RECENTLY, the information exchange using wired and

Fast Dedicated Retransmission Scheme for Reliable Services in OFDMA Systems Howon Lee and Dong-Ho Cho Department of Electrical Engineering and Computer Science Korea Advanced Institute of Science and Technology (KAIST) 373-1, Guseong-dong, Yuseong-gu, Daejeon, Korea Telephone: +82-42-869-3467, Fax: +82-42-867-0550 Email: hwlee@comis.kaist.ac.kr, dhcho@ee.kaist.ac.kr Abstract In OFDMA systems, multicast data packets are always transmitted with control messages that identify mutlicast data packets. In case of reliable multicast services, if a multicast data packet is destroyed, the BS must retransmit this packet with a control message. In this case, a lot of downlink resources are wasted, because the control message uses a very robust modulation and coding scheme (MCS) level. Therefore, to solve this problem, we propose a new retransmission scheme for reliable multicast services in OFDMA systems. Using our proposed scheme, we can save a lot of downlink resources because of the reduction of control messages for multicast data packet retransmissions. In this paper, we analyze the downlink cell throughput and the required resources of our proposed scheme for reliable multicast services, and show, via OPNET simulation, that the throughput of our proposed scheme is superior to that of the original scheme in OFDMA systems. I. INTRODUCTION RECENTLY, the information exchange using wired and wireless communication systems has become an important part of everybody s life and is spreading rapidly in our society. Moreover, the explosive growth of Internet has given rise to demands for higher capacity, higher data rate, and more advanced multimedia services. The orthogonal frequency division multiple access (OFDMA) system has been proposed and extensively investigated to satisfy these demands. The OFDMA system has a lot of advantages. Its main advantage is that multiple access interference (MAI) within a cell can be mitigated by using frequency division multiple access (FDMA) techniques at the subcarrier level. In this system, the orthogonality of the subcarriers facilitates a subcarrier division of different users, where one OFDM symbol contains many users. Also, this system is more robust compared with singlecarrier multiple access systems, because OFDMA techniques offer robustness to narrow band interference. And, this system does not need adaptive time domain equalizer, since channel equalization is performed in the frequency domain through one-tap multipliers [1]. The MAC layer multicast services are used in case that a base station (BS) sends the same data packets to many users. In this case, if the BS sends this data packets to the users using unicast services, it causes a waste of a lot of downlink This research was supported in part by University IT Research Center Project. resources. Then, if the BS uses the MAC layer multicast services, it can use downlink resources efficiently. In case of MAC layer multicast services, the BS sometimes needs ACK or NACK packets based on ARQ (automatic repeat request) mechanism. For example, in case that the BS sends multicast data packet to the users included in multicast group, if the users do not send ACK or NACK packets, the BS cannot know whether the users receive the data packets or not. Then, the BS which has to know the status of the users, can not continue multicast services. To solve this problem, ARQ mechanism must be supported for MAC layer multicast services in OFDMA systems like unicast services using ARQ mechanism. Also, we can obtain some merits by using ARQ mechanism in MAC layer multicast services. The BS can use less robust data burst profiles (high coding rate, less robust modulation) in multicast services using ARQ mechanism compared with multicast services that do not use ARQ mechanism. Hence, the BS can save downlink resources in multicast services by using ARQ mechanism. In this paper, we define a MAC layer multicast service using ARQ mechanism as a reliable multicast service. The remainder of this paper is organized as follows: In Section II, we introduce feedback methods using general feedback scheme and fast feedback schemes. Fast feedback scheme is possible by using a fast feedback channel, and CA(Cumulative ACK)/AFR(ARQ Feedback Request) CDMA codes and multicast slot [2] [3]. Also, in this section, we specify the problem of multicast packet retransmissions in reliable multicast services. In Section III, we propose an efficient scheme for packet retransmissions of reliable multicast services by using a fast dedicated retransmission scheme in OFDMA systems. In Section IV, we show that our proposed scheme has the better performance than the original scheme using a general retransmission method. Finally, in Section V, we make conclusions. II. RELATED WORKS AND PROBLEM STATEMENT A. Related Works There are many ARQ feedback schemes for reliable multicast services in OFDMA systems, such as a general feedback scheme using unicast ARQ feedback messages, a fast feedback scheme using a fast feedback channel, and a fast feedback scheme using CA/AFR codes and multicast slots [2] [3]. 0-7803-9152-7/05/$20.00 2005 IEEE 1108

1) General Feedback Scheme: Reliable multicast services are possible by using unicast ARQ feedback messages like general unicast services using ARQ mechanism [2]. This is a basic and simple scheme. In this case, for the transmissions of ARQ feedback messages of the users included in multicast groups, the BS periodically assigns the uplink resources to the multicast users. However, this scheme cannot be used as the fast feedback scheme, because the uplink resources for ARQ feedback messages are not small. Hence, if the BS uses this scheme as a fast feedback scheme, it causes a waste of a lot of uplink resources. 2) Fast Feedback Scheme I: Reliable multicast services can be possible by using fast feedback channels like unicast services using fast feedback channels in IEEE 802.16 task group (TG) d system [3]. The fast feedback channels are dedicatedly allocated to each user in a multicast group. The fast feedback channel is an indication channel. This channel consists of 1 OFDM subchannel (frequency domain) and 3 OFDM symbols (time domain). By using this channel, the multicast users cannot transmit the ARQ feedback messages, because the size of this channel is not enough to send the ARQ feedback message of the multicast user. It can be used only for indications whether the users included in multicast gourps, well receives multicast data packets or not. In case that the multicast service user wants to send the ARQ feedback messages, the user requests the uplink resources for ARQ feedback messages by using this fast feedback channel. 3) Fast Feedback Scheme II: Reliable multicast services are possible by using CDMA codes (CA and AFR codes) [2]. The reliable multicast services based on CDMA codes can save a lot of uplink feedback resources compared with the general feedback scheme. Also, the fast feedback mechanism is possible by using these CDMA codes, since a multicast region and multicast slots are always allocated in uplink MAC frame in case that multicast services are activated [2]. In other words, to support this fast feedback scheme, we have to define the multicast region, multicast slot, CA code and AFR code. The multicast region is total area that is used for transmissions of CDMA multicast codes. It consists of the several multicast slots. In case of the multicast services, the BS assigns this multicast region and multicast slots. The multicast users can transmit its multicast codes by using one of this multicast slots in the multicast region. The multicast slot that will be used, is randomly selected by the users included in multicast groups to send CDMA multicast codes. If the CDMA codes sent by the SSs at the same time are orthogonal, these codes do not collide and destroy. In this scheme, in the beginning of the muticast services using our proposed scheme, the BS assigns two orthogornal CDMA codes (CA code, AFR code) to the SSs. The CA code is an indication code. This code informs the BS of the fact that the multicast user has well received multicast data packets transmitted after previous correctly received packet. The AFR code is a bandwidth request code to obtain bandwidth for sending ARQ feedback message to the BS. In general, if the packet errors occur, the multicast user has to send ARQ feedback message to notify its status. In this case, the multicast 1109 Frequency Fig. 1. Transmission Retransmission Burst Burst overhead Burst DL MAC frame Time Burst Control message overhead of general retransmission scheme user transmits the AFR code for the bandwidth request to send the ARQ feedback message in our proposed scheme. And, the BS can know the multicast user that sent the AFR codes, because CA code and AFR code are uniquely assigned to each user included in multicast groups. Then, the BS allocates uplink resources for the transmission of the ARQ feedback message to the multicast user. B. Problem Statement In case of MAC layer multicast services, the users in multicast groups have the different channel environments. In other words, each user has different signal-to-interference plus noise ratio (SINR) and propagation loss. Also, these channel conditions vary frequently. Although the BS uses the robust burst profile (robust modulation and coding scheme (MCS) level), the multicast data packets can be often destroyed. Therefore, the retransmissions of the destroyed multicast data packets can not be avoided. The retransmissions in the reliable multicast services are possible by using general retransmission scheme. This general scheme is using the multicast data packet and the control message that identifies the multcast data message. This control message is the same as a in IEEE 802.16d/e OFDMA system. The position, MCS level and other information of the multicast data packets are contained in the control messages. In case that the channel condition is not good and many multicast services are activated, the BS may send a lot of retransmission packets because many multicast data packets are destroyed. In this case, if we use the general retransmission scheme, a lot of downlink resources are wasted, since the control messages use a very robust MCS level, such as QPSK modulation and 1/12 coding [3]. Fig. 1 shows the operation of the general retransmission schemes for supporting reliable multicast services and the overhead of the controbl messages in general

Transmission subframe service initiation Frequency Retransmission Burst Burst No overhead Burst Burst DL MAC frame BS informs multicast users of a region of multicast subframe in DL MAC frame. BS transmits multicast data packet in multicast subframe to all users included in a multicast group. In case that packet of user 1 is destroyed, this user immediately has to notify his status to BS by using a fast feedback scheme. BS retransmits multicast data packet in the same region of the multicast subframe in next DL MAC frame. Fig. 2. Time subframe structure of proposed scheme user can receive multicast data packet successfully. retransmission scheme. Since this scheme causes a waste of a lot of donwlink resources, this scheme is not suitable as a retransmission scheme in reliable multicast services. III. PROPOSED SCHEME To solve this problem, we propose a new retransmission scheme for reliable multicast services in OFDMA systems. Our proposed scheme can be utilized in reliable multicast services using fast feedback schemes, such as the feedback scheme based on the fast feedback channel and the feedback scheme based on CDMA codes [2]. That is, if the users receive multicast data packets from the BS, they must immediately respond to the BS with any fast feedback scheme in our proposed scheme. With the proposed scheme, we can save a lot of downlink resources, since our proposed scheme uses fast dedicated retransmissions. In fast dedicated retransmissions, control messages are not needed. For applying our scheme to reliable multicast services, we have to define the multicast subframe in downlink MAC frame, as shown in Fig. 2. The multicast subframe is designed to support reliable multicast services based on our proposed retransmission scheme in OFDMA systems. As shown in Fig. 2, multicast data packets are only transmitted or retransmitted using this multicast subframe. Sometimes, this multicast subframe can be used for other services, in case that there is no activated multicast services. The region of the multicast subframe can be changed occasionally. In this case, the BS has to inform the users of the changed region in the multicast subframe. The basic operation of the proposed scheme is shown in Fig. 3 In case of the reliable multicast services using our proposed retransmission scheme, the retransmission packet is sent to the same region which is used for the transmission of original packet. Because the BS transmits the retransmission packets 1110 Fig. 3. Operation of the proposed scheme in the dedicated region of the multicast subframe, the control message for the retransmission of the multicast data packet, such as DL- message in IEEE 802.16 OFDMA system, is not required in our proposed scheme. Therefore, we can solve the problem of the original scheme and save a lot of downlink resources in reliable multicast services. A. Numerical analysis of required downlink resources In case of the original scheme, by using the number of multicast groups (N), the size of the DL- message (L ), the size of the multicast data packet (L PK ), the size of basic resource unit for DL- message (L BU ) and the size of basic resource unit for the MCS level of the multicast group (L i BU ), the total number of required downlink resources for multicast packet retransmissions (R r ORI ) can be calculated as N L R r ORI = + L PK. (1) L BU L i BU Similarly, the total number of required downlink resources for multicast packet retransmissions of our proposed scheme (R r PRD ) can be obtained as R r PRD = N L PK L i BU. (2) In equation (2), there is no overhead for control messages that identify the multicast data packets, because our proposed scheme uses fast dedicated retransmissions by using the multicast subframe in a downlink MAC frame. Therefore, we can save a lot of downlink resources using our proposed scheme for reliable multicast services in OFDMA systems.

Fig. 4. Downlink cell throughput vs. multicast groups B. Numerical analysis of downlink cell throughput By using MAC frame duration (T MF ) and the size of the multicast data packet (L PK ), the downlink cell throughput of the original scheme and our proposed can be calculated by S = 1 T MF N L PK + S PRD. (3) The destroyed multicast data packets and control messages are not considered for the downlink cell throughput. Our proposed scheme can save a lot of downlink resources compared with original scheme in reliable multicast services. These saved resources can be utilized for other unicast, multicast and broadcast services. Hence, the proposed scheme obtains an additional downlink cell throughput (S PRD ) compared with the original scheme. In the original scheme, S PRD =0. IV. PERFORMANCE EVALUATION Simulation results are provided to demonstrate the performance of our proposed scheme, via OPNET simulation. For performance evaluation, we define some simulation parameters as shown in Table I. 1 basic downlink resource unit consists of 3 OFDM symbols (time domain) and 1 OFDM subchannel (frequency domain). 1 OFDM subchannel consists of 16 OFDM subcarriers. We assume that multicast data packets are transmitted by QPSK modulation and 3/4 coding, and control messages are transmitted by QPSK modulation and 1/12 coding. And, for the multicast data traffic, we assume that the interarrival time for a multicast data packet generation has an exponential distribution with mean time, 5ms. Fig. 4 shows the downlink cell throughput against the number of multicast groups in case that the packet error rates 1111 TABLE I SIMULATION PARAMETERS Parameters Value Total number of downlink resource 384 MAC frame duration 5ms Size of multicast data packet (L PK ) 50 bytes Size of control message () (L ) 6 bytes Size of basic resource unit of control message (L BU ) 1 bytes Size of basic resource unit of multicast packet (L i BU ) 9 bytes (PERs) of the multicast group users are 0.1, 0.2 and 0.3. These PER values are not reasonable in unicast services. However, in multicast services, many users included in multicast group have to receive a same packet in a same region. In this case, if a certain multicast user does not receive this packet, or the received packet of a certain user is destroyed, the BS should retransmit this multicast data packet in reliable multicast services. Thus, the PER value of the multicast data packet can be calculated by PER MS =1 (1 PER US ) Nuser. (4) In equation 4, PER MS and PER US are the PER values in case of a multicast service and an unicast service. And, N user is the number of users included in a multicast group. For example, in case that PER US =0.01, and N user =30, PER MS 0.26. Therefore, we can prove that these PER values are reasonable. In case that the number of multicast groups are less than 30, the saturation of the 384 downlink resources does not occurred.

Fig. 5. Number of DL required resources vs. multicast groups Thus, the downlink cell throughputs of the original scheme and the proposed scheme are nearly the same. However, as shown in Fig. 4, in case that the number of multicast groups are more than 30, the saturation of the downlink resources occurs. So, in this case, the downlink cell throughput of our proposed scheme is larger than the throughput of the original scheme. In other words, The more the number of multicast groups are, the larger the difference of the downlink cell throughput between the proposed scheme and the original scheme is. Also, the larger the value of PER, the larger the difference of the throughput is. Fig. 5 shows the number of required downlink resources against the number of multicast groups in case that the packet error rates (PERs) of the multicast group users are 0.1, 0.2 and 0.3. For reliable multicast services, our proposed scheme always requires less downlink resources than the required downlink resources of the original scheme, because the proposed scheme does not require the resource for control messages in multicast packet retransmissions. Therefore, by using our proposed scheme for reliable multicast services, we can save a lot of downlink resources compared with the original scheme. The more packet errors occur, the more the downlink resources can be saved. These saved resources can be utilized for other unicast, multicast and broadcast services. Hence, we can obtain an additional downlink cell throughput. Since, our proposed scheme does not cause downlink MAC overhead and can obtain additional downlink cell throughput, we can use our proposed scheme for reliable multicast services in OFDMA systems. information of the control messages is not required, the proposed scheme can solve the problem of the original scheme and save a lot of downlink resources in reliable multicast services. Through the OPNET simulation, we can show that the proposed scheme obtains the additional downlink cell throughput and saves a lot of downlink resources compared with original scheme. Our proposed scheme can be utilized compatibly in OFDMA systems that support the reliable multicast services. REFERENCES [1] Michele Morelli, Timing and frequency synchronization for the uplink of an OFDMA system, IEEE Trans. Communications, vol. 52, no. 2, pp. 296-306, Feb. 2004. [2] Howon Lee and Dong-Ho Cho, Reliable multicast services using CDMA codes in IEEE 802.16 OFDMA system, IEEE VTC 05 Spring, to be published. [3] IEEE 802.16-REVd/D5-2004, IEEE Standard for Local and Metropolitan Area Networks Part 16: Air Interface for Fixed Broadband Wireless Access Systems, May. 13, 2004. V. CONCLUSIONS Consequently, in our proposed scheme, a retransmission packet is sent in the same region used by the original packet in multicast subframe of downlink MAC frame. Since the 1112