Management of the Power Control Operation in HIPERLAN/2 Networks

Soft Computing j (2004) 1 – 15 DOI 10.1007/s00500-003-0354-3

FOCUS

A. Oikonomou P. Demestichas K. Tsagkaris G. Koundourakis M. Theologou Management of the power control operation in HIPERLAN/2 networks

Published online: jjj Springer-Verlag 2003

Abstract BRAN/WLAN systems, e.g., HIPERLAN/2, circuit-switched voice services. The migration of second IEEE 802.11a, etc., are seen as a promising solution for generation (2G) towards the 2.5G era [1] and the covering residential, business, transport, tourism, etc., development of third generation (3G) mobile wireless environments, and generally areas of high demand, systems aim to enable networks to provide users with characterized as ‘‘hot spots’’. This paper presents man- instantaneous bit rates of up to 2 Mbit/s, significantly agement functionality for augmenting the potential improving packet-data transmission and mobile multi- exploitation of one of these systems, HIPERLAN/2, by media applications. This will be materialized through making feasible their (loose or tight) integration in a the gradual introduction of the Universal Mobile Tele- composite radio (CR) context. The approach will be the communications System (UMTS) [2, 3, 4]. following. The first point will be to revisit the main In addition, even higher data rates can be obtained features of a HIPERLAN/2 system. Next, the architec- for local area networks using novel short-range wire- ture of a general Service and Network Management less technologies. Bandwidth demanding, real-time and System (SNMS), which has been developed for assisting interactive multimedia services, such as high-quality vi- wireless systems in their operation in a CR context, will deo distribution, client-server multimedia applications, be briefly presented. The next main point will be the and data-bank access, are typical applications for this presentation of the functionality of the SNMS compo- technology. Therefore, new wireless networks with nent that is tailored to the managed HIPERLAN/2 broadband capabilities are being sought to provide high- technology and specifically to the configuration of the speed integrated services (data, voice, and video) with Power Control (PC) functionality. An algorithm for cost-effective support for Quality of Service (QoS). This configuring the PC operation, based on a greedy algo- leads to the introduction of systems collectively called rithm and a neural network, will be presented. A rele- Broadband Radio Access Networks (BRANs) and/or vant resource management problem, which should be Wireless Local Area Networks (WLANs). efficiently solved for exploiting HIPERLAN/2 networks, A particular class of such systems, operating at the will be addressed. Numerical results will be presented. 5 GHz band, promises to offer high data rates at adequate capacity volumes, for short-range communica- Keywords HIPERLAN/2 IEEE 802.11a tions with limited mobility. Therefore, they are seen as a IEEE 802.11h Power control promising solution for covering residential (home), corporate (business, office, etc.), transport (e.g., airport, train, etc.), and other environments, which are often 1 Introduction characterized as hot-spot areas. The class includes the IEEE 802.11a and 802.11h systems, the High Perfor- Present day wireless telecommunications networks, mance Radio LAN Type 2 (HIPERLAN/2) [5–11], which are primarily narrowband, are mostly used for specified by the European Telecommunications Stan- dards Institute (ETSI), and Japan’s High Speed Wireless Access Network (HiSWAN). The spectrum allocation in A. Oikonomou(&) the three systems is presented in Fig. 1 National Technical University of Athens, Moreover, a recent trend, often called ‘‘wireless be- Electrical and Computer Engineering Department, yond 3G’’, assumes that cellular, BRAN/WLAN and Telecommunications Laboratory, 9 Heroon Polytechneiou Street, DVB (Digital Video Broadcast) systems can be co- Zographou 15773, Athens, Greece operating systems in a composite radio (CR) infra- E-mail: [email protected] structure [12–14]. According to the CR concept, a

50000354 Dispatch: 12.1.2004 Journal : Soft Computing No. of pages: 15 Journal number Manuscript number B Author’s disk received 4 Used 4 Corrupted Mismatch Keyed 2

Fig. 1 Current spectrum allocation at the 5 GHz band

network provider (NP) can rely on diverse radio tech- will pose various requirements. The presented manage- nologies for efficiently covering service area regions. ment functionality aims at the fast adaptation of the This may mean that the NP either possesses licenses for system to the new requirements. deploying and operating diverse radio systems (tight The control actions in a HIPERLAN/2 network are integration between the radio technologies), or cooper- mainly Link Adaptation, Dynamic Frequency Selection ates with other NPs that own alternate radio networks (DFS), and Power Control (PC). The quality of the (loose integration between the radio technologies). Effi- radio link, which is dependent on the radio environment, cient coverage means offering as high as possible (re- changes over time and in accordance with traffic in quired) Quality of Service (QoS) levels, at adequate surrounding radio cells. To cope with variations, a Link capacity volumes, in a cost-effective manner. Adaptation scheme is applied. In addition, the DFS A typical CR scenario will include 2.5G/3G mobile operation allows several operators to share the available networks, BRAN/WLAN and DVB systems. In this spectrum and avoids the use of interfered frequencies. context, BRAN/WLAN systems should properly man- Frequency selection is based on interference measure- age their resources, so as to have capacity available, ments performed by the access point and associated which can be used for cooperating with other networks mobile terminals. The functionality of this paper is in the of the infrastructure. The cooperation is materialized direction of managing the PC operation, in order to through the agreement of absorbing traffic from other ensure that the traffic handled by the network is served networks of the CR infrastructure, in order to assist in a most efficient way, reducing, nevertheless, the gen- them to the handling of new service area conditions (e.g., erated interference as much as possible. hot-spot situations, traffic demand alterations, etc.), or The reference system for this paper will be service management requests. Achieving this operation, HIPERLAN/2, however, our practices are applicable to however, requires upgraded service and network man- the IEEE 802.11a, IEEE 802.11h and HiSWAN systems agement systems (SNMSs). This paper will present that have similar specifications. Our approach in this (essential parts of ) such an SNMS. It will be assumed paper is the following. Section 2 revisits the basic fea- that a BRAN/WLAN network, operated by an arbitrary tures of the HIPERLAN/2 system. Section 3 briefly NP, is covering a given area. The proposed management presents a management system that enables HIPER- functionality extends the exploitation possibilities (and LAN/2 networks to act as parts of a CR environment. therefore, the chances of success) of the BRAN/WLAN Section 4 presents the management functionality that network, by enabling its (loose) integration in an overall will configure the PC operation [15, 16]. It will be based CR infrastructure, which comprises also other NPs that on a greedy algorithm (Sect. 4.4) and a neural network operate various types of networks. (Sect. 4.5). Sections 5 and 6 include sample numerical It should be noted that BRAN/WLAN systems pos- results and concluding remarks. sess a ‘‘self-sufficient’’ mode of operating, in the sense, that they are adequately dynamic (autonomous) for adapting to the environment conditions. This is mainly 2 HIPERLAN/2 Overview motivated by the fact that these systems will operate in a license-exempt spectrum band. Therefore, under these This section revisits the basic features of the HIPER- conditions, the introduction of management function- LAN/2 system [5–11]. ality can be essential. The reason is the provision of Figure 2a depicts the reference architecture of statistical guarantees regarding performance and QoS, HIPERLAN/2 networks. Each Access Point (AP) con- towards the CR infrastructure. The CR infrastructure trols a cell. It offers wireless connectivity to the mobile 3

Fig. 2 a Reference architecture of HIPERLAN/2 networks. b Figure 2b presents, in a high level manner, the func- Functionality (protocol stacks) of an AP. c Structure of the medium tionality (protocol stacks) of an AP. The convergence access control (MAC) frame layer realizes the mapping between the core network protocols and the lower layers of the HIPERLAN/2 terminals (MTs) of the cell, and is, therefore, the inter- system. The physical layer uses Orthogonal Frequency face between the radio and the fixed network. Typically, Division Multiplexing (OFDM). The carrier spacing is a HIPERLAN/2 system will comprise a fixed network 20 MHz, which means that in Europe there can be 19 segment that enables the interworking with core net- carriers available (most likely, 12 will be for outdoor/ works, as well as the communication between APs. indoor use, and 7 only for indoor use). Each carrier is 4 split into 52 sub-carriers (48 are used for data and 4 are physical layer mode that can be selected, and conse- pilots). Table 1 lists the seven physical layer modes quently, the throughput that can be achieved. The provided regarding the sub-carrier modulation and appropriate physical layer mode is selected through the code-rate schemes, and the resulting bit-rates. The link adaptation operation. The transmitter power con- modulation schemes supported are binary phase shift trol operation is a means for improving the CIR levels. keying (BPSK), quaternary PSK (QPSK), 16-quadrature Section 4 presents management functionality for man- amplitude modulation (16-QAM), and 64-QAM. The aging (properly configuring) the power control proce- code-rates are 1/2, 3/4, and 9/16. dure, and therefore improving the CIR. The medium access control (MAC) is based on a time division duplex (TDD) and time division multiple access (TDMA) scheme, controlled by the AP. Figure 2(c) 3 Service and network management system depicts the structure of the MAC frame. It has a fixed duration of 2 ms and consists of several phases. The This section briefly presents the SNMS that enables a broadcast phase contains control information. The HIPERLAN/2 network to be loosely integrated in a CR frame channel phase describes the (allocation of re- infrastructure (Fig. 3). The detailed description of the sources in the) current MAC frame. The access feedback platform can be found in [14]. channel phase contains information on previous random The system is composed by three entities: (i) Moni- access attempts. The uplink and downlink phases con- toring, Service management interworking and Resource tain data from/to MTs. The direct link phase contains Brokerage (MSRB); (ii) Resource Management Strate- information exchanged between MTs, without the direct gies (RMS); Network and Environment Simulator involvement of the AP, according to the ad-hoc network (NES). paradigm. The random access phase is used by MTs for Firstly, the MSRB entity [17], identifies new service establishing associations with APs, requesting resources, area conditions (e.g., new traffic demand patterns), and once associated with an AP, and finally, conducting accepts and responds to service management requests. handovers. Through these capabilities the SNMS is capable of act- The data link control (DLC) layer is divided into a ing in a reactive (to new service area conditions) and control and a user plane part. The control part supports proactive (accepting and satisfying service management a number of procedures: (i) association control, i.e., requests) mode. The resource broker capability is im- association establishment, authentication, encryption posed by the CR concept. It enables the generation, and key exchange, disassociation, etc.; (ii) DLC control, e.g., negotiation on, a set of offers. References [18–22] are connection set-up/release/modify, multicast join/leave, some samples of background material, on which this etc.; (iii) radio resource control, i.e., link adaptation, component can be based. power control, dynamic frequency selection (DFS) and Whenever one of the triggers above emerges (new handover. service area condition, service management request, or The DFS operation, which enables an AP to auto- resource brokerage request) the MSRB initiates the matically select its frequency, based on interference RMS operation. The RMS component finds cost-effec- measurements, will not be further analyzed in this paper. tive reconfigurations of the managed network that The link adaptation operation enables the AP to select achieve the required capacity figures. Using the input the highest possible physical layer mode (Table 1), for from the MSRB component certain management actions both the uplink and downlink, based on radio link are taken, to ensure that the status of the network re- quality measurements, and mainly from the Carrier to mains adequate. These actions, as already stated, can be Interference Ratio (CIR), conducted by both the MT the proper configuration of the operations in the control and the AP. The power control operation decreases the interference caused in other cells, or other systems, in the same band. The CIR is one of the important resources of the system. The higher the CIR in a cell, the higher the

Table 1 Physical layer modes for HIPERLAN/2

Physical layer Modulation Code rate Physical layer mode bit rate (Mbps)

1 BPSK 1/2 6 2 BPSK 3/4 9 3 QPSK 1/2 12 4 QPSK 3/4 18 5 16 QAM 9/16 27 6 16 QAM 3/4 36 7 64 QAM 3/4 54 Fig. 3 Deployment of the management system for the HIPERLAN/2 network 5 domain. The current paper investigates the configura- component managing the power control procedure can tion of the power control operation. assist to the achievement of the target CIR levels by The Network Environment Simulator (NES) enables properly configuring the power control related opera- validation of some management decisions prior to the tions and parameters. application in the network, off line testing and demon- The algorithm for configuring these parameters is stration. It is not further analyzed in this paper. presented below. The algorithm evolves in two phases. The first phase intends to find a solution fulfilling a minimum CIR requirement for all cells. This is accom- 4 Management of the power control operation plished using a neural network approach. The second phase of the algorithm performs computations, which This section presents the ETSI standard requirements result in the increase of certain cells’ requirements. The for the PC procedure, and our approach to the man- first phase algorithm is then invoked again to generate agement of the PC operation. new results.

4.1 Constraints and requirements for the power 4.3 General problem formulation control procedure The input for the algorithm is the set of cells (V ) and The HIPERLAN/2 standard ([9]) defines the acceptable their requirements in terms of total mean bandwidth for levels of transmission, between which the APs and MTs all services required for each cell (bwULðÞv , are allowed to operate. For both, the transmission bwDLðÞ8v v 2 V ) expressed for the uplink and downlink. power must be greater than )15 dBm and lower than Another required piece of information is the carrier 30 dBm (or even lower in accordance to regulatory allocation to cells, as well as information on the service requirements). area layout and cell coverage, the propagation condi- The AP transmission power (AP Tx Level) ranges tions and the equipment capabilities. from )15 dBm to the maximum, with an increase step of It should be noted at this point, that the exact posi- 3 dB. However, the standard rather than defining a tion of the users requiring the services is not taken under transmission power for the MTs, defines a desired re- consideration both because the users are not considered ceived power level at the AP, from the MT. This power to be static and because we intend to minimize the level (AP Rx UL Level), ranges from )71 to )43 dBm. necessary input data for the algorithm. Therefore, we The MT transmission power must be therefore such that consider the cell to be divided in a certain number of the received power at the AP reaches its desired value, areas, and users to be uniformly distributed in each area without however exceeding the transmission power of its according to the size of the area. If we assume the associated AP nor of course its maximum transmission division of each cell in n areas, each area will be defined capabilities. To accomplish that, the MT makes mea- between the homocentric circles with radius m ðR=nÞ, surements on the link to estimate the current pathloss, and ðm þ 1ÞðR=nÞ, with 0 m n À 1. If we mark the and defines its power at such a level so that the received total demand in each cell as bwðÞ v , the demand in each pR2ðÞmþ1 2ÀpR2m2 ðÞmþ1 2Àm2 power at the AP is the desired one. area is then bwðÞ v pR2n2 ¼ bwðÞ v n2 . Thus, Therefore, the power control procedure defines two each cell is divided in areas, marked from 0 to n À 1. values for each AP, the transmission power and the re- The total number of available frequencies is F , and quired reception power (namely AP Tx Level and the exact allocation of carriers to cells is provided by a AP Rx UL Level). In the following, the algorithm for vector ACAP. The service area layout is described through configuring these two values will be presented. the positions of the APs and the radius of the cells. These are given as posAPðÞv , belonging to cell v (v 2 V ) and rAPðÞv correspondingly. From these given informa- 4.2 Basic assumptions tion, if we mark P as the total area covered by the network, for each pixel p (p 2 P) belonging to the area two The proper configuration of the target, uplink and functions can be identified. (i) A function returning the downlink, transmission powers allowed, constitutes the set of pixels, pAPðÞv , belonging to cell v (v 2 V ); (ii) a management of the power control operation. The man- function, cAPðÞp , returning the cell to which pixel p be- agement functionality should ensure fast adaptations to longs. severe traffic alterations, involving large segments of the The propagation conditions are described through an network. The new traffic assigned can be the outcome of attenuation model typical for HIPERLAN/2 systems the handling of new service area conditions by the ([23]), resulting in a function set of pixel-level attenua- 2 management infrastructure. tion values, where each value aPðÞp1; p2; k ðÞ2p1; p2 P , The throughput (capacity) of HIPERLAN/2 cells 0 k jjÞF provides the attenuation of a transmission is influenced by their CIR, which depends on the allo- that originates from pixel p1 and terminates at pixel p2, cation of carriers to APs, and on the (uplink and when the distance of the carriers used in cAPðÞp1 and downlink) transmission powers used. Therefore, the cAPðÞp2 is k. The equipment capabilities specify the 6 maximum transmission powers of MTs and APs, pMT The IPMTðÞv notation represents the aggregate interfer- and pAP, respectively. ence sensed by the reference transmission (MT) of cell v. Likewise, IPAPðÞv is the aggregate interference sensed by the AP of cell v. It should be noted here that in both 4.4 First phase of the algorithm formulations the MT is considered to be in the n À 1 area of the cell v (worst case scenario). The solution After gathering the above-mentioned information, the algorithm is influenced by the scheme analyzed in [16, first phase of the algorithm intends to find a solution 25, 26]. It employs a greedy algorithm proceeding in an fulfilling the minimum requirements for all cells in the iterative manner. In each round there are computations coverage area. Using results in bibliography ([24]) it can of the aggregate interference in each cell. Moreover, be found that 5 dB is the minimum CIR for the per- there are assessments on the compliance with the con- mission of services in a HIPERLAN/2 system, therefore, straints and on the convergence of the algorithm. The the minimum requirement for each cell is reaching at solution algorithm is shown in Fig. 5. least a 5 dB Carrier to Interference (CIR) at the edges of the cell. The first target for the power control procedure is ensuring that the n À 1 area of each cell v (which is the 4.5 Soft computing enhancement for the first most difficult area to ensure the desired CIR), reaches phase algorithm DL UL DL the cirnÀ1ðÞ¼v cirnÀ1ðÞ¼v 5dB8v 2 V , where cirnÀ1ðvÞ is the CIR in the n À 1 area of v cell in the downlink (and Even though the greedy algorithm provides a solution, it UL cirnÀ1ðvÞ in the uplink). Obviously the other areas of the still needs some time to achieve it. In real time condi- cells will receive a higher CIR. tions this period could be crucial for the stability of the The solution of the problem provides the allocations network. A solution would be to implement a neural of transmission power to APs and MTs, APAP ¼ network, trained off-line under the supervision of the fj8tpAPðÞv v 2 V g and APMT ¼ fj8tpMTðÞv v 2 V g, respec- greedy algorithm. tively. The notation tpMTðÞv and tpAPðÞv corresponds to Trying to select the most appropriate input vector for the maximum, uplink and downlink, transmission the neural network, a mandatory observation is that powers, which should be allowed, in cell v, by the power only the cells that share the same frequency and thus control operation. The objective function is targeted to contribute to the co-channel interference have to be ta- the minimization of the aggregate transmission powers. ken into account. Thus, the input sequence consists of The assigned powers should maintain the required CIR the CIR targets of the cells that interfere to the examined levels and be compatible with the equipment capabilities. cell plus the CIR target for the examined cell itself. The Moreover, the uplink power should not exceed the output of the neural network will be the transmission downlink power in each cell. power of the AP that controls the cell. Fig. 4a and b describes the formulations of two sub- The output power values are quantized into the pre- problems that compute the APAP and APMT allocations. defined, discrete power levels [9]. Therefore, the

Fig. 4 Power control conﬁg- uration. a Downlink. b Uplink 7

Fig. 5 Solution algorithm for the ﬁrst phase

proposed neural network shall be used for classification units. It further consists of neurons allocated in three and more specifically will perform multiple-class identi- layers after the input: fication. – pattern layer: there is one pattern node for each Several types of neural networks could be used for training example. Each pattern neuron forms a dot that purpose with the feed-forward backpropagation product of the weight vector and the input pattern network and the probabilistic network being the best candidates [27]. The latter one is selected and applied to the solution of the downlink problem, as presented in the previous section.

4.5.1 Probabilistic neural network

In general, Probabilistic Neural Networks (PNNs) are a class of radial basis function networks, which combine some of the best attributes of statistical pattern recog- nition and feed-forward neural networks [28]. The PNN architecture is shown in Fig. 6. The neurons in the input layer distribute the input to the pattern Fig. 6 PNN architecture 8 vector. After that, the product is passed through a The simulation below concentrates on a specific cell, selected activation function. Each pattern neuron cell=1, and aims at finding the transmitted power of the computes a distance measure between the input and corresponding AP. As stated before, the input consists the training case represented by that neuron. of the CIR targets of the interfering cells (i.e. cells 10, 18, – summation layer: the summation layer has one neuron 20 and 28) plus the CIR target for cell 1. Therefore, for each class (possible –discrete- output). Each the training data is a sequence that contains 3000 vec- summation neuron, associated with a single class, tors of type: ink ¼ ½cirk1 cirk10 cirk18 cirk20 cirk28 , sums the pattern layer neurons corresponding to k ¼ 1,…,3000. It is important to state that, since the numbers of that summation neuron’s class. It pro- number of cells sharing the same frequency is not pri- duces at its net output a vector of probabilities. marily known, the size of vector ink is not fixed and – output layer: The output neuron is a threshold dis- therefore multiple PNNs, deferring in this size, have to criminator that picks the maximum of these proba- be trained. bilities and produces one for that class and zero for The output target i.e. the power transmitted by the other classes. AP ¼ 1, is defined by the greedy algorithm and plays the role of the network supervisor. The only factor that Working as a classifier, the main problem is to determine needs to be appropriately configured is the factor r, the class membership of a k-sized multivariate random which is optimized by trying many values and select the vector X ¼ ½x ; ...; x , into one of N ¼ðn ; n ...n Þ 1 k 1 2 m best one in terms of increase in network performance. possible groups. If the probability density functions The trials showed that too small values for r lead up to (PDF), p ðÞX , are known for all populations, then i poor generalization capabilities, while too large r values according to the Bayes optimal decision rule [29], each X conceal details. is classified into population i if Figure 7 shows that the network performs perfectly hicipiðÞX > hjcjpjðÞX 8j 6¼ i when dealing with a sample within the training sequence. where h : the prior probability of a sample being drawn Nevertheless, it is unfair to judge a classifier based on its i performance in classifying its training set. Consequently, from population i. ci: the cost from misclassifying a sample from population i. the next step is to pick up a sample of 100 unknown The above decision rule can be also applied even if vectors of type ink and observe the neural network’s the PDFs are not known, provided that Parzen’s tech- performance in the general population. This procedure is followed for two different sequences of such vectors. nique is used on the training sample to find the piðÞX estimates of the density function of each population i Figure 8 and 9 show the response of the selected PNN, [30]. The Parzen’s PDF estimator uses a weight function which results in 89% success in the first case and 83% that maximizes its values for small distances between the success in the second case. training and unknown points and decreases toward zero xÀxi for high distances, respectively (WF r ). Actually, it is an average of that weight function across the training 4.6 Second phase of the algorithm set. The common density estimator, using the ‘‘Gauss- ian’’ weighting function, is given by After the first phase of the algorithm reaches convergence, the second phase of the algorithm is triggered. XnÀ1 1 2 2 ÀjjxÀxi =2r The results are stored and the cell noted as vMAC,is pxðÞ¼ r e 2 r found. Cell v is selected from all cells as the cell with ðÞ2p r n t¼0 MAC the maximum usage in time of its MAC frame. This where r stands for the dimensionality of the input pat- calculation is feasible, as both the required bit-rate is terns, n denotes the number of training patterns and r provided and the Physical Rate is calculated through the (sigma) is the scaling parameter that controls the width achieved CIR levels. Using link level simulations ([23]) a of the area of influence. The larger sample size is used, function phyrate(cir) connecting the CIR and the the smaller r values to be set. Physical Rate is derived (This function can be defined In the sequence, a PNN is applied to the problem of through the Physical Rate versus CIR graph). The cal- downlink allocation of transmitted powers (see culation of vMAC is the following. Sect. 4.4). Numerical results are also presented. 2 XnÀ1 ðÞmþ1 Àm2 bwULðÞv n2 vMAC ¼ v 2 V : max UL 4.5.2 Simulation and results m¼0 phyrateðcirm ðvÞÞ !! 2 2 bw ðÞv ðÞmþ1 Àm We will assume the network topology depicted in DL n2 þ DL : Fig. 11. The number in parenthesis next to the AP phyrateðcirm ðvÞÞ number, corresponds to the frequency used by the Ac- DL UL cess Point. The total number of frequencies is 7. The The requirement cirnÀ1ðÞ¼vMAC cirnÀ1ðÞvMAC (in Fig. 4a network will be described in more detail in a later sec- and b) for the cell vMAC is then raised by a reasonable tion. step (i.e. 1 dB) and the first phase of the algorithm is 9

Fig. 7 Training set classiﬁ- cation – 100% success

invoked again. A greater raise step will lead to faster either case, the last stored solution is the outcome of the however more approximate results. Each time conver- algorithm. For the uplink, the transmission power of gence is achieved, the new vMAC with the maximum usage the MTs is converted, through the pathloss formula, to of its MAC frame, is calculated and the algorithm of the the desired level of reception at the AP. The second ﬁrst phase is rerun, while the last found working solution phase is presented in Fig. 10 is stored. This procedure continues until either the ﬁrst phase algorithm cannot provide a solution, or the MAC usage for all cells v 2 V , is lower than a minimum 4.7 Discussion on the management algorithm threshold, ensuring that even under unpredicted raises in the demand, the MAC frame will not be fully covered As it is seen, the behavior of the algorithm guarantees, resulting in blockage of users demanding services. In not only that the mobile terminals which are near the

Fig. 8 General set classiﬁca- tion 1 – 89% success 10

Fig. 9 General set classiﬁca- tion 2 – 83% success

edge of the cell will receive the required CIR to operate ture of the network. Other networks of the similar size normally, but also that the cells with the heaviest traffic and connectivity degree could have been used instead. will lower their usage of the MAC window, allowing Only co-channel interference is taken into account in the more mobile terminals to be served at a certain area. The test case. Our focus is restricted to co-channel interfer- algorithm also provides the functionality to lower as ence for facilitating the presentation of our methods (re- much as possible (without exceeding the AP and MT sults or whatever). Our work can readily be expanded to capabilities) the usage of MAC frame if serious altera- include more general interference conditions, e.g., based tions in traffic are anticipated, or to keep the mean on [31]. In accordance with the standardization, the power at a low level (satisfactory, nevertheless) if no maximum transmission power of APs is assumed 30 dBm serious variations are anticipated and therefore the and the maximum allowed uplink interference is set to usage of the MAC frame can be kept at a high level. )43 dBm. It will be assumed that the APs of the network can access only 7 frequencies, due to the background interference induced to the other carriers by 5 Results other systems, using the 5 GHz band in the area. The assignment of frequencies to the cells is depicted also in This section includes indicative results, mainly, on the Fig. 11 (the number in the parenthesis next to the AP behavior and efficiency of the HIPERLAN/2 configu- number). ration component, and the Power Control itself. An At the initial condition of the network it is assumed indicative test case will be realized. It is assumed there is that there are (approximately) 600 users (subscribers) in an initial condition that corresponds to a certain load, each cell. The users access two services through the performance and configuration for the network. At a HIPERLAN/2 network. Half of the users access service next phase there is a new condition, caused by the s1 and the other half access service s2 (300 users per additional traffic that should be absorbed, possibly in service). Service s1 is analogous to conversational video. order to assist another network of the CR infrastructure. It requires 64 Kbps on both directions (uplink and Most likely a 2.5G or 3G mobile network will face such downlink). Service s2 is analogous to a streaming service problems, and therefore, will need the assistance of the requiring an average of 128 Kbps, on the downlink, and HIPERLAN/2 network. The HIPERLAN/2 configura- 8 Kbps for the uplink. Each user generates 0.02 Erlangs tion component, using the PC scheme, is applied to (20 mErlangs) for the service he is subscribed to. Be- adapt the network to the new condition. cause of the uniformity of the demand in traffic as well Figure 11 depicts the service area and the network as the low level of the common demand, the Power used in our test case. There are 36 cells, organized in a Control procedure would produce a common power 6 · 6 structure. The radius of each cell is assumed level for transmission. 100 m. The overall service area covered by the network Figure 12 is focused on our test case. It presents the is 1 km2. Our study does not depend on the exact struc- distribution of the additional traffic load that should 11 c Fig. 12 The test case. a Distribution of additional traffic load in the service area. b Outcome of the configuration of the power control operation on the downlink. c Outcome of the configuration of the power control operation on the uplink

be accommodated, and the outcome of the handling of the HIPERLAN/2 configuration component. Fig- ure 12a shows that the service area can be split in three sets. In the heavily shaded area there are 600 more users for each service, therefore the demand is three times more than the initial case. In the less shaded area the additional users compared to the initial condition are 300 more for each service. Finally in all the other cells, the demand remains as it was in the initial condition. Figure 12b shows the outcome of the configuration of the power control operation and parameters on the downlink. Specifically, it depicts the maximum transmission power per AP that can be imposed by the power control operation, in order to preserve the target CIR levels. The values range from 6 dBm, in moderately loaded cells in the periphery of the network, to 12 dBm, in more heavily loaded cells. These values are significantly lower than the maximum possible values, which have been used as a reference in the configuration of the frequency selection operation. Figure. 12c shows the outcome of the configuration of the power control operation and parameters on the uplink. As it was expected, higher power values are allocated to the cells where the demand is increased. What is really interesting is to investigate the results of the proposed power allocation when it is applied to the HIPERLAN/2 network. In order to achieve this, two simulations are run (in the NES component of the Fig. 10 General presentation of the second phase algorithm management system). In both simulations, the configuration for the network is the one corresponding to the number of users, user activity and type of services mentioned above. However, in the first simulation, fixed power levels for the Power Control procedure are used (and the PC scheme proposed is not applied). In the second, the results of the PC scheme proposed are applied. During the simulations, statistics on the performance of the network are kept, and at the end of the simulation the mean measured values are calculated. The mean rate of transmission for the Physical Layer for both downlink and uplink is calculated (including mistakes and retransmissions), and the time used out of the 2 ms of the MAC frame for the DL and UL are produced, for every AP. In Fig. 13, the mean Physical Layer Rate is depicted both without and with the PC for all APs. Next, in Fig. 14, the time used in the MAC frame is provided for both cases. Conclusions on the performance of the PC procedure can be extracted. First of all, the Physical Layer transmission rate is increased for the cells with higher demand, while it is slightly reduced in some of the cells with lower requirements. Fig. 11 Network and service area considered in our test cases This was not only anticipated, but is also one of the 12 13

Fig. 13 a Mean physical bit-rate for the downlink. b Mean physical they would get, and therefore the demand is better bit-rate for the uplink served where needed. Very important is also the fact that the utilization of the MAC frame is lowered in major beneﬁts of the PC scheme proposed. Cells the cells in which the demand is higher, and slightly heavily loaded, achieve better link quality than what increased in cells with lower demand. That leads to a 14

Fig. 14 a Time consumed in each MAC frame in the downlink. b users, if needed, even at cells which are more loaded, Time consumed in each MAC frame in the uplink and in case the PC management procedure was not used, would have to block users trying to establish more uniform usage of the MAC frame in all cells, connection. Last but not least, using the neural and creates the possibility to accommodate more network approach, the Power Control procedure 15 accomplishes these in a fast and self-operating way, Type 2; Data link control layer; Part 1: Basic data transport using power levels significantly lower than the maxi- functions (TS 101 761-1) 9. European Telecommunications Standards Institute (ETSI); mum allowed values. Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data link control layer; Part 2: Radio link control (TS 101 761-2) 10. European Telecommunications Standards Institute (ETSI); 6 Conclusions Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data link control layer; Part 4: Extension for home This paper presented management functionality for environment (TS 101 761-4) augmenting the potential exploitation of BRAN/WLAN 11. European Telecommunications Standards Institute (ETSI); Broadband Radio Access Networks (BRAN); HIPERLAN systems through the use of a Network Management Type 2; Network management (TS 101 762) System using a Power Control scheme. The approach 12. IST project MONASIDRE (Management of Networks used was the following. The first point was to revisit the and Services in a Diversified Radio Environment) Web site main features of a HIPERLAN/2 system. Next, the http://www.monasidre.com architecture of a general SNMS, which has been devel- 13. IST project CREDO (Composite Radio for Enhanced Service Delivery During the Olympics) Web site oped for assisting wireless systems in their operation, http://www.ist-credo.org was presented. The last main point was the functionality 14. Demestichas P, Papadopoulou L, Stavroulaki V, Theologou M, of the SNMS component that is tailored to the managed Vivier G, Martinez G, Galliano F (2002) Wireless beyond 3G: HIPERLAN/2 technology and specifically the Power managing services and network resources. IEEE Computer 15. Bambos N (1998) Toward power-sensitive network architec- Control functionality integrated to the Management tures in wireless communications: Concepts, issues, and design System. The algorithm for configuring the PC operation, aspects. IEEE Personal Commun 5(3): June based on a greedy algorithm and a neural network, was 16. Demestichas P, Kotsakis G, Tzifa E, Demesticha V, Anag- presented. A resource management problem, which was nostou M, Theologou M (2002) Power allocation in the context of dimensioning the air-interface of third-generation efficiently solved with the use of PC for exploiting HI- W-CDMA-based cellular systems. Inter J Commun Sys 15:375– PERLAN/2 networks, was addressed. Numerical results 400 were presented. 17. Demestichas P, Koutsouris N, Koundourakis G, Papadopou- Issues for further study are the following. First, the lou L, Stavroulaki V, Theologou M (2002) Brokerage of wire- development of a combinative scheme, which will less systems’ resources in a composite radio context. Challenges and achievements in e-business and e-work. IOS Press implement both DFS and PC to lower (if possible) the 18. Dutta P (1999) Strategies and games: theory and practice. MIT power used. The second issue is the report of further Press, Cambridge, Massachussets experience that will be obtained from the experimenta- 19. Lewicki R, Saunders D, Minton J (1999) Negotiation: readings tion with the SNMS platform and the PC algorithm. exercises and cases. McGraw-Hill, Boston 20. Shell G (1999) Bargaining for advantage: negotiation strategies for reasonable people. Viking, Penguin Books, New York Acknowledgements. This work was partially funded by the Com- 21. Ghosh S (1998) Making business sense of the Internet. Harvard mission of the European Communities, under the Fifth Framework Business Review 76(2): Program, within the IST project MONASIDRE (IST-2000-26144: 22. Kalakota R, Whinston A (1997) Electronic commerce: a man- Management of Networks and Services in Diversified Radio ager’s guide. Addison-Wesley Publishing Company Environment). 23. Lin Z, Malgren G, Torsner J (2000) System performance analysis of link adaptation in HiperLAN type 2. VTC 24. Doufexi A, Armour S, Butler M, Nix A, Bull D (2001) A study of the performance of HIPERLAN/2 and IEEE 802.11a physical layers. VTC References 25. 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