相河 聡

近年，大電力機器用電磁遮へい材の要求が増している．本研究では，一面を開口面とした金属きょう体内に送信コイルを配置し，きょう体から漏洩する近傍磁界について検討した．また，電磁遮へい効果を向上できる穴あき金属板について検討した．

The fingerprint technique is used as an indoor localization method. This study uses a CNN-based indoor fingerprint localization method. The estimation accuracy of CNN improves as the number of AP information (AP identifiers and received signal strength indicator) increases. However, gathering AP information is time-consuming and costly. The problem can be solved using UD (AP information users measured). However, the UD measuring method does not know the user’s exact location. Therefore, it is essential to choose UD that is accurately estimate and use it for CNN training. In this study, we propose a method for selecting UDs that makes use of the RSSI similarity between AP information and UD.

In this study, we performed indoor location estimation using wireless LAN. The estimation method is based on the Finger Print method [1]. We measure the database (DB) and user data (UD) using wireless LAN radio waves to improve the location estimation accuracy of Finger Print indoor location estimation. The Neural Network (NN) that compares UD and DB is ResNet (Residual Network), which is a derivative of CNN (Convolutional Neural Network). The number of layers that provide the best accuracy varies depending on the environment. To confirm this, we experimentally verified the relationship between the number of layers and the estimation accuracy in different environments, and clarified the design method.

Today, we can know our outdoor location through global positioning system (GPS). However, it is not easy to estimate our indoor location because satellite signals are difficult to reach. Therefore, we study indoor localization using wireless local area network (LAN). In this work, we attempted to rectify our position estimated using a convolutional neural network (CNN). CNN of indoor localization is based on fingerprinting, and it can only estimate pre-measured coordinates. Simultaneously, CNN is not considered relation by time series. Thus, we suggest using filters like the Kalman filter or particle filter. We can estimate the pre-measured coordinates and inter-coordinates among them using filters. Additionally, we can improve the position estimation accuracy based on the temporal dependency of the user assuming a pedestrian. Our experimental validation shows that the proposed method improves the accuracy. We evaluated and enhanced the position estimation accuracy using CNN with the particle filter. Consequently, we obtained that the mean, median, and max errors decreased 0.27, 0.24, and 4.72 m, respectively, compared with only CNN.

This paper scopes a method that applies CNN to Fingerprint indoor localization. AP information are used to train the CNN. As the number of AP information with correct labels increases, the estimation accuracy by the CNN improves. However, it costs a lot to collect AP information with correct labels. UD (User Data) can be used to solve the problem. The UD is unlabeled data because the measuring method of the UD does not know the user’s place exactly. We perform semi-supervise assuming the estimation result of the UD as the correct label. However, the estimation result of the UD may be incorrect. Therefore, it is needed to select UD which is correctly estimated and use it for training the CNN. In this study, we propose a way to select UD by feature value using CNN feature extractor.

金属格子を誘電体で挟んだ構造は，ある帯域の電磁波のみを透過する空間フィルタとして動作することを確認している．本研究では，導電膜シートを格子状に配列したシートを用い，高誘電率材料と積層することで空間フィルタが構成できるか実験・理論の両面より検討した．

We are investigating the fingerprint method using wireless LAN access points (APs) to estimate user location. We focused on database update as a way to prevent the loss of localization accuracy. Updating a database using user-measured data presents the issue of update errors. In this paper, we have proposed and verified this issues.

電波吸収体は不要電磁波除去対策として様々な箇所で使用されている．特に，メタマテリアルを構成材料とする電波吸収体は従来型電波吸収体では実現できない画期的な特徴を有することが実証されている．本研究では，金属パターン周期配列構造と誘電体から構成される新たな電波吸収体を提案し，その構造パラメータを変化させることで整合条件を満たす電波吸収体を設計した．更に，本電波吸収体について，垂直入射時での評価に加え，実用を想定し斜入射時における電波吸収特性を実験的に評価した．

As the newly EMC materials, the authors have been proposed the bandpass space filters consisting the metal wire grid with negative relative permittivity and dielectric materials. In this study, the novel space filters using conductive film grid, instead of a metal wire grid, are proposed. Next, the transmission characteristics of the proposed space filters for a Wifi router in the 2.4GHz band or the 5.0GHz band are evaluated by the measurement in free space and the calculation of transmission line theory.

Currently, many researches are proceeded on location estimation using wireless LAN access points. In this research, we apply a particle filter, which is one of the time series filters, to examine the design method of its parameters and evaluate its accuracy. As a result of adjusting the parameters in the proposed method, the estimation error could be reduced. It was also confirmed that the proposed method can improve the accuracy more than the conventional method even if the DB coordinate interval increases.

In our laboratory, we study “area estimation” by Finger Printing for indoor commercial facilities and buildings. We study a method for updating DBs with user-acquired AP information. There is a problem of lowering the estimation accuracy due to the bias of the updated DBs. In order to solve this problem, we proposed a method to create a large distribution by mixing multiple databases.

Currently, research is being conducted on location estimation using wireless LAN access points. In this research, we apply a particle filter, which is one of the time series filters, to examine the design method of its parameters and evaluate its accuracy. As a result of adjusting the parameters in the proposed method, the estimation error could be reduced. It was also confirmed that the proposed method can improve the accuracy more than the conventional method even if the DB coordinate interval increases.

We are investigating the finger print method using wireless LAN access points (APs) to estimate user location. We focused on the APs coincidence rate (APCR) and database update as a way to prevent the loss of estimation accuracy. The APCR is a method to prevent estimation errors by narrowing down the candidate estimation positions. However, depending on the conditions, there is an issue that causes of no estimates. Updating a database using user-measured data presents the issue of update errors. In this paper, we have proposed and verified these two issues.

Recently, EM wave absorbers are frequently applied to prevent undesired EM waves. In our previous works, EM wave absorbers consisting of a artificially designed material and a ferrite rubber mixture have been proposed. As the newly materials, the authors propose the grid sheets consisting of the conductive material and the metal foil, and evaluate their equivalent relative permittivities. Next, EM wave absorbers consisting of a proposed sheet and a permalloy composite material are designed, and their reflection characteristics are experimentally evaluated.

This contribution describes a fingerprint position estimation using RSSI (Received Signal Strength Indicator) of wireless LAN Access Points (APs) that employs Neural Network (NN). We propose position estimation using the weighted intermediate point to estimate the exact position. In this method, RSSI of UD (User Data) is collated with the RSSI in DB (data base). RSSI of DB was measured on coordinates in advance. Therefore, one of the coordinates is selected as the estimation result. The coordinate closest to correct point is selected in case of estimation at the intermediate point of the coordinates. In this study, we propose to estimate the intermediate point using weighting with the existence probability distribution of limited coordinate output from NN. The accuracy of proposed position estimation method was verified using three-layer NN based on measured data.

屋内位置推定アプリケーションは，商業施設などで需要は高い．現在，屋内商業施設には，多数の無線LANアクセスポイント(AP)が設置されている．また，ネットワークに接続していなくともAPからの受信電波強度(RSSI)を観測することができる．本論文では，APが多く普及している商業施設における無線LANによる屋内位置推定を検討する．位置推定対象となるスマートフォンを信号取得端末（ユーザ端末）とし，APからのRSSIを取得することにより位置推定を行う．導入対象となる商業施設エリアは，シャドーイングやマルチパスフェージングの影響を無視できない．また，APは各店舗が自由に設置しているため，設置場所も既知ではない．Laterationのように，RSSIからの距離を求める方式では遮蔽物による減衰が影響する．このため，商業施設ではFingerPrintが有効である．求められる位置推定精度は，1店舗の大きさである4〜10mとなる．商業施設内でデータ測定を行った後，その場ですぐに位置推定可能となることが求められている．このためデータベース作成，データ加工に時間をかけることができない．これに対して，ベイズ推定を用いたアルゴリズムでは，データベース作成に時間を要する．商業施設でデータベースを作成する際，観測されたAPの電波分布はガウス分布に従うことが知られている．これを利用し少ない観測データからデータベースを補間することができる．本論文では，このガウス分布をAPごとの信頼度と定義して位置推定を行うAP信頼度合算法を提案した．一方，屋内位置推定を行う際に，RSSIが小さく観測できる回数の少ないAPが原因で誤った位置に推定されることがある．これらに対して本論文では，ポアソン分布を利用して，観測回数に基づいてAP信頼度に重みづけすることで位置推定精度を向上する手法を提案する．以上に述べた低コストで実現できる位置推定に関する二つの提案手法を商業施設における実験により検証し，データベース作成に必要となる時間の削減と精度向上を確認した．

屋内・屋外問わず常に高精度な位置推定を行うには，位置推定を行う人物の位置に応じて位置推定手法を切り替える必要がある．本論文ではディープラーニングを利用することで，無線LANアクセスポイントからの電波による位置推定の精度を予測する手法を提案する．

This paper study to realize navigation system with Localization using GPS (Global Positioning System) and wireless LAN-Finger printing. GPS is effective in outdoor and wireless LAN is effective in indoor. So, in this study, examine evaluate high accuracy position estimation by switching GPS and wireless LAN.

Communication using smartphone devices connected to the cellular network in a bullet train is increasing and there is currently a limited capacity in the train-unit entrance circuit. In the Sequentially Switched Antenna Array Receiver (SSAAR), the array antenna fixes the receiving point by sequentially switching the elements against the direction of travel. The authors propose a multiple antenna combining scheme that satisfies the requirements for the sequentially switching, and show its effectiveness by computer simulations using experimental data.

Mobile traffic from many smartphone devices to base stations is rapidly increasing. In a case many users communicate in moving vehicles such as a bullet train, it is desirable to multiplex their traffic and relay it to a base station located along the railroad line for improving spectral efficiency. However, high speed channel variation occurs on the entrance circuit of a bullet train. A sequentially switched antenna array receiver is known as a promising technology for compensating such channel variations. This technology slows down the channel variation by switching the antenna element in the opposite direction of the train travel in order to practically fix the receiving point. When designing such a receiver, spatial intervals between the antenna elements and temporal switching intervals need to be considered. In this paper, we propose a multiple antenna combining scheme to satisfy the requirements for the sequentially switching, and show its effectiveness via computer simulations.

Anterior cruciate ligament (ACL) injury causes knee joint instability, and affects on sports performance. Therefore, ACL reconstruction is essential to keep their performance high and to prevent osteoarthrosis. It is well known that the outcome of ACL reconstruction is strongly related to the placement and orientation of the bone tunnel. 2-D X-ray radiograph and CT images have been used to evaluate the placement and orientation of the bone tunnel. Quadrant method evaluates the bone tunnel placement based on the Blumensaat's line which has high intensity on 2-D X-ray lateral radiograph. There is problem of invasiveness using X-ray radiograph or CT image. Therefore, we have proposed an MR image based computer-aided surgical planning of ACL reconstruction. The system evaluates the bone tunnel placement and orientation based on Quadrant method. The remained problem of our system is Blumensaat's line is manually determined. This paper proposes that a method to synthesize the pseudo lateral radiograph from MR images, and extract the Blumensaat's line on the synthesized pseudo lateral radiograph. The experimental results showed that the proposed method successfully determined the Blumensaat's line on the pseudo lateral radiograph.

This paper proposes a new synchronization scheme for packet mode orthogonal frequency division multiplexing (OFDM) signals employing space division multiplexing (SDM) it targets IEEE 802.11 TGn wireless LANs that offer over 100 Mbit/s. In addition, this paper shows the packet format of OFDM-SDM signals that ensures backward compatibility with IEEE 802.11a. The proposed synchronization scheme has simple open-loop construction and consists of automatic frequency control (AFC), symbol timing detection, channel estimation and phase tracking. The AFC and symbol timing detection are carried out in the time-domain. After OFDM demodulation, the proposed scheme processes channel estimation and phase tracking in the frequency-domain. Considering all the above synchronization tasks, we evaluate the packet error rate (PER) performance using the TGn-defined multi-input multi-output (MIMO) channel model. The proposed scheme shows superior performance it suppress the required Eb/N0 degradation to within 0.4 dB (0.3 dB) with 64 byte (1000 byte) packets compared to the performance achieved if only channel estimation is considered the RMS delay spread = 50ns.

In recent years, personal navigation attracts attention due to spreading of smart phones. The navigation widely used for the car until now. The car navigation displays its position on a map using GPS and driving distance. A personal navigation with a smart phone can expect business for sightseeing guidance to a stranger, or providing O2O (Online to Offline) service in a shopping mall. Personal navigation systems have been used until now, however, some problems are left behind in order to further large spread. This contribute will show some countermeasures.

自律移動ロボットが位置推定に無線LANの受信レベルを用いる場合,周辺環境により各位置での推定精度が異なる.一方,移動ロボットは複数箇所で位置推定を行うことができる.このことから,各点での位置推定結果に移動履歴を加えて得られる現在位置を複数平均する到達精度向上手法を提案する.具体的には,位置推定履歴と移動履歴から複数の推定現在地を算出し平均することで特異点を平坦化させる.更に自律移動ロボットに提案法を搭載し,屋内において実際に移動実験を実施することで提案手法の有効性の検証を行った.

近年,モバイル無線LANルータに代表される移動性のある無線LANネットワークが広まりつつある.無線LANのAPとなるルータ自体に移動性があるため,同じチャネルを選ぶ端末の接近が避けられず,スループット低下を引き起こす.本論文ではモバイル無線LANルータの動的なチャネル変更を提案し,シミュレーションにより手法の評価を行う.

電波伝搬路特性を用いた秘密鍵生成に関する研究が行われている.この手法では鍵ビットの一致率を向上させるためにしきい値付近削除処理が必要となるが,そのために鍵長以上のRSSIを取得する必要がある.我々は使用するアンテナパターンを限定することで,しきい値付近削除処理量を減らし,鍵一致率が向上する手法を提案し,評価を行う.

Recently, secret key generation schemes for wireless communication systems using propagation performance are researched. RSSI, signal strength indications received at AP (Access Point) and UT (User Terminal) are same value in TDD systems because of reciprocity theorem, and are controlled by the array antenna pattern. However, RSSI at TP (Tapping Point) is not same as RSSI at AP or UT that depend on their location. The secret keys are generated from binarized RSSIs. This paper describes a method for improving the probability of key agreement between AT and UT by using array antennas control. © 2011 IEEE.

車車間通信では交差点などでの事故防止のための通信と走行車線で渋滞情報などを送受する通信がある.車両間の位置関係から最適なアンテナ配置はそれぞれの場合で異なる.本論文では,いずれの場合でも通信が可能なアンテナ配置方法及びダイバーシチ方式を提案する.

The wireless systems that establish multiple input multiple output (MIMO) channels through multiple antennas at both ends of the communication link, have been proved to have tremendous potential to linearly lift the capacity of conventional scalar channel. In this paper, we present two efficient decision feedback equalization algorithms that achieve optimal and suboptimal detection order in MIMO spatial multiplexing systems. The new algorithms combine the recursive matrix inversion and ordered QR decomposition approaches, which are developed for nulling cancellation interaface Bell Labs layered space time (BLAST) and back substitution interface BLAST. As a result, new algorithms achieve total reduced complexities in frame based transmission with various payload lengths compared with the earlier methods. In addition, they enable shorter detection delay by carrying out a fast hybrid preprocessing. Moreover, the operation precision insensitivity of order optimization greatly relaxes the word length of matrix inversion, which is the most computational intensive part within the MIMO detection task.

In rich scattering environments, multiple antenna systems designed to accomplish spatial multiplexing have enormous potential of lifting the capacity of corresponding multiple input multiple output channels. In this paper, we present a new low complexity algorithm for decision feedback equalization detector in the SM scheme. The basic idea is to reduce the joint optimization problem to separate optimization problems to achieve better performance-complexity tradeoffs. Concretely, we separately optimize the detection order and the detector filters so that the complexity of the entire signal detection task is reduced. The new order search rule approximates the optimal Bell Labs layered space time (BLAST) approach from a geometrical perspective, and the detector filters are derived using a Cholesky based QR decomposition. The new algorithm is able to switch from zero forcing to minimum mean square error without additional operations and the computational effort is a small fraction of that in the optimal BLAST algorithm. Despite its low complexity, the error performance of new detector closely approximates that of the standard BLAST.

Recent theoretical and experimental studies indicate that spatial multiplexing (SM) systems have enormous potential for increasing the capacity of corresponding multiple input multiple output (MIMO) channels in rich scattering environments. In this paper, we propose a new recursion based algorithm for Bell Labs layered space time (BLAST) signal detection in SM systems. The new algorithm uses an inflated recursion in the initialization and a deflated recursion in the iteration stage: as a result, the complexity is greatly reduced and the irregularity issues are completely avoided. Compared with the conventional fastest recursive approach, the complexity of our proposal is lower by a factor of 2 and it is also very implementation friendly.

This paper proposes a simple and feasible decision-feedback channel tracking scheme for multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems designed for wireless local area networks (LANs). In the proposed scheme, the channel state matrix for each subcarrier is tentatively estimated from a replica matrix of the transmitted signals. The estimated channel matrices, each derived at a different timing, are combined, and the previously estimated channel matrices are replaced with the latest ones. Unlike conventional channel tracking schemes based on a Kalman filter, the proposed scheme needs no statistical information about a MIMO channel, which makes the receiver structure quite simple. The packet error rate (PER) performances for the proposed scheme are evaluated on computer simulations. When there are three transmit and receive antennas, the subcarrier modulation scheme is 64 QAM, and the coding rate is 3/4, the proposed scheme keeps the SNR degradation at PER of 1e-2 less than 0.1 dB when the velocity of receiver is 3 km/h in an indoor office environment at 5 GHz band. In addition, compared to the conventional channel tracking scheme based on known pilot symbols, the proposed scheme improves throughput performance by 13.8% because it does not need pilot symbols. These results demonstrate that the proposed channel tracking scheme is simple and feasible for implementation in MIMO-OFDM systems based on wireless LANs.

This paper proposes a new multi-task synchronization scheme for packet mode orthogonal frequency division multiplexing (OFDM) signals in multi-input multi-output (MIMO) transmission systems; it targets high-rate wireless LANs that offer over 100Mbit/s. In addition, this paper introduces a packet format for MIMO-OFDM signals that ensures backward compatibility with IEEE 802.11a. The proposed synchronization scheme has simple open-loop construction and consists of automatic frequency control (AFC), symbol timing detection, MIMO channel estimation, and phase tracking. AFC and symbol timing detection are carried out in the time-domain. After OFDM demodulation, the proposed scheme performs MIMO channel estimation and phase tracking in the frequency-domain. Considering all of the above synchronization tasks, we evaluate the packet error rate (PER) performance using the IEEE 802.11 TGn-defined channel model-D and model-E. In channel model-D with the RMS delay spread = 50ns, the proposed scheme shows superior performance; it suppress the required E-b/N-0 degradation to within 0.4 dB with 1000 byte packets compared to the performance achieved if only MIMO channel estimation is considered. Moreover, in channel model-E with the RMS delay spread = 100 ns, it is found that the proposed scheme degrades the required E-b/N-0 only by approximately 1.5 dB compared to the MIMO channel estimation only case, even if the packet length is 1000 bytes with 64QAM and coding-rate = 7/8.

The mesh topology based on the standard IEEE 802.11 for wireless LANs (WLANs) appears to be a very promising architecture on the way to realizing an ubiquitous high-speed wireless Internet access in the future. However, the current IEEE 802.11 protocol is aimed at single Access Point (AP) environments and many problems related to the wireless meshed interconnection of APs and Mobile Terminals (MTs) remain unsolved. Some proposed solutions to build such mesh architectures are based on ad-hoc-oriented single-channel schemes that modify IEEE 802.11 protocol. The main problem with this type of schemes, however, lies in the very low performance of the single-channel architecture itself when the network becomes larger or the offered traffic load increases. The task group IEEE 802.11s is currently discussing and working out a standard for IEEE 802.11 -compliant mesh architectures for various usage scenarios including residential, office and campus/community/public access network but much work is ahead since the group was recently established. In this paper we propose a new multi-radio multi-channel mesh architecture for WLAN hot spots, which works using a Distributed Coordination Function (DCF)-based technique for interconnecting APs, and also a radio-aware packet forwarding scheme among APs. A major advantage of the system is that, putting routing issues aside, it introduces no changes into the MAC protocol of IEEE 802.11. The simulation results obtained in OPNET v. 10.0 show the great potential of our mesh architecture to support real-time traffic with any packet size, and the effectiveness of the radio-aware forwarding scheme in improving the delay performance of the mesh network.

This paper describes the experimental performance of eigenbeam multi-input multi-output with orthogonal frequency division multiplexing (MIMO-OFDM) systems as measured in a testbed implemented with field programmable gate arrays (FPGAs). The FPGA-testbed, characterized by the software-defined radio (SDR) technique, offers 1/5-scale real time signal processing. Extensive experiments on the testbed confirm the basic operation and performance of eigenbeam, MIMO-OFDM with quadrature phase-shift keying (QPSK) and 16 quadrature amplitude modulation (QAM). From the packet error rate (PER) performance, we confirm that the eigenbeam 16QAM/MIMO-OFDM scheme with permutation matrix and three transmit antennas (M-t = 3) drastically improves the required carrier-to-noise power ratio (CNR) by approximately 5.6 dB over the scheme without eigenbeam with M-t = 2. Furthermore, to determine the impact of Doppler frequency f(d), we focus on the transmission interval between the MIMO channel estimation and data transmission. To suppress the required CNR degradation to within 1.5 dB, it is found that the eigenbeam 16QAM/MIMO-OFDM scheme with permutation matrix and M-t = 3 permits a transmission interval of approximately 68.5 ms when f(d) = 1 Hz for a 1/5-scale model.

The gateway access point (AP) in a wireless mesh network becomes the natural bottleneck node around which all the traffic relayed by APs that is exchanged among the terminals and the Internet tend to concentrate. So far most of the practical deployments of mesh wireless local area networks (WLANs) focused on public safety and public access have taken place in rural or suburban areas where the low density of users and the low data-rate,applications in use do not impose stringent traffic conditions, making the conventional single-radio DCF-based system defined by IEEE 802.11 a feasible implementation option. However, under relatively high traffic-load conditions, the large number of packet collisions produced by the accumulation of traffic in the vicinity of gateway APs may greatly reduce the overall network throughput and largely increase the delay, especially in case of packets that traverse several hops, thus affecting real-time applications like voice over IP (VoIP). To cope with this problem a polling mechanism compliant with the IEEE 802.11 e hybrid-coordination-function controlled channel access (HCCA) which operates in a single network interface card (NIC) in the vicinity of gateway APs has been proposed in this paper. The polling scheme is complemented with a Distributed Coordination Function (DCF) channel access that also operates in the vicinity of gateway APs in a different NIC and on a different channel. The HCCA NIC allows any gateway AP to exchange data frames with its surrounding APs in a scheduled and bidirectional way while the DCF NIC provides gateway APs a contention-based way to receive data frames from their respective surrounding APs. Computer simulations carried out in OPNET version 10.0 to evaluate the combination of both contention-based and contention free access schemes in the area surrounding gateway APs show that the proposed mechanism can largely increase the total throughput while providing low transmission delay. As no changes to the IEEE 802.11 related protocols are required, the proposed scheme represents an attractive option to implement a mesh WLAN.

This paper presents a reduced-complexity maximum likelihood detection (MLD) scheme for orthogonal frequency division multiplexing with space division multiplexing (OFDM-SDM) systems. Original MLD is known to be an optimal scheme for detecting the spatially multiplexed signals. However, MLD suffers from an exponentially computational complexity because it involves an exhaustive search for the optimal result. In this paper, we propose a novel detection scheme, which drastically reduce the complexity of MLD while keeping performance losses small. The proposed scheme decouples the spatially multiplexed signals in two stages. In stage one, the estimated symbols obtained from zero-forcing (ZF) are used to limit the candidate symbol vectors. In stage two, to form a final estimate of the transmitted symbol vector, the Euclidean or original defined likelihood metric is examined over all symbol vectors obtained from stage 1. Both the bit error rate (BER) and packet error rate (PER) performances are evaluated over a temporally and spatially uncorrelated frequency selective channel through the computer simulations. For a four-transmit and four-receive OFDM-SDM system transmitting data at 144 Mbit/s and 216 Mbit/ss i.e., employing 16 Quadrature Amplitude Modulation (16QAM) and 64QAM subcarrier modulation over 16.6 MHz bandwidth channel, the degradation in required SNR from MLD for PER = 1% are about 0.6 dB and 1.5 dB, respectively. However, the complexity of MLD is reduced to 0.51000% and 0.01562%.

This paper proposes a new effective data transfer method for IEEE 802.11 wireless LANs [1] by integrating priority control and multirate mechanism. The IEEE 802.11 PHY layer supports a multirate mechanism with dynamic rate switching and an appropriate data rate is selected in transmitting a frame. However, the multirate mechanism is used with the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) protocol, low rate transmissions need much longer time than high rate transmissions to finish sending a frame. As a result, the system capacity is decreased. The proposed method assumes the same number of priority levels as the data rates, and a data rate is associated to a priority level. Priority of a transmission goes up with the used data rate. For this purpose, we have modified the CSMA/CA protocol to support prioritized transmission. By selecting the appropriate priority depending on the data rate and giving more transmission opportunities for high rate transmission, the system capacity is increased. The effect of the proposed mechanism is confirmed by computer simulations.