In this paper, we present a direction of arrival (DOA) estimation method for multiple-input and multiple-output frequency-modulated continuous wave radar systems considering range-angle migration and piecewise sub-array model. In general, DOA estimation methods based on an exact wavefront model can achieve highly accurate results. However, the methods also entail considerable computational complexity, hindering real-time radar data processing in automotive driving. To reduce the computational complexity caused from the nonlinearity of wavefront curvature, we use a piecewise sub-array model and FFT-based methods in both the range and angle domains. The proposed method combines the advantages of far-field-based methods and near-field-based methods, thereby offering enhanced resolution at a reduced computational cost. We confirm that the DOA spectrum generated by the proposed method achieves higher resolution compared to conventional far-field-based methods and demonstrates approximately four times greater computational efficiency (in terms of FLOPs) than the back-projection algorithm.

Phase modulated continuous wave (PMCW) radar is one of promising options for the enhanced sensing capability and the multiple access availability. In this paper, we focus on the advantage of the high-resolution multi-input multi-output (MIMO) radar with fast-time code division multiplexing (CDM). First, we formulate the signal model of the fast-time CDM-MIMO PMCW radar by assigning different code sequences to each transmitter (Tx). We investigate the level of self-code interference induced by simultaneously transmitting Tx signals, as a function of the number of Tx and the type of sequence. For self-code interference cancellation, we propose compressive sensing-based greedy algorithms with significantly reduced computation complexity using the property of the circulant matrix. We show via simulation that the proposed methods can effectively mitigate self-code interference and generate radar images by implementing virtual antenna array with significantly enhanced peak to sidelobe ratio and range/angle estimation accuracy. Since the proposed method can fully leverage the advantages of CDM whose transmit signals share time-frequency resources, it can be utilized for high density Internet of Things networks and increase the fundamental performance of sensing while the reduced computations are suitable for the real-time operation.

For high-level autonomous driving, the development of an effective vehicle-to-vehicle (V2V) communication system is an utmost concern. Among several candidates, the millimeter-wave (mmWave) V2V communication system is considered as a promising one, as it is expected to provide high data rate. To overcome the high path loss at mmWave frequencies, the codebook-based beam-alignment approach has been widely studied as a means of generating narrow and strong beams. Despite its potential, many existing codebooks are designed to generate codewords with equal beamwidths, which may not be optimal for V2V communications. Our focus is on designing a beamforming codebook for V2V communications, taking into account the angular distribution characteristics of vehicles on the road. To achieve this, we propose an analytical model to represent the traffic density of vehicles on roads. Based on the proposed traffic density model, we derive the angular distribution of directional communication links between vehicles by considering realistic road structures, including straight roads, intersections, and roundabouts. Utilizing the angular distribution between vehicles, we propose a codebook design framework specifically tailored for V2V communication systems. Simulation results verify that our model fits well for realistic urban grid road environments, and the proposed codebook outperforms codebooks with equal beamwidths.

Modern autonomous driving vehicles are equipped with a number of sensors to perceive the surrounding environment and enable automation of various systems. Especially, the passenger occupancy detection system can be utilized to detect a child left unattended in a parked vehicle, and efficiently manage the energy inside the vehicle. In this study, we propose a method of detecting the occupancy of passengers inside a vehicle using multichannel 60-GHz frequency-modulated continuous-wave (FMCW) radar. The received signal from the radar is converted into a range-angle map, and clutter suppression is performed to eliminate reflections from stationary objects. Then, by applying classification algorithms, such as the support vector machine (SVM), multilayer perceptron (MLP), and convolutional neural network (CNN), various arrangements of passengers inside a vehicle are identified. The classification results demonstrated that the proposed method can detect the location and number of passengers with an accuracy of 97.68%.

We develop a vehicle motion state estimation algorithm based on an orthogonal frequency division multiplexing radar system suitable for vehicle-to-infrastructure (V2I) communications scenarios. Due to the transverse movement with respect to the direction from the target to roadside unit (RSU), the measured velocity is distorted as the Doppler shift depends on the angle formed between the vehicle position and RSU. To compensate for the distorted velocity, the exact vehicle position is required. However, this cannot be obtained in a system with a single RSU. In this paper, we develop an efficient cooperative vehicle state estimation algorithm that reduces the number of RSUs required for localization compared to conventional schemes by eliminating unnecessary RSUs to overcome the degradation of target detection performance in low signal-to-noise ratio regimes. Furthermore, the singularity problem due to the absence of Doppler shift when the vehicle travels near the tangential direction can be addressed with spatially distributed RSUs. Numerical results verify that the proposed estimation algorithm outperforms the existing schemes in V2I scenarios.

In a quadrature receiver, mismatch between the I and Q channels and DC offsets result in I/Q imbalance of the received signal. The I/Q imbalance creates an image target in the signal spectrum and can cause false alarms at the receiver. Therefore, I/Q imbalance needs to be corrected for reliable target detection. This paper proposes an effective method for correcting I/Q imbalance in an frequency modulated continuous wave radar system. The proposed method is based on geometric sequence decomposition of the received signal and can be applied regardless of the number of targets. In addition, a target pairing method utilizing the phase information is presented. The simulation results demonstrate that the proposed method can successfully reconstruct the signals distorted by I/Q imbalance. After the reconstruction of the signal, the image target was eliminated and target information was correctly estimated.

This paper presents an efficient mutual interference suppression method using wavelet denoising in automotive radar systems. When a radar signal transmitted from another radar-equipped vehicle flows into our radar system, it acts as an interference signal and degrades the target detection performance. With wavelet denoising, the interference signal can be extracted from the time-domain low-pass filter output. Then, the effect of the interference can be mitigated by subtracting the interference signal from the original low-pass filter output. As a result, the beat frequency containing the target information can be estimated accurately. Simulation results show that the proposed method can enhance the estimation accuracy of the target's distance, velocity, and angle. In addition, the performance of the proposed method is verified through actual experiments using heterogeneous radars. In the measurement results, even though the exact specifications of the radar signal transmitted from the other vehicle are not identified, the interference is effectively suppressed. Unlike other existing methods, the proposed method using wavelet denoising does not need to generate a specific radar waveform and it can mitigate interference only by signal processing without changing the existing hardware.

The temporal characteristics of millimeter wave (mm wave) wireless channels at 28 GHz in an urban environment have been analyzed for use in millimeter wave wireless communications systems. In this paper, we discuss the characteristics of mm wave signals in outdoor radio channels by using extensive simulation results generated with ray tracing simulator. Using this simulation data, we calculate the root-mean-square delay spread and analyze the results. Also, we apply Saleh-Valenzuela channel model to analyze the channel characteristics. The channel is modeled by using various probability distributions, and the similarity of each distribution is analyzed. In addition, using the Saleh-Valenzuela channel model, the inter cluster delay and the peak power of the power delay profile are modeled, and the optimal parameter values are calculated. Finally, the differences between the distribution of the sample and the model are analyzed.

In this paper, we propose a new algorithm for predicting the path loss exponent of outdoor millimeter-wave band channels through deep learning method. The proposed algorithm has the advantage of requiring less inference time compared to existing deterministic channel models while concretely considering the topographical characteristics. We used three-dimensional ray tracing to generate the outdoor millimeterwave band channel and path loss exponent. We trained a neural network with generated path loss exponent. To evaluate the performance of the proposed method, we analyzed the influence of the hyperparameters and environmental features, for example, building density and average distance from the transmitter.

In IEEE 802.11ad, single carrier (SC) and orthogonal frequency-division multiplexing (OFDM) transmissions are supported as mandatory and optional transmission schemes, respectively. Both transmission schemes can be implemented with a minimum increase in complexity, because the frequency-domain equalization (FDE)-aided packet structure of IEEE 802.11ad enables SC receivers to operate using a mechanism similar to OFDM receivers. However, when the transmitter and receiver suffer phase noises, SC and OFDM receivers should mitigate the phase noise in different manners, because phase noise has different characteristics in time- and frequency-domains. Consequently, conventional phase noise mitigation methods for SC and OFDM transmissions are usually applicable only for a given transmission scheme, and these lead to an increase in complexity for integrated receiver designs for IEEE 802.11ad. To mitigate the phase noise of both transmission schemes with a minimum increase in complexity, we investigated an OFDM receiver design that can be added to the conventional SC receiver that includes precise signal-to-noise ratio (SNR) estimation for efficient phase noise mitigation. Link-level simulations were conducted to evaluate the packet-error rate (PER) performance of the highly modulated OFDM transmissions, 16QAM and 64QAM. The simulation results show that the SNR required to achieve 1% PER decreases by 1.2 dB and 0.8 dB for 16QAM and 64QAM, respectively, when the precise SNR estimation developed for SC transmission is applied.

This paper proposes an efficient road structure recognition method using statistical characteristics of received signals in automotive frequency-modulated continuous wave radar systems. Generally, roads consist of various structures, some of which, such as tunnels and soundproof walls made of iron, generate undesired echoes, called clutter. When the clutter flows into the radar system, the target detection performance cannot be guaranteed completely. This causes great danger to the driver using the radar function such as adaptive cruise control. Thus, an efficient method to recognize the structures that deteriorate the radar detection performance is desired. Depending on the types of road structures, frequency components of the received signals have distinctive distributions. Focusing on this point, parameters that reflect statistical properties of each distribution are extracted. These parameters can be used as standards for the recognition because they show different values according to the road structures. For more enhanced recognition, we use a support vector machine method with a linear classifier or a Gaussian kernel, and the resulting confusion matrices are derived. According to the results, the proposed method successfully classifies the structures with high accuracy. If the recognition of the road structures that degrade radar's function is performed effectively, the safety of the driver in the radar-equipped vehicle can be ensured by applying additional signal processing or giving a warning message to the driver.

This study demonstrates a signal‐to‐noise ratio (SNR) analysis and estimation algorithm for efficient phase noise mitigation that can be practically applied to single‐carrier frequency‐domain‐equalisation (SC‐FDE) systems that operate in millimetre‐wave bands. First, the effect of phase noise in SC‐FDE systems is investigated on each of the packet reception processes, namely, channel estimation, SNR estimation, and data‐field reception. According to the analysis, an SNR estimation algorithm is proposed. The performance of minimum‐mean‐square‐error equalisation and conventional phase noise mitigation algorithm can be enhanced using the proposed SNR estimation. The effectiveness of the proposed analysis and SNR estimation algorithm is verified through the link‐level simulation. Compared with the conventional SNR estimation and the iterative phase noise mitigation algorithms, the proposed algorithm provides a lower packet‐error rate without any iterative decoding process.

Inter-cell interference coordination (ICIC) is a promising technique to improve the performance of frequency-domain packet scheduling (FDPS) in downlink LTE/LTE-A networks. However, it is difficult to maximize the performance of FDPS using static ICIC schemes because of insufficient consideration of signal-to-interference-and-noise ratio distribution and user fairness. On the other hand, dynamic ICIC schemes based on channel state information (CSI) also have difficulty presented in the excessive signaling overhead and X2 interface latency. In order to overcome these drawbacks, we introduce a new concept of ICIC problem based on geometric network information (GNI) and propose an adaptive sector coloring game (ASCG) as a decentralized solution of the GNI-based ICIC problem. Furthermore, we develop an ASCG with a dominant strategy space noted as ASCG-D to secure a stable solution through proving the existence of Nash equilibrium. The proposed scheme provides better performance in terms of system throughput gain of up to about 44.1%, and especially of up to about 221% for the worst 10% users than static ICIC schemes. Moreover, the performance of the CSI-based ICIC, which require too much computational load and signaling overhead, is only 13.0% and 5.6% higher than that of ASCG-D regarding the total user throughput and the worst 10% user throughput, respectively. The most interesting outcome is that the signaling overhead of ASCG-D is 1/144 of dynamic ICIC schemes' one.

To evaluate the technical feasibility of the millimeter-wave (mmWave) band for cellular network, several channel measurement experiments have been performed in recent years. To reduce the burden of repetitive channel measurement campaigns, ray tracing simulations have been widely utilized to evaluate the characteristics of wireless propagation channels for outdoor environments. However, existing simulation studies have not addressed the permittivity effects of building materials on the mmWave band propagation channel because it is difficult to precisely model the overall building surfaces. In this paper, we investigate the variation of a propagation channel at non-line of sight (NLOS) points in urban micro-cell environments at the 28 GHz mmWave band using a three- dimensional (3D) ray tracing tool. The simulation results are compared with actual field measurement data. It is demonstrated that the path loss and channel impulse responses in the NLOS region fluctuate approximately 10-20 dB in urban environments owing to high-rise building blocks. Further, the impulse responses of the NLOS points indicate an increasing trend of the number of multipath components and the received signal strength as the permittivity increases. Therefore, the dielectric models of building materials must be considered for simulation analysis on mmWave channel properties such as path loss, mean excess delay, and delay spread.

In heterogeneous cellular networks, users are sometimes forcibly redirected into a low power base station (BS) for the purpose of data offloading. In order to guarantee an acceptable performance for such users, who suffer from severe inter-tier interference, enhanced inter-cell interference coordination (eICIC) was proposed. Using this technique, macro BSs periodically mute their data transmission by using an almost blank subframe (ABS). In this paper, we formulate a joint optimization problem incorporating user association and ABS portion tuning to increase network-wide utility. In the development of our algorithms, we particularly consider the time-varying characteristics of wireless propagation channels in order to reflect practical signal transmission environments, and we derive a throughput estimation equation that is compatible with the eICIC operation. Based on this analysis, we separately develop algorithms for user association and ABS tuning, and the performance enhancement achieved by our proposed methods is verified through extensive system-level simulations.

In this paper, radar cross section (RCS) measurement for human subjects and vehicles in 77 GHz automotive frequency modulated continuous wave (FMCW) radar system is presented. In this system, it is impossible to utilize conventional RCS definition due to high frequency band and the modulation technique used. Therefore, we introduce a new parameter called pseudo-RCS that can replace the conventional RCS. Then, we conduct actual experiment in the road to measure the RCS of the human subjects and the vehicles. In the measurement, data of four human subjects and four different kinds of vehicles are recorded with the 77 GHz FMCW radar. From the actual measured data, we find RCS distributions of the human subjects and the vehicles. For the human subjects, the RCS values are distributed following the Nakagami distribution. On the other hand, the log-normal distribution is well fit for the RCS values in the case of the vehicle.

Phase noise is known to be a major bottleneck in full- duplex wireless systems. Current self-interference cancellation schemes with phase noise mitigation assume the perfect estimation of the self-interference channel. However, self-interference channel estimation schemes in the presence of phase noise have not been reported. In this paper, we propose an EM-based joint estimation scheme for self-interference channels with common phase errors. For simplicity, only the strongest echo path is considered. Simulation results show that the proposed scheme outperforms the conventional ML- based self-interference channel estimation scheme.

Ultra-wideband (UWB) indoor channel characteristics which incorporate human body effects are analyzed. Channel impulse responses (CIRs) are measured in the frequency band of 3 GHz to 4.6 GHz in four different classrooms. For each measurement site, the proportion of occupied seats is changed to observe the effect of the number of people on the wireless propagation channel. First, path-loss exponents are calculated from the measurement data. After that, using Inverse Fast Fourier Transform (IFFT), CIRs are derived. Based on the CIRs, the comparison among the bit error rate performance, partial Rake receiver, and selective Rake receiver are investigated. The result shows that the human body effect including changes of population on UWB channel should be considered when the performances of UWB communication system are evaluated.

Small cells are considered an emerging technology for increasing the potential capacity of cellular networks. However, as the density of infrastructure increases, users have many choices for connection, and therefore, selecting an appropriate base station (BS) becomes an important issue. This study aims to provide an improved user association rule, where each user autonomously chooses the best among all the BSs in the vicinity, while considering their congestion levels. As the first step, an optimization problem is formulated, which emphasizes both the time-varying nature of the wireless channel and fairness among users. On the basis of this formulation, the influence of a specific handover event on system performance is investigated, and then, two versions of the handover frameworks are developed. Simulation results show that the proposed algorithms increase the throughput of every user in the network by 2.8–12% compared to the best conventional scheme. Furthermore, these schemes especially enhance the performance of users having low service quality by 5.0–85%, through efficient utilization of the pre-installed infrastructures.

Body area network considering the external areas of human body is a promising new application. In order to develop efficient body area network system, a communication channel modeling is essential. However, there are few models considering the effect of human body. This paper reports the empirical Ultra wideband(UWB) channel model for body area network. Using the frequency domain measurement system, channel responses in anechoic chamber have been obtained. From the measurement results, the path loss property has been configured to the propagation environments. Furthermore the frequency dependent UWB channel correlation characteristics are investigated.

In the recent cellular network environment, data traffic has increased rapidly by virtue of the development of a wide variety of wireless technology, such that it is hard for mobile service providers to satisfy the QoS (Quality of Service) requirement. In the real-world, users are not evenly distributed throughout cells; the base stations with multiple users have a great deal of difficulty of limited resources. In order to resolve this load imbalance problem, relay stations which can increase the transmission rates of cell boundary users by service area expansion have been an attractive technique in current networks. In addition, since users at the cell edge suffer from low throughput because of the inter-cell interference (ICI), we adopt additional scheme to mitigate this problem. In this paper, we propose an adaptive relay scheme which can improve the performance of the overall network by utilizing relay stations to distribute heavy traffic to adjacent cells. Each user calculates the network utility function with the neighboring relay stations and base stations, and each relay station also calculates the utility with adjacent cells. The central node controls the associations of users and relay stations through the utility information gathered from base stations. Simulation results show that our proposed algorithm improves the throughput and utility of the lower 5\% users without loss of the total network throughput.

In this paper, we propose an effective user association rule for resolving the unbalanced loading problem in multi-cell cellular networks. We first derive an improved user throughput estimation formula for the proportional fair (PF) scheduling algorithm under the Rayleigh fading environment. Our simple and accurate throughput estimation can be regarded as an extended version of the conventional scheme which is only valid when the data rate function is linear. Based on this result, we also develop a centralized user association framework, where a central node periodically determines the overall association of the system. The simulation results show that our load balancing algorithm achieves almost 90% of the upper limit of the system.

In this paper, a ray-tracing technique taking into consideration the effects of user-body shadowing is investigated. With the rapid growth in the applicability of mobile devices, accurate received signal predictions are increasingly important. The user's body especially has a great impact on the received signals, because it is nearly always located close to the mobile device. We consider two deterministic shadowing models based on anechoic chamber measurements and an existing diffraction theory. The both models are established as the power loss on a single propagation path depending on the user's postures and relative positions. A combined method of the user-shadowing models and multipath channel profiles obtained by ray tracing is then introduced and the results are validated by a comparison with in-building measurements.

The opportunistic spatial orthogonalization (OSO) scheme, proposed by Cong Shen and Michael P. Fitz, allows the existence of secondary users during the period in which the primary user is occupying all licensed bands. This paper introduces an active secondary user selection algorithm which mitigates the interference from the primary user transmitter to the secondary user receiver. In order to select the active secondary user, we adopt a fusion center concept, which is utilized for the cooperative spectrum sensing. A proposed algorithm guarantees the minimum average throughput of the primary user and overcomes the average sum throughput of a conventional OSO. We have numerically analyzed the average throughput under various constraints.

In this paper, shadow fading (SF) in an indoor multipath environment is measured and the spatial correlation of SF is modeled statistically. To effectively extract SF from the measured field strength, we use frequency averaging (FA) method instead of spatial averaging (SA) method which has generally been used. SF is modeled as a random variable with a log-normal distribution. The spatial correlation model for SF is expressed by an exponential decay function with convergence to non-zero value.

In this paper, we introduce a full-duplex relay (FDR) based on block diagonalization (BD) and analyze the corresponding sum rate in a multiantenna multiuser relay system. The poposed precoding scheme can suppress the self-interference as well as to support multiple users at the same time slot. Simulation results show that the proposed FDR scheme enhances significantly the sum rate performance compared to HDR.

The capacity of downlink multi-input multi-output (MIMO) systems is significantly limited by inter-cell co-channel interference (CCI). Improving the performance of cell edge users who suffers strong inter-cell CCI is still a crucial issue. In this paper, coordinated beamforming (CBF) algorithm for multicell multiuser MIMO system is proposed to cope with the presence of intel-cell CCI. In multicell environment with MIMO channel, limited base station (BS) cooperation is considered to design optimal transmit and receive beamformers which make interference-free at the receiver. Simulation results show that the proposed algorithms can improve the capacity of cell edge user effectively as well as the overall system capacity for multiuser multicell MIMO system.

This paper reports the variation of indoor radio channel caused by metal door. The simulation results using the finite difference time domain (FDTD) method and measurement results using the vector network analyzer in frequency domain are used for the characterization of received signal strength variation by metal door. Target frequency bands are three - sensor band, 802.11b ISM band, and 802.11a UNII band. From the simulation and measurement results, the effect of door angle to the received signal strength in three frequency bands and effect of radio frequency to variation are investigated. And FDTD simulation parameters for different environments are suggested.

This paper reports the empirical ultra wide band (UWB) channel model for short range outdoor environments. The measurements were performed based on the frequency sweep method in frequency domain at two different parking lot environments. From the measured channel transfer functions, parameters of log-distance path loss law and the effect of frequency to the path loss exponent are extracted. The distribution of received signal is analyzed to the existence of line-of-sight (LOS) path also. For the characterization of frequency dependent UWB channel parameters, standard deviation and correlation properties of channel gain in frequency domain are analyzed. The standard deviation increases with transmitter-receiver separation in LOS condition and the correlation is regressed to the double-slope model.

In this letter, we propose an iterative time and frequency synchronization scheme for orthogonal frequency-division multiple access (OFDMA) uplink system using the space-alternating generalized expectation-maximization (SAGE) algorithm. In the proposed scheme, the expectation step of the SAGE algorithm is intended to remove the multiple-access interference (MAI) caused by the other asynchronous users. Then, the maximization step is utilized in the timing and frequency offset estimation of each asynchronous user using the MAI-canceled signals. Simulation results present that the proposed scheme shows performance enhancement in the presence of multiple asynchronous users

In orthogonal frequency-division multiplexing (OFDM)-based wireless local area networks (WLANs), the phase noise (PHN) and a residual frequency offset (RFO) can cause the common phase error (CPE) and the inter-carrier interferences (ICI), which seriously degrade the performance of systems. In this paper, we propose a combined pilot symbol assisted and decision-directed scheme based on the least-squares (LS) and maximum-likelihood (ML) algorithms. In the proposed scheme, the CPE estimator is derived using the pilot symbols embedded in each OFDM symbol. Then, data symbols and channel response including the CPE are estimated using the CPE estimator. Simulation results present that the proposed scheme significantly improves the performance of OFDM-based WLANs.

This paper reports the empirical path loss model of ultra wide band (UWB) communications in office environments. The channel transfer functions of 46 transmitter-receiver location pairs are acquired using channel measurement system based on frequency sweep method. From the measured data, parameters of log-distance path loss formula are extracted considering propagation environments and existence of a line of sight path. The effect of frequency to the path loss properties of ultra wideband signal is presented also. Finally, statistical properties of received signal power with respect to the receiver conditions are analyzed

In this letter, we propose an iterative detection scheme in the presence of residual frequency offset (RFO) for orthogonal frequency-division multiplexing systems using an approximate application of the space-alternating generalized expectation-maximization (SAGE) algorithm. In the proposed scheme, the expectation step intends to divide the received signal into the desired signal and the interference signal. In the maximization step, the desired signal is used to estimate required parameters (i.e., RFO, data symbols, and channel state information) sequentially. Simulation results present that the proposed scheme shows almost ideal performance as long as the normalized RFO value is within 0.2.

The strongest ray analysis based on the computer simulation of wave propagation between a base station(BS) and mobile stations(MSs) is carried out to obtain the spatial-time wideband channel characteristics. The strongest rays are responsible for about 70% of the total received power for BS-MS pairs on the average and their arrival angles rarely deviate from the shortest path lines between a BS and MSs. However, this does not mean that the strongest ray is necessarily the first arrived ray for each BS-MS pair. The AOA statistics reveal that AOAs of the strongest and the second strongest rays are very close to each other for uplink channel, while those are quite apart for downlink channel.

ATM-UBR service responds to congestion by dropping cells when switch buffers become full. TCP connections running over UBR experience low throughput and high unfairness. For 100% TCP throughput, each switch needs buffers equal to the sum of the window sizes of all the TCP connections. Intelligent drop policies can improve the performance of TCP over UBR with limited buffers. The UBR+ service proposes enhancements to UBR for intelligent drop. The early packet discard scheme improves throughput but does not attempt to improve fairness. The selective packet drop scheme based on per-connection buffer occupancy improves fairness. The fair buffer allocation scheme further improves both throughput and fairness.

We extend our earlier studies of buffer requirements of TCP over ABR in two directions. First, we study the performance of TCP over ABR in an ATM backbone. We find that the TCP queues are at the edge router and not inside the ATM network. The edge router requires buffering equal to the sum of the receiver window sizes of the participating TCP connections. Second, we study the performance when ABR capacity is variable due to the effect of various patterns of VBR background traffic. The key factors in this study are the VBR traffic pattern, ABR feedback delays and the sensitivity of the ABR switch scheme to variance. We present our experiences in refining the ERICA+ switch scheme to handle these conditions.