The present paper proposes orthogonal variable spreading factor codes over finite fields for multi-rate communications. The proposed codes have layered structures that combine sequences generated by discrete Fourier transforms over finite fields, and have various code lengths. The design method for the proposed codes and examples of the codes are shown.

The network load is increasing due to the spread of content distribution services. Caching is recognized as a technique to reduce the peak network load by storing popular content into memories of users. Coded caching is a new caching approach based on a carefully designed content placement to create coded multicasting opportunities. Coded caching schemes in single-rate networks are evaluated by the tradeoff between the size of memory and that of delivered data. For considering the network with multiple transmission rates, it is crucial how to operate multicast. In multicast delivery, a sender must communicate to intended receivers at a rate that is available to all receivers. Multicast scheduling method of determining rates to deliver are evaluated by throughput and delay in multi-rate wireless networks. In this paper, we discuss coded caching in the multi-rate wireless networks. We newly define a measure for evaluating the coded caching scheme as coded caching delay and propose a new coded caching scheme. Also, we compare the proposed coded caching scheme with conventional coded caching schemes and show that the proposed scheme is suitable for multi-rate wireless networks.

Wireless Local Area Networks (WLANs) based on the IEEE 802.11 standard have been increasingly used. Access Points (APs) are being established in various public places, such as railway stations and airports, as well as private residences. Moreover, the rate of public WLAN services continues to increase. Throughput prediction of an AP in a multi-rate environment, i.e., predicting the amount of receipt data (including retransmission packets at an AP), is an important issue for wireless network design. Moreover, it is important to solve AP placement and selection problems. To realize the throughput prediction, we have proposed an AP throughput prediction method that considers terminal distribution. We compared the predicted throughput of the proposed method with a method that uses linear order computation and confirmed the performance of the proposed method, not by a network simulator but by the numerical computation. However, it is necessary to consider the impact of CSMA/CA in the MAC layer, because throughput is greatly influenced by frame collision. In this paper, we derive an effective transmission rate considering CSMA/CA and frame collision. We then compare the throughput obtained using the network simulator NS2 with a prediction value calculated by the proposed method. Simulation results show that the maximum relative error of the proposed method is approximately 6% and 15% for UDP and TCP, respectively, while that is approximately 17% and 21% in existing method.

This paper proposes the concept of adaptive multi-rate (AMR), which jointly employs switching between two links and adaptive rate on each link, for hybrid free-space optical/radio-frequency (FSO/RF) systems. Moreover, we present the cross-layer design of AMR switching, which is based on both the physical and link layers with an automatic-repeat request (ARQ) scheme. We develop an analytical framework based on a Markov chain model for system performance analysis. System performance metrics, including frame-error rate, goodput and link switching probability, are analytically studied over fading channels. Numerical results quantitatively show how the proposal significantly outperforms conventional ones with physical layer-based design and/or fixed-rate switching operation.

This paper presents a new generalized single threshold model that can be used in communications and cellular networks. In the proposed model, called Single Hysteresis Model (SHM), it is assumed that the amount of resources accessible for a new call of a given class can depend on two load areas of the system. The switching between areas is modulated by the two-state Markov chain which determines the average time the system spends in a particular load area, i.e. the area in which calls of selected classes with a reduced amount of resources (high load area) and with the initial amount of resources (low load area) are serviced. The results obtained for the discussed analytical model are compared with the results of the simulation of an exemplary WCDMA radio interface carrying a mixture of different multi-rate traffic streams. The research study confirms high accuracy of the proposed model.

In wireless networks, the mechanism to adaptively select a link transmission rate based on channel variations is referred to as RA (rate adaptation). The operation may have a critical impact on the upper-layer application, specifically video streaming which has strict QoS requirements. Thus, RA should consider the QoS requirements and radio conditions at the same time. In this paper, we present a CV-RA (cross-layer video-oriented rate adaptation) scheme for video transmission over multi-rate wireless networks. The transmission rate is switched in a cross-layer optimized way, by simultaneously considering video R-D (rate-distortion) characteristics as well as wireless conditions. At the radio link layer, transmission rate selection is made using cross-layer optimization. As a result of RA, the effective link throughput dynamically changes. At the application layer, video source rate is adaptively controlled using cross-layer adaptation. CV-RA is compared to three traditional RA schemes. It can realize the highest possible visual communications for any channel condition. For the previous schemes, the variations of visual quality is high due to dynamic packet error rates. In contrast, for CV-RA, visual quality improves with the channel condition.

This paper describes an efficient image enhancement method based on the Multi-Scale Retinex (MSR) approach for pre-processing of video applications. The processing amount is drastically reduced to 4 orders less than that of the original MSR, and 1 order less than the latest fast MSR method. For the efficient processing, our proposed method employs multi-stage and multi-rate filter processing which is constructed by a x-y separable and polyphase structure. In addition, the MSR association is effectively implemented during the above multi-stage processing. The method also modifies a weighting function for enhancement to improve color rendition of bright areas in an image. A variety of evaluation results show that the performance of our simplified method is similar to those of the original MSR, in terms of visual perception, contrast enhancement effects, and hue changes. Moreover, experimental results show that pre-processing of the proposed method contributes to clear foreground object separation.

Switching a communication path from one Access Point (AP) to another in inter-domain WLANs is a critical challenge for delay-sensitive applications such as Voice over IP (VoIP) because communication quality during handover (HO) is more likely to be deteriorated. To maintain VoIP quality during HO, we need to solve many problems. In particular, in bi-directional communication such as VoIP, an AP becomes a bottleneck with the increase of VoIP calls. As a result, packets queued in the AP buffer may experience a large queuing delay or packet losses due to increase in queue length or buffer overflow, thereby causing the degradation of VoIP quality for the Mobile Nodes (MNs) side. To avoid this degradation, MNs need to appropriately and autonomously execute HO in response to the change in wireless network condition, i.e., the deterioration of wireless link quality and the congestion state at the AP. In this paper, we propose an HO decision strategy considering frame retries, AP queue length, and transmission rate at an MN for maintaining VoIP quality during HO. Through simulation experiments, we then show that our proposed method can maintain VoIP quality during HO by properly detecting the wireless network condition.

In this paper, to realize the low delay and high throughput route discovery in multi-rate ad hoc networks, we propose a novel on-demand routing using signal strength, called signal strength aware routing (SSR). SSR is based on the on-demand routing with the route request (RREQ) and route reply (RREP) procedure. In SSR, a node measures the signal strength of a received RREQ, and calculate the appropriate data transmission rate. Nodes also calculate the standby time for the RREQ forwarding proportionally to the medium time at the data transmission rate. A RREQ through higher data rate links arrives at the destination earlier, and the destination can select a low delay and high throughput route easily. We evaluate the performance of SSR in terms of delay, throughput and route discovery delay by means of QualNet network simulator. As a result, we show that SSR can discover the lower delay and higher throughput route than the conventional shortest hop routing without increasing control overhead in multi-rate ad hoc networks.

We develop an enrichment protocol, called a 2-hop Path Selection Protocol (2PSP) for a set of nodes, in which data can be sent faster using adaptive rate control capability of IEEE 802.11a/b/g MAC protocol via a relaying concept than via a direct connection. The main objective of this protocol is to build upon opportunistic rate adaptation in order to assist a sender, a relay node, and a receiver to reach a higher rate data transmission through Medium Access Control (MAC) layer relaying. We propose a relay mechanism to further improve the performance of 2PSP protocol. In the relay mechanism, new contention window, called a Short Backoff Internal (SBI) is applied for a set of relay nodes. A potential node that succeeds as a relay is allowed to send a Ready-To-Relay (RTR) message. In this paper, two collision resolution algorithms are proposed to deal with the case when the potential relay node is more than one. Simulation results indicate that the proposed 2PSP protocol can achieve high reduction of delay and power consumption and also an improvement in the throughput compared to both Receiving-based Auto Rate (RBAR) and Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocols.

Multiple-access interference (MAI) limits the bit error rate (BER) performance of CDMA uplink transmission. In this paper, we propose a generalized chip-interleaved CDMA with 2-dimensional (2D) spreading using orthogonal variable spreading factor (OVSF) codes to minimize the MAI effects and achieve the maximum available time- and frequency-domain diversity gains. We present the code assignment for 2D spreading to provide users with flexible multi-rate data transmission. A computer simulation shows that by the joint use of 2D OVSF spreading and chip-interleaving, MAI-free transmission is possible for the quasi-synchronous DS- or MC-CDMA uplink, and hence the single-user frequency-domain equalization based on the MMSE criterion can be applied for signal detection. The BER performance in a time- and frequency-selective fading multiuser channel is theoretically analyzed and evaluated by both numerical computation and computer simulation.

In this paper, we propose a novel peak-to-average power ratio (PAPR) reduction method for multi-rate orthogonal frequency division multiplexing (OFCDM) based on an orthogonal variable spreading factor (OVSF) code. In the method, the base station assigns two signs of code sequence to each user to maintain the orthogonality among the code sequences that have different lengths. After that, one sign of the code sequence is selected for reducing the PAPR of the OFCDM symbol. Based on theoretical analysis and computer simulation, we show that the proposed method reduces the PAPR for two data rate classes.

As an enhancement of the state-of-the-art solutions, a high-throughput architecture of a decoder for structured LDPC codes is presented in this paper. Thanks to the peculiar code definition and to the envisaged architecture featuring memory paging, the decoder is very flexible, and the support of different code rates is achieved with no significant hardware overhead. A top-down design flow of a real decoder is reported, starting from the analysis of the system performance in finite-precision arithmetic, up to the VLSI implementation details of the elementary modules. The synthesis of the whole decoder on 0.18µm standard cells CMOS technology showed remarkable performances: small implementation loss (0.2dB down to BER = 10-8), low latency (less than 6.0µs), high useful throughput (up to 940Mbps) and low complexity (about 375 Kgates).

In this letter, adaptive multi-stage parallel interference cancellation (PIC) receiver is considered for multi-rate DS-CDMA system. In each stage of the adaptive multi-stage PIC receiver, multiple access interference (MAI) estimates are obtained by the sub-bit estimates from the previous stage and the adaptive weights for the sub-bit estimates. The adaptive weights are obtained by minimizing the mean squared error between the received signal and its estimate through normalized least mean square (LMS) algorithm. It is shown that the adaptive multi-stage PIC receiver achieves smaller BER than the matched filter receiver, multi-stage PIC receiver, and multi-stage partial PIC receiver for the multi-rate DS-CDMA system in a Rayleigh fading channel.

A scheme to evaluate the number of users and cell coverage of a WCDMA supporting multi-rate traffic is newly presented through calculation of the realizable Erlang capacity from a derived blocking probability and the path loss from the COST231 Walfisch-Ikegami (WI) model. Based on this analytical scheme, we evaluate the voice-data Erlang capacities at various data rates of 15 kbps to 480 kbps and the relationship between the cell coverage and the number of active users from them. When the value of Eb/Io is low from 4 dB to 3 dB under voice user capacity of 50 Erlang at 8 kbps, the result shows that the data user capacity is increased to 10 Erlang at low rate of 15 kbps and the cell coverage is enlarged to 100 m, and it is also shown that its capacity is increased to 0.2 Erlang at high rate of 480 kbps and its coverage to 50 m.

In this paper, we perform some experiments to show that the quantization noise caused by low-bit-rate speech coding can be characterized as a white noise process. Then, the signal-to-quantization noise ratio of the decoded speech for a given bit-rate is estimated by observing the perceptual speech quality equivalent to the artificially generated noisy speech obtained by adding a white Gaussian noise source. This information is incorporated into the parameter tuning of a noise-robust compensation algorithm for speech recognition so that the compensation algorithm can be performed better under a range of the estimated SNRs. Finally, we apply the compensation algorithm to a connected digit string recognition system that utilizes speech signals decoded by the GSM adaptive multi-rate (AMR) speech coder. It is shown that the noise-robust compensation algorithm reduces word error rates by 15% or more at low bit-rate modes of the AMR speech coder.

Multi-rate capabilities are supported by the physical layers of most 802.11 devices. To enhance the network throughput of MANETs, transfer rate adaptation schemes at MAC layer should employ the multi-rate capability at physical and the information of previous transmissions provided by MAC and physical layers. In this paper, we propose a transfer rate adaptation scheme plus back-to-back frame transmissions, and fragmentation at MAC layer, named TRAF. TRAF adopts a bi-direction-based approach with an extended option to select an appropriate rate for frame transmission under fast changing channel conditions. Consecutive back-to-back frame transmissions to fully utilize good channel quality during a coherent time interval and fragmentation algorithm to maintain high throughput under worse channel conditions are recommended in TRAF. Extensive simulation is experimented to evaluate the performance of TRAF. Regarding simulation results, frame delivery ratio, network throughput, and fairness of TRAF are significantly improved by comparing to that of fix rate, ARF, RBAR, OAR, and AAR protocols.

In this letter, we propose a groupwise successive interference cancellation (GSIC) receiver with gradient descent search for multi-rate DS-CDMA system. Proposed receiver incorporates iterative gradient descent search algorithm into conventional GSIC receiver for multi-rate DS-CDMA system. It is shown that the receiver achieves significant performance improvement over the matched filter (MF) receiver, GSIC receiver, multi-stage parallel interference cacnellation (PIC) receiver, multi-stage partial PIC receiver, and GSIC receiver with PIC in a Rayleigh fading channel.

In this paper, we propose a Distributed Request based CDMA Reservation ALOHA protocol to support multi-class services, such as voice, data, and videophone services, efficiently in multi-rate transmission cellular systems. The proposed protocol introduces a frame structure composed of an access slot and an transmission slot and an adaptive access permission probability based on the estimated number of contending users for each service, in order to control MAI by limiting the access to slots. It can provide voice service without the voice packet dropping probability through the proposed code assignment scheme, unlike other CDMA/PRMA protocols. The code reservation is allowed for voice and videophone services. The low-rate data service basically uses the remaining codes among the codes reserved for the voice service, but it can also use the codes already assigned to voice calls during the their silent periods to utilize codes efficiently. On the other hand, the high-rate data service uses the codes reserved for itself and the remaining codes among the codes reserved for the videophone service. Using the analytic method based on the Markov-chain subsystem model for each service including the handoff calls in uplink cellular systems, we show that the proposed protocol can guarantee the constant GoS for the handoff calls even with a large number of contending users through the proposed code assignment scheme and the access permission probability. Also, we show that the data services are integrated efficiently on the multi-rate transmission environment.

In this paper, two types of multi-stage partial parallel interference cancellation (PIC) receivers are considered for multi-rate DS-CDMA system: multi-stage PIC receiver with partial cancellation factors and multi-stage PIC receiver with decision thresholds. Bit error rate (BER) of the multi-stage partial PIC receivers is obtained by simulation in a Rayleigh fading channel. It is shown that the multi-stage partial PIC receivers achieve smaller BER than the matched filter (MF) receiver, multi-stage PIC receiver, group-wise successive interference cancellation (GSIC) receiver, and extended GSIC receiver (EGSIC) for the multi-rate DS-CDMA system in a Rayleigh fading channel.