of service and good indoor and outdoor coverage. In order to coexist with other technologies in the unlicensed bands, several new coexistence functionalities for LAA LTE have been introduced, including long term channel selection, short-term channel sensing based on listen-before-talk (LBT), and discontinuous transmission on a carrier with limited maximum transmission duration. LAA will be supported for both Downlink (DL) and Uplink (UL) transmissions. This paper presents a detailed overview of the design challenges for UL LAA access on unlicensed bands due to the scheduling aspects of LTE. We present solutions to address grant and scheduling request (SR) overhead and delays. We also discuss possible UL LBT options to increase the performance and the flexibility of scheduled LTE when operating on unlicensed bands. A range of simulations show fair coexistence between LAA and Wi-Fi. Furthermore, this paper sheds lights on possible future enhancement for UL LAA using unscheduled UL transmission.

Dynamic base station downlink discontinuous transmission (DTX) operations have been proposed and under discussion in the standardization body as a candidate to reduce interference in the LTE network to support rapidly increasing demand for mobile broadband services and traffic volumes [1]. In this paper, we analyze the energy efficiency improvement of the dynamic base station DTX operations from three perspectives. First, we discuss the complementary operation aspect of the dynamic base station DTX and existing LTE dormant mode. Secondly, we provide an analysis showing substantial energy efficiency benefits at all loads of dynamic base station DTX operation with near- future base station hardware components. Thirdly, we provide an analytical framework to assess the energy saving potential of the dynamic base station DTX concept for all possible future hardware improvement possibilities.

To evolve the LTE network to cope with ever and rapidly increasing demand for mobile broadband services and traffic volumes, interference avoidance designs have been identified as one major area to address in the next LTE release. In both traditional homogeneous macro network and heterogeneous network with mix of macro and small cells, intercell interference can be substantially reduced by temporarily halting unneeded downlink transmission. In this paper, we analyze several design aspects of such base station downlink discontinuous transmission (DTX) operations. We provide extensive performance evaluation results and analysis on (1) the time scale of such DTX operations; and (2) whether the DTX operations should be restricted to the small cells only. The results provide guideline for an ideal combination of these design dimensions.

Cellular wireless systems have focused, to date, on system capacity, user data rates, latency and coverage as primary design criteria. Energy efficiency has recently received increased attention as another important criterion to be considered. The potential for energy savings in 3GPP LTE systems has been investigated in [1]-[3]. An important design element that determines the potential for energy efficient transmission is the structure of reference signals that need to be transmitted independent of the load in the network, such as the cell-specific reference signals (CRS) in 3GPP LTE systems. This paper discusses modifications to the reference signal structure in LTE that can enable greater energy efficiency. The time and frequency estimation performance implications of such modifications are evaluated and discussed.

In this paper we investigate the feasibility of using microwave frequencies for fixed non-line-of-sight wireless backhauling connecting small-cell radio base stations with an aggregation node in an outdoor urban environment, i.e. a typical heterogeneous network scenario. We study system level simulations for a point-to-point system where the wave propagation is based on diffraction over rooftops. We further investigate the effects of carrier frequency, interference, antenna height, rain, and tolerance to antenna alignment errors. It is found that the higher frequencies offer not only larger bandwidths but also higher antenna gains which would ideally work to their advantage. However, these advantages may be lost when taking antenna alignment errors and rain into account. Different frequencies simply have their different trade-offs.

Link level performance results for physical antenna port muting in LTE are presented. In this paper we examine the block error rate performance of the LTE downlink control channels (PBCH, PCFICH, PDCCH, and the PHICH) for different antenna muting configurations. Also the throughput performance of the LTE downlink data channel (PDSCH) is studied for 2Tx and 4Tx transmit diversity formats. The results show that the combination of logical antenna port merging with physical antenna port muting provides a very minor performance loss. Since antenna muting should only be used when the traffic in a cell is low, this performance loss is not significant in practice. With antenna muting we can save energy by activating radio units only when motivated by the traffic load in the cell.

In this paper, we present a pilot design for an OFDM cellular system based on Costas arrays. The design provides pilot symbols for both common and dedicated pilots for MIMO transmission. The pilot symbols are designed so that pilots from different cells intersect minimally. The design results in a large number of available distinct pilot symbol patterns with minimally overlapping resource elements that may be assigned to different cells which simplifies cell planning. The design is described in the context of IEEE 802.16m, but is applicable to any OFDM system. Performance of the pilot design in noise and interference limited environments is presented and is shown to be on par with currently used pilot designs while providing a larger number of patterns.

We develop a physical layer abstraction to model the performance of MIMO with maximum likelihood demodulation (MLD) in a system simulation. MLD is inherently a non-linear process, and thus it is difficult to find a linear abstraction to its performance. The abstraction is based on bounds on performance obtained from the QR & QL decompositions of the channel. The accuracy of the abstraction is demonstrated using link simulations. In addition, the abstraction has low complexity. The model is used in a system simulation to evaluate the gains obtained with MLD.

The error detection performance of CRC coding in LTE with general two-level early stopping algorithms for turbo decoding is investigated. Analytical models for the probability of block error and undetected block error at the code block and transport block levels were developed. Simulations were used to verify the model for shorter CRC lengths and block sizes. The analytical models show the setting of 24-bit CRCs in LTE allows low-complexity, robust and reliable early stopping algorithms to reduce turbo decoding complexity.

Channel estimation is critical to receiver performance in an OFDM system. Pilot symbols are typically interspersed with data symbols among the sub-carriers to aid channel estimation. Standard algorithms to estimate the channel based on these pilot symbols include minimum mean-square error (MMSE) and maximum-likelihood (ML) estimation. With a uniform density of pilots in frequency among the sub-carriers, the ML estimate exhibits a boundary effect whereby the quality of the estimates on sub-carriers towards the edges of the signal band degrades. While the use of MMSE mitigates this effect significantly it requires precise knowledge of channel statistics which are difficult to obtain. Also, it is often too complex to implement for common numbers of sub-carriers. This paper proposes mitigation of the boundary effect described above by transmitting extra pilot symbols in the sub-carriers towards the edges of the band in addition to the regularly spaced pilots, thereby increasing pilot density at the edges. Channel estimation is performed using a two-step algorithm. ML estimates are first formed using the uniform pilot set. The output of the ML estimator is then used to bias an MMSE estimator which is applied only to the edge pilot set. The two-step approach enhances performance as compared to ML estimation while keeping complexity much lower than that incurred when MMSE is used across the entire band.

Two methods for accurate estimation of time or angle of arrival that use successive cancellation techniques are presented. The first method performs successive cancellation with a correlation approach used for each iteration. The second method uses the multiple signal identification and classification (MUSIC) approach for each iteration. The first method exhibits enhanced performance as compared to straight-forward correlation techniques and has lower complexity than high resolution techniques such as MUSIC. The method also performs better than Capon's (1969) minimum variance method and MUSIC when the total time duration of the received signal is small or when the signal-to-noise ratio is low. The second method performs better than MUSIC and Capon's minimum variance approach but is more complex than both of them. The methods are explained in the context of time of arrival estimation.

Multiple signal identification and classification (MUSIC) is a well known approach to time of arrival estimation of band-limited signals received through multi-path channels. MUSIC uses the eigen vectors of the correlation matrix for the received data that are associated with noise components. We compare the performance of MUSIC to an algorithm that is based on the eigen vectors associated with the signal components of this correlation matrix. The performance comparison is made using a simple two path channel model where the second path delay is assumed known and the statistics of the error in measuring the delay of the first path are compared. The optimal algorithm for this estimation problem is the maximum-likelihood algorithm (ML) and the performance of MUSIC and the signal eigen vectors algorithm are compared to the ML approach.

Positioning services in cellular systems have received increased attention recently. Solutions of the positioning problem requiring time and angle of arrival measurements have been proposed. The problem of estimating the time of arrival (TOA) of the line of sight (LOS) component of narrowband digital signals in a multipath environment is addressed. A TOA estimation algorithm designed to operate under low SNR is proposed. The algorithm combats multipath propagation by exploiting the fading statistics of the mobile channel. Performance of the algorithm obtained through simulations using COST propagation models for GSM and IS-136 signals is shown and is promising.

A locally invertible convolutional encoder has a local inverse defined as a full rank w/spl times/w matrix that specifies a one-to-one mapping between equal-length blocks of information and encoded bits. In this correspondence, it is shown that a rate-1/n convolutional encoder is nondegenerate and noncatastrophic if and only if it is locally invertible. Local invertibility is used to obtain upper and lower bounds on the number of consecutive zero-weight branches in a convolutional codeword. Further, existence of a local inverse can be used as an alternate test for noncatastrophicity instead of the usual approach involving computation of the greatest common divisor of n polynomials.