In this paper, we present an open source simulation model for the ns-3 simulator that allows the simulation of LTE handover scenarios, to support the design and evaluation of handover decision algorithms. In addition to the features supported by other publicly available open source LTE simulators, such as mobility, propagation, channel, PHY and MAC modeling, our model provides additional features such as the modeling of the RRC protocol, the MAC random access procedure, and the X2, S1 and S11 interfaces. On top of these features, the LTE handover procedure is modeled, following closely the 3GPP specifications. We present in detail the characteristics of each component of the simulation model, highlighting the modeling assumptions that were made in each case.

In this paper, a distributed power adjustment algorithm for downlink Long Term Evoluation (LTE) mobile networks is proposed, in which the Quality of Service (QoS) requirement is taken into account. By tailoring the downlink transmit power to match the aggregate service requirement, it is possible to eliminate unnecessary power, and thereby reducing inter-cell interference to the neighbouring cells. The reduction of inter-cell interference further lowers the required power to meet a certain QoS target. Simulation results have shown that such an approach can significantly reduce the energy consumption with negligible impact on the system performance.

In this paper, the issue of multi-user radio resource scheduling on the downlink of a Long Term Evolution (LTE) cellular communication system is addressed. An optimization model has been proposed in [1], where radio resources for multiple users are jointly allocated at the air-interface. It has been shown that an optimal solution to such a problem may provide reasonable gain over a simply greedy approach. However, the complexity of such an optimal approach could be prohibitively high. By exploiting meta-heuristic method such as Simulated Annealing (SA), the results in this paper show that significant reduction in complexity can be obtained while achieving near optimal solutions.

Link adaptation is an important technique in achieving good system spectral efficiency. However, due to the effect of frequency selective fading, the SNRs of adjacent sub-carriers in an OFDMA system can potentially be different. To provide a reasonable signalling overhead, the smallest unit of channel quality feedback is represented in terms of a group of sub-carrier SNRs, or effective SNR for short. The knowledge of the PDF or CDF of the effective SNR can be very useful in performance analysis of systems involving frequency selective feedback. However, closed-form expressions of these statistical quantities are very difficult to obtained. In this paper, we propose to approximate these quantities using the method of Pearson system. Numerical results show that the proposed approximation is very accurate.

The Probability Density Function (PDF) or Cumulative Distribution Function (CDF) of the effective Signal to Noise Ratio (SNR) is an important statistical characterization in the performance analysis of an Orthogonal Frequency Division Multiple Access (OFDMA) system using Exponential Effective SNR Mapping (EESM). However, the exact closed form of PDF is extremely difficult to obtain. A general approximation method known as Moment Matching Approximating (MMA) is used to approximate the distribution of effective SNR by a simple expression. In this paper, the approximation by Gaussian, Generalized Extreme Value (GEV) and Pearson distribution are studied. Results show that Gaussian approximation is very useful when the number of sub-carriers is sufficiently large. Both GEV and Pearson approximation are accurate enough in approximating the distribution of effective SNR in a general case.

Admission control (AC) is an important feature in a wireless network by which radio resource usage is optimized whilst maintaining the Quality of Service (QoS) of existing users. In this paper, a novel admission control mechanism is proposed for the Long-Term Evolution (LTE) cellular networks. Simulation results show that the proposed AC can provide a more flexible trade-off between network and user performances compared to simpler approaches based on resource utilisation. It is also shown that a predictive approach provides a significant overall network performance gain at the expense of cell-edge blocking.

In this paper we present simulation results to demonstrate that a simple distributed intra frequency load balancing algorithm based on automatic adjustment of handover thresholds can significantly reduce the call blocking rate and increase cell-edge throughput in an LTE network.

In this paper, we consider the related issues of pre-emption and congestion control in Long Term Evolution (LTE) networks. A load-reduction method which can be used for either pre-emption or congestion control is described and evaluated by simulation. The simulation results show that the proposed mechanism can significantly improve dropping and blocking probabilities, depending on the bearer priorities.

In an orthogonal frequency division multiple access (OFDMA) system such as LTE and WiMAX, EESM provides an efficient way to estimate the link performance of a resource block/sub-channel contains a number of sub-carriers by a single value (effective SNR). In order to learn the performance of an OFDMA system involving EESM, it is of interest to investigate the statistics of effective SNR over frequency selective channels. In this paper, general expressions including the MGF and average of EESM effective SNR over arbitrary number of sub-carriers are proposed. Subsequently, the analytical result of average effective SNR over Nakagami-m fading channel is also derived. The numerical results show that the approaches are quite useful in characterizing the average effective SNR over frequency selective channels.

Novel methods of obtaining the moment generating function (MGF) and moments of the exponential effective SNR mapping (EESM) signal-to-noise ratio (SNR) over N jointly distributed fading channels are presented. Based on these methods, novel explicit expressions of the MGF and moments over arbitrary number of correlated Nakagami-m fading channels are proposed. Numerical evaluation of these expressions shows that the proposed approach can be a useful and efficient analytical tool in characterizing the statistics of EESM over correlated Nakagami-m fading channels.

The problem of allocating resources for user transmissions on the downlink of a Long Term Evolution (LTE) cellular communication system is studied. A novel optimal multiuser scheduler is proposed and its performance is evaluated. Numerical results show that the system performance improves with increasing correlation among OFDMA sub-carriers. It is found that a limited amount of feedback information can provide a relatively good performance. A sub-optimal scheduler with a lower computational complexity is also proposed, and shown to provide good performance. The sub-optimal scheme is especially attractive when the number of users is large, as the complexity of the optimal scheme may be unacceptably high in many practical situations.

A novel expression of the moment generating function (MGF) of the exponential effective SNR mapping (EESM) signal-to-noise ratio (SNR) over two correlated but not necessarily identically distributed Nakagami-m fading channels is presented. Based on the MGF, a novel expression for the average effective SNR is also presented. Numerical evaluation of these expressions shows that the proposed approach can be a useful and efficient analytical tool in analyzing the characteristics of EESM over correlated Nakagami-m fading channels.

Multiuser scheduling is an important aspect in the performance optimisation of a wireless network as it allows multiple users to efficiently access a shared channel by exploiting multiuser diversity. For example, the 3GPP cellular standard supports multiuser scheduling in the high speed downlink packet access (HSDPA) feature. To perform efficient scheduling, channel state information (CSI) for users is required, and is obtained via their respective feedback channels. Multiuser scheduling is studied assuming the availability of perfect CSI, which would require a high bandwidth overhead. A more realistic imperfect CSI feedback in the form of a finite set of channel quality indicator values is assumed, as specified in the HSDPA standard. A global optimal approach and a simulated annealing (CSA) approach are used to solve the optimisation problem. Simulation results suggest that the performances of the two approaches are very close even though the complexity of the simulated annealing (SA) approach is much lower. The performance of a simple greedy approach is found to be significantly worse.

The challenge of scheduling user transmissions on the downlink of a long term evolution (LTE) cellular communication system is addressed. A maximum rate algorithm which does not consider fairness among users was proposed in . Here, a multiuser scheduler with proportional fairness (PF) is proposed. Numerical results show that the proposed PF scheduler provides a superior fairness performance with a modest loss in throughput, as long as the user average SINRs are fairly uniform. A suboptimal PF scheduler is also proposed, which has a much lower complexity at the cost of some throughput degradation.

A novel expression of the moment generating function (MGF) of the exponential effective SNR mapping (EESM) signal-to-noise ratio (SNR) over two correlated but not necessarily identically distributed Nakagami-m fading channels is presented. Based on the MGF, a novel expression for the average effective SNR is also presented. Numerical evaluation of these expressions shows that the proposed approach can be a useful and efficient analytical tool in analyzing the characteristics of EESM over correlated Nakagami-m fading channels.

Generalized selection combining (GSC), whereby the receiver selects M out of N received replicas of the same signal for combining, is an effective mean to achieve reliable transmission in fading channel. However, in order to perform coherent combining, the selected signals must be individually tracked for sufficiently long time before accurate channel estimates can be produced for combining purpose. This tracking operation, unfortunately, is incompatible with the inherently antenna switching that takes place inside the GSC receiver. In this paper, we extend the switching rate analysis of M out of N GSC receivers in (J. Cavers et al., 2007) to the case of independent but statistically non-identical branches. Despite the fact that non-identical branches introduces a correlation between the difference of the M-th and the M+1-th strongest signals, u'(t) , and its derivative, u'(t) , we were able to derive an analytical expression for the switching rate of the GSC receiver under this condition (independency between u'(t) and u'(t) is crucial in obtaining the simple results in (J. Cavers et al., 2007)). Our numerical results agree with the intuition that having non- identical branches reduces the switching rate. The more dissimilar the branches are, the larger the reduction. While this lowering of the switching rate allows the GSC receiver more time to dwell on the selected signals and hence producing more accurate channel estimates for coherent combining, the bit-error rate of GSC, unfortunately, is higher when the branches are not identical.

Bit or frame error rates are commonly used as performance measures in wireless communication systems. However, in emerging applications such as voice over IP (VoIP), the bit rate outage probability is often a more useful performance measure. In this paper, an accurate method is proposed for approximating the probability distribution of the downlink received signal-to-interference ratio (SIR) in the high speed downlink packet access (HSDPA) channel of a 3GPP network. Based on this distribution, a power adjustment scheme is proposed to minimize the weighted sum of bit rate outage probabilities for multiple users. It is shown that the proposed method can greatly improve outage probability fairness without incurring a very large degradation in throughput.

The problem of efficiently allocating resources to high speed uplink packet access (HSUPA) users in a 3GPP network is studied. A discrete optimization problem is formulated in which user channel qualities and buffer sizes are used at the base station to jointly allocate uplink resources. To circumvent the possibility of local optima, a linearized version of this formulation is also presented. It is shown that knowledge of user buffer sizes can significantly improve the overall achievable bit rate.

The impact of channel state estimation errors in a CDMA system employing adaptive modulation and coding in conjunction with multicodes is studied. The channel is modelled as a finite-state Markov chain and the performances using (1) a simple moving average (SMA) filter (2) a hidden Markov model (HMM) filter to estimate the channel state are compared. The results show that the HMM filter is more robust and provides a significant throughput improvement over the SMA filter, especially when the channel estimate is quite noisy or the normalized Doppler rate is small.

In this paper, an expression for the joint probability density function (pdf) of the order statistics for a set of arbitrarily correlated Nakagami-m fading channels is derived. This result is useful in analyzing the performance of a variety of diversity schemes which involve branch selections. The derivation also yields a computationally efficient method for obtaining the marginal pdf of the p-th order statistic for a set of arbitrarily correlated Nakagami-m random variables. As an illustration, the derived result is applied to the performance analysis of the generalized selection combining (GSC) scheme with arbitrarily correlated branches.

In performance analyses of wireless communication systems, expressions for the probability distributions of certain random variables are often needed. While exact efficiently computable closed-form expressions are desirable, they are often very difficult, if at all possible, to obtain. In this paper, the Pearson system of distributions is studied as a means to obtain approximate expressions for such distributions. The usefulness of this approach is illustrated for a number of diversity techniques employed on Nakagami and Weibull fading channels. The resulting relatively simple approximations are shown to be quite accurate.

In this paper, some analytical results for general order selection (GOS) over independent but not necessarily identically distributed (i.n.d.) Weibull and Nakagami fading channels are presented. The Weibull fading parameters are assumed to be equal whereas the Nakagami fading parameters are assumed to be integer-valued. It is shown that the pdf of the q-th order statistic can be expressed as a linear combination of Weibull and Nakagami pdf's respectively. Closed-form expressions for the moment generating functions and general moments are derived. In addition, exact closed-form expressions for the symbol error rate are obtained for a number of modulation schemes. Numerical results show that for the same average channel gains, the performance on i.n.d. channels may be better or worse than on i.i.d. channels.

For the problem of allocating radio resources in the downlink of a CDMA network studied in [1], the modulation and coding schemes, numbers of multicodes, and transmit powers used for all mobile stations (MS's) are jointly chosen so as to maximize the total transmission bit rate during each scheduling interval, subject to certain constraints. The approach employs discrete mathematical programming, which is potentially complex, and analytical solutions are not available. In this paper, sub-optimal approaches are examined. In particular, analytical expressions are derived for optimal resource allocation involving a single MS. Based on the single-MS solution, a sub-optimal, sequential optimization procedure for multiple MS's is presented. Numerical results show that when the order for allocating resources to MS's is judiciously chosen based on the MS channel conditions and/or traffic loads, the sequential solution is an attractive alternative to the more complex joint optimization. This is especially true when radio resources are scarce.

In performance analyses of wireless communication systems, expressions for the probability distributions of certain random variables are often needed. While exact, efficiently computable closed-form expressions are desirable, they are often very difficult, if at all possible, to obtain. In this paper, the Pearson system of distributions is studied as a means to obtain approximate expressions for such distributions. The usefulness of this approach is illustrated for a number of diversity techniques employed on Nakagami and Weibull fading channels. The resulting relatively simple approximations are shown to be very accurate

In this paper, some analytical results for general order selection (GOS) over independent but not necessarily identically distributed (i.n.d.) Weibull and Nakagami fading channels are presented. The GOS model is important in multiuser scheduling. By transforming the probability density function (pdf) of the selected channel signal to noise ratio (SNR) into an appropriate form, exact closed-form expressions for the corresponding moment generating function (MGF) and general moments are derived for Weibull fading channels. Using the derived results together with those in R. Kwan and C. Leung (2005), exact closed-form expressions for the symbol error rate are obtained for a number of modulation schemes over the i.n.d. Weibull and Nakagami fading channels. Numerical results show that for the same average channel gains, the performance over i.n.d. channels may be better or worse than over i.i.d. channels

Adaptive modulation and coding (AMC) has been adopted in the 3GPP standard in order to improve spectral efficiency. In order to increase the granularity of the adaptation and to provide higher bit rates, multicode transmission is employed. Since the use of AMC requires knowledge of the channel state, the accuracy of this information is important. In practice, errors in estimating the channel state are inevitable, resulting in performance degradation. The average bit rate performance of AMC with multicodes is studied for a CDMA system experiencing Nakagami fading and channel estimation errors. The results are obtained in terms of the generalized Marcum Q-function. Numerical results are provided to illustrate the performance degradations due to inaccuracies in estimating the channel.

The problem of allocating radio resources in the downlink of a CDMA network is studied. The modulation and coding schemes, numbers of multicodes, and transmit powers used for all mobile stations (MS's) are jointly chosen so as to maximize the total transmission bit rate, subject to certain constraints. Based on the discrete and nonlinear nature of the proposed model, a mixed-integer nonlinear programming optimization problem is formulated. It is shown that the nonlinear relationship between bit rate and transmit power due to the use of different modulation and coding schemes and the maximum number of multicodes which can be assigned to an MS generally result in an optimal allocation which involves simultaneous transmission to several MS's. In addition, it is shown that the optimal scheduler can yield a significant improvement in throughput by taking the MS traffic loads into account.

The high-speed downlink packet access (HSDPA) is currently an important research topic to enhance the downlink performance of a WCDMA network. It is a set of schemes built on top of the downlink shared channel (DSCH), resulting an extremely high bit rate for a single user in the downlink. When the channel condition is good, a high utilization of the code resource for a single user can be effectively achieved. However, when the channel condition is bad, a fraction of the code resource would be wasted because only a user can use a small fraction of the code resource. Thus, code multiplexing offers a solution to utilize the limited code resource more effectively. In this paper, the effect of code multiplexing on the HSDPA performance is studied in a dynamic simulator platform. It means that the code resource is shared among a few users. Multicode transmission is also considered in the study. Two, semi-static and dynamic code sharing, multiplexing cases are studied. The number, n, of the code multiplexing users is one of the study parameters. The simulation results show that code-multiplexing can increase the throughput in a WCDMA network by improving code utilization while also decreasing the transfer delay between the base station and the terminals.

The problem of optimally detecting a desired BPSK modulated bit in the presence of another bit unsynchronized BPSK modulated interfering signal and additive white Gaussian noise is considered. Given certain information about the desired and interferer signals, the minimum achievable bit error rates for two types of receiver models are obtained and compared with that for a conventional matched filter. It is found that knowledge of the interferer signal's carrier phase is important when the signal-to-interference ratio is low. The effect on bit error rate of a frequency offset between the desired and interferer signals is also studied. Finally, performance sensitivity to errors in estimating the interferer signal's carrier phase and the bit timing difference between the desired and interferer signals is examined.