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Adaptive Multimedia Traffic Multiplexing for Dedicated Channels in the UMTS System

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Adaptive Multimedia Traffic Multiplexing for Dedicated Channels in the UMTS System Adaptive Multimedia Traffic Multiplexing for Dedicated Channels in the UMTS System J.R. Gállego, A. Hernández-Solana, M. Canales, A. Valdovinos Communications Technologies Group (GTC). Aragon Institute for Engineering Research (I3A). María de Luna 1, 50018, University of Zaragoza, SPAIN {jrgalleg, anhersol, mcanales, toni}@unizar.es Abstract WCDMA UMTS environment that considers the different aspects concerning the physical layer, In this paper, a multiservice transmission scheme is functionalities of the UMTS protocol stack and system evaluated for the Dedicated Channel of the Universal capabilities. Errors in the Signal to Interference Ratio Mobile Telecommunications System. The transmission (SIR) estimated by the power control are also rate for each service is determined according to its QoS considered. A new strategy to select transmission rates requirements by means of an adaptive Transport Format of the multiplexed services is proposed and evaluated. selection. The proposed selection is based on buffer This strategy takes into account buffer occupation, delay occupation, delay requirements and target bit rate, requirements and target bit rate keeping power keeping power constraints. This power restriction constraints. UMTS also allows to multiplex different depends on the estimated residual capacity and the services in upper layers (Logical Channels) sharing a contribution of this particular mobile user to the system common transmission rate (Transport Channel). This load. Service multiplexing in upper layers (Logical multiplexing is proposed for video and game Channels) sharing a common transmission rate transmission, and two strategies to share the common (Transport Channel) has been evaluated by means of rate are evaluated. two different dequeueing strategies. The remaining paper is organized as follows. In Section 2, a description of the UMTS system functionalities, services and service capabilities is given 1. Introduction and the proposed strategies are described. In Section 3, One of the key requirements of the UMTS air the system model is presented and main parameters are interface is the support for multiplexing different described. We discuss performance results in Section 4 services with different QoS on a single connection. The and, finally conclusions are provided in Section 5. main advantage of the WCDMA air interface to provide 2. QoS for the UMTS Air Interface these differentiated services is the option of variable transmission data rates through different spreading Provision of QoS in the UMTS air interface is related factors, multicode transmission and coding schemes. to functionalities of the radio interface protocol The acceptance of a new user connection must be architecture, which is shown in Fig. 1. The WCDMA conditioned by the fact that target signal to interference physical layer offers data transmission services to ratio (Eb/No) values can be achieved by each existing Medium Access Control (MAC) layer [1] by means of connection once a new one is activated. Therefore, Transport Channels (TrCh). The set of specific attributes getting the required QoS for each user is closely of the physical layer (channel coding, interleaving, and connected with power allocation. A good interference transmission rate) is referred to as the Transport Format handling by radio resource allocation schemes plays an (TF) of the considered TrCh, and it determines the important role to guarantee the performance and to transmission quality for the data information to be sent. increase the system capacity. When a connection is The MAC entity is responsible for mapping Logical accepted, certain resources are allocated for this user. Channels (LCh) onto TrChs, selection of TF, priority Different services can be multiplexed over this same handling and dynamic scheduling. A set of LCh types is connection with different QoS requirements. According defined for the different kinds of data transfer services to these QoS requirements and the available resources, offered by MAC. They can be dedicated, shared and transmission rates for each multiplexed service must be common channels. A LCh is defined by the type of determined. In this paper, an analysis of service transferred information. Each of the multiplexed LChs multiplexing in the dedicated channel (DTCH) for both may have variable data rate on a TTI (Transmission uplink and downlink is carried out in a realistic Time Interval) by TTI basis. Each combination of rates on the individual channels results in a certain data format PDUs with a lower WT are firstly dequeued. to be transmitted, defining the total number of bits per In order to guarantee QoS in terms of Bit Error Rate frame and their assignment to the individual channels (BER) or Block Error Rate (BLER), a specific bit energy (Transport Format Combination – TFC) [2]. to interference ratio (Eb/No) must be provided for each Mobile Station Base Station RRM Entity User information User information service. A joint symbol energy to interference ratio Mapping QoS according type of service (Es/No) needs to be computed according to the TFC so Call Admission Control according type of service. that all Eb/No are met. Rate matching (RM) [2] provides Resource Allocation PDCP PDCP the required Eb/No for each of the multiplexed services in RLC RLC RLC RLC RLC RLC RLC RLC RLC RLC RRC the CCTrCH. Equations (2) and (3) show the basic DCCHDTCH DTCH DTCH Logical channels DTCHDTCH DCCH process in the uplink. MUX MAC MAC MUX TFC selection § Ec · DCH DCH Transport channels DCH DCH ¨ N ¸ Ni ǻNi © 0 ¹i § E · § E · Rb Physical Layer (Hidden Markov Model) , c b (2) ¨ N ¸ ¨ N ¸ Function (Propagation, mobility, power control accuracy,coding, interference level) Ni Es © 0 ¹i © 0 ¹i Rc N0 Figure 1. UTRA-FDD Radio Interface protocol for i = 1,..., I (number of TrChs) architecture I I § § E c · · E s In addition to physical procedures, upper layers ¨ Ni u ¨ ¸ ¸ Ndata u , Ndata Ni 'Ni (3) ¦¨ N0 ¸ N0 ¦ provide other feasibilities depending on the service i 1© © ¹i ¹ i 1 requirements. In particular, the Radio Link Control where Rb and Rc are transmission rates before and after (RLC) [3] protocol can provide a reliable service channel coding, Ndata is the number of bits transmitted dependent transmission by selecting its operating mode. over the CCTrCH in a frame with rate Rs (bauds), Ni are In fact, UMTS system considers three different modes in the bits associated to each service and 'Ni the added bits RLC configured by the Radio Resource Controller to match the total rate to Rs. RM is similar in both links, (RRC) [4]: based on FEC are Transparent Mode (TM) although the process is less dynamic in the downlink. and Unacknowledged Mode (UM) whereas the Spreading factor is fixed and DTX (Discontinuous Acknowledged Mode (AM) is based on joint FEC and Transmission) is used in addition to repetition and ARQ. The RLC mode also allows the use of early puncturing in order to match the variable transmission discard, which allows to drop in the transmitter packets rate of each service. Power transmission is determined that have exceeded the maximum tolerable delay, by the required Es/No, channel conditions and system reducing the delay of the following packets. The time a interference level in different ways for both links. These packet is allowed to stay in the RLC buffer is controlled dependences are shown in (4) and (5). and signalled by upper layers. We have considered a K W F U P h P 0 i T BS idown P (4) packet-dependent value according to (1) T BS,i W MAX BS,i U § Es · Li Rs ¨ ¸ TDisc,i Tmargin TTX ,i Tmargin (1) © N0 ¹ R target where PT-BS,i is the transmitted power by the Base Station where Li is the length of packet i and TTX,i the time to (BS) associated to user i, PMAX-BS,i its maximum allowed transmit packet i at Rtarget, and Tmargin an additional value, and PT-BS the total transmitted power, K0 the provided margin. thermal noise spectral density, W the available Several LChs, belonging to different services (e.g. bandwidth in the cell, Fi the intercell interference video, audio, etc) can be jointly transmitted, using observed by the user i, U the orthogonality factor, and hi- different TrChs (with their corresponding TF) down the path loss between BS and user i. multiplexed over the Coded Composite Transport K0 W (5) Channel (CCTrCH) [2], [4]. Some of the multiplexed PT UE,i PMAX UE,i § · TrChs can also convey several LChs sharing the same ¨ ¸ ¨ W ¸ TrCh attributes. When different LChs are transmitted Cres hiup 1 ¨ § Es · ¸ ¨ Rs ¨ ¸ ¸ over the same TrCh, the MAC entity has to multiplex © © N0 ¹ ¹ upper layer PDUs from the different RLC entities into the where PT-UE,i is the transmitted power by user i, PMAX-UE,i transport block sets delivered to the physical layer. In this is the maximum transmitted power, Cres is the residual paper, two different dequeueing strategies have been capacity in the system, and hi-up is the path loss between implemented in order to evaluate how the multiplexing user i and the BS. The residual capacity is given, on strategy affects the system performance: average, by the expression (6), 1) First In First Out (FIFO): RLC PDUs are mapped onto -1 § · transport blocks in the same order they arrive at the RLC ¨ ¸ N ¨ W ¸ queues, regardless of the LCh they belong to. Cres 1 (1 f ) ¦ ¨1 ¸ t K (6) j 1 § Es · 2) Lowest Waiting Time First Out (LWTFO): RLC PDUs ¨ D j Rs, j ¨ ¸ ¸ ¨ © N0 ¹ ¸ are stamped with a maximum Waiting Time (WT) that © j ¹ depends on the QoS requirements of the service. RLC where Dj is the activity factor of source j and f is the ratio between inter and intracell interference. Cres is lower this delay, which is decreased each TTI the RLC packet limited by K (equal to 0.1 is considered in order to limit remains in the queue.
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