Performance Comparison of d OFDMA, TD-CDMA, cdma2000 1xEV-DO and a WLAN on Voice over IP Service

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1 Performance Comparison of 82.16d OFDMA, TD-CDMA, cdma2 1xEV-DO and 82.11a WLAN on Voice over IP Service Jee-young Song, Hyun-ho Choi, Hyu-dae Kim, Sang-wook Kwon and Dong-ho Cho* Hong-sung Chang, Geunwhi Lim, Jun-hyung Kim** *Communication and Information Systems Lab., Dept. EECS KAIST, Guseong-dong, Yuseong-gu, Daejeon, Republic of Korea Phone: , **Telecommunication System Division, Telecommunication Network Samsung Electronics Co., Ltd. Abstract We analyzed four systems designed for high-data rate transmission such as IEEE 82.16d OFDMA, 3GPP TD- CDMA, 3GPP2 cdma2 1xEV-DO, and IEEE 82.11a WLAN, and compared their efficiency, the number of VoIP user, and the normalized number of VoIP user per 1MHz. It is shown that the system serves the most number of VoIP user, and the normalized number of user per 1MHz is also the most. Even though 82.11a system is operated over the widest band, it supports the less number of user compared to the system with smaller bandwidth, due to the inefficiency of CSMA/CA scheme. TD-CDMA and 1xEV-DO serve the same number of VoIP user per 1MHz bandwidth, and TD-CDMA performs with the best efficiency among four systems. Comparing with TD- CDMA and 1xEV-DO, we concluded that 82.16d OFDMA system is the most flexible to assign its resource to users due to its finest granularity among three systems. Keywords OFDMA, TD-CDMA, 1xEV-DO, WLAN, VoIP I. INTRODUCTION For wireless mobile communication system beyond 3 rd generation, efficient utilizing of limited spectrum is strongly needed to increase the number of bit per Hz and total number of user, and to assure higher per-user throughput. In addition to this, it is expected that VoIP-related services become one of the most important services for the next generation wireless communication systems. In this paper, the number of VoIP service user and the efficiency of four system 82.16d OFDMA, TD-CDMA, cdma2 1xEV- DO and WLAN are analyzed. Section II describes the basic operations and the frame structures of four systems. Performance measures such as the number of VoIP user, the number of VoIP user per 1MHz, and the efficiency of system are defined and analyzed in Section III. Finally Section IV concludes this paper. II. OVERVIEW OF THE SYSTEMS Basic features of OFDMA, TD-CDMA, 1xEV-DO and WLAN are shown in Table 1. Table 1. Basic Features Feature OFDMA TD-CDMA 1xEV-DO WLAN Bandwidth [MHz] x 2 2 Multiple DS-CDMA DS-CDMA OFDMA Access TDMA TDMA CSMA/CA Duplex TDD TDD FDD - Resource Code & Code & 1 slots Unit Time slot Time slot Time slot Max. 2.4M (Fwd) 3M 2M Rate [bps] 153.6k (Rvs) 54M Frame Length 5ms 1ms 26.67ms - BPSK BPSK QPSK QPSK QPSK 16QAM QPSK 8PSK 16QAM 64QAM 16QAM 64QAM FEC Conv. Conv. Conv./Turbo Turbo Modulation Standard A. OFDMA IEEE Rev.d 3GPP Release 6 cdma2 1xEV-DO Rev. A IEEE 82.11a/e The frame structure of OFDMA system is shown in Fig. 1. User data with the same MCS level are grouped to be coded and be modulated together. Grouped user data consist of DL and UL bursts, and beside the data bursts, overhead information such as preamble, FCH, DL/UL MAP, and Ranging subchannels should also be transmitted with data burst. B. TD-CDMA A radio frame of TD-CDMA system, which is 1ms-sized, consists of 15 time slots. Each time slot carries traffic bursts as much as the number of allocated codes. (1~16 codes are /5/$2. (c)25 IEEE

2 available according to the spreading factor.) One burst consists of two parts of, Midamble, TPC, TFCI and Guard Period as shown in Fig. 2. Random backoff MAC Packet DATA Fig. 4 Transmission by WLAN MAC III. PERFORMANCE EVALUATION Fig. 1 Frame Format of OFDMA Now we compare the performance of the four systems, in view of the number of VoIP users, the number of VoIP users per 1MHz bandwidth, and the efficiency of system. We assume that the packetized voice over IP traffic of 2 bytes is generated by every 2ms, and the 4-byte header is attached to every VoIP packet by RTP, UDP, and IP protocols. In case of using header compression, the header size of the packet is shortened as 2 bytes. of a system is defined as the ratio of the amount of resource used in VoIP packet transmission to the amount of total resource. The number of VoIP user means the total number of user that a system can support by using its total resource, and the number of VoIP user per 1MHz bandwidth indicates the normalized number of user divided by given bandwidth. A. OFDMA Fig. 2 Frame and Burst Structure of TD-CDMA C. 1xEV-DO 1xEV-DO system uses two 1.25MHz bands for forward link and reverse link in a FDD-fashion. Forward link channels consist of preamble, pilot, MAC channel, Control channel, and channel, and are operated in a timedivision fashion. Unlikely to forward channels, reverse channels are operated as code-division mode, so users are classified by Walsh code assigned by the base station. Frame structure of 1xEV-DO is shown in Fig. 3. From the Eqn. (1), the efficiency of OFDMA system is calculated by the ratio of the amount of uplink and downlink data burst to the amount of total resource. Except uplink and downlink data burst, overhead information such as preamble, DL/UL-MAP, FCH, Ranging subchannel, guard band, and unused resource are also included in a frame. The portion of each part is shown in Fig. 5 and Fig. 6, and the efficiency of the system and the number of user are shown in Table 6 and Table 7 for uncompressed VoIP packet and compressed one respectively. number of slots used for data E =.(1) total number of slots (Uncompresed VoIP) Remain UL Burst Used UL Burst Ranging,CQICH,H Remain DL Burst Used DL Burst UL-MAP DL-MAP FCH D. WLAN Fig. 3 Frame Structure of 1xEV-DO Number of VoIP users per frame Fig. 5 Ratio of Payload to s : 82.16d OFDMA, Uncompressed VoIP Differently from aforementioned three systems, WLAN system uses Carrier Sensing Multiple Access with Collision Avoidance (CSMA/CA) as its multiple access scheme. To avoid collision in wireless link, a sender and a receiver exchange RTS/CTS packets to set Network Allocation Vector (NAV), and wait during Inter-Frame Spacing (IFS) before transmitting packet. When packets collide with each other, mobile stations start backoff procedure to resolve contention. Fig. 4 shows an example of data transmission in WLAN system. E fficiency (Com pressed VoIP) Num ber of VoIP users per fram e Fig. 6 Ratio of Payload to s : 82.16d OFDMA, Compressed VoIP Remain UL Burst Used UL Burst Ranging,CQICH,H Unused DL Burst Used DL Burst UL-M AP DL-M AP FC H

3 The amount of resource for data burst is calculated by the difference between the total amount of resource of one frame and the amount of overhead. The number of VoIP user can be obtained by the ratio of the amount of resource for data burst to the size of one VoIP packet. With using 124-FFT, one frame consists of 124 subcarries and 42 symbols. (One symbol consists of 16 slots.) By using the parameters shown in Table 2, we evaluate the number of VoIP user. The OFDMA system serves 25 uncompressed VoIP users and 39 compressed VoIP users per one frame. That is, during 2ms, 1 uncompressed VoIP users and 156 compressed VoIP users can be served by OFDMA system. Table d OFDMA Protocol 1 symbol FCH 2 slots DL-MAP 96 bits + DL-MAP-IE* UL-MAP 56 bits + UL-MAP-IE** Ranging, CQICH, H 3 symbols *DL-MAP-IE : 32 bits x number of burst **UL-MAP-IE : (56+36) bits x number of burst B. TD-CDMA In a TD-CDMA system, total amount of data bits to be transmitted in one radio frame is 25 chips per chip duration, that is 6624 bits per frame in case of QPSK with spreading factor 16. Time slots assigned for signaling channels, midamble, TPC, TFCI and Guard Period in a data burst are counted as overhead, and headers for each protocol layer (i.e., PHY, MAC and RLC) are also considered as overhead. The portion of each part is shown in Fig. 7 and Fig. 8, and the efficiency of the system is shown in Table 6 and Table 7. Differently from the OFDMA system, the TD- CDMA system and the 1xEV-DO system have unused assigned resource, named wasted, even under the fully loaded situation, since the basic resource units of the two systems are unnecessarily large compared to the VoIP packet size. total number of slot of data E =..(2) total amount of resource Table 3. TD-CDMA RLC (TM) Header MAC TCTF 4 bits PHY CRC 16 bits Midamble 256 chips Guard Period 96 chips TPC 2 bits TFCI 16 bits Signaling BCH 1 time slots FACH, PCH 1 time slots RACH 1 time slots VoIP (Uncompressed) Number of Users Fig. 7 Ratio of Payload and s : TD-CDMA, Uncompressed VoIP VoIP (Com pressed) Num ber of Users Fig. 8 Ratio of Payload and s : TD-CDMA, Compressed VoIP unused wasted data BCH+PCH/FACH+RACH PHY+MAC+RLC TPC+TFCI midamble+gp unused w asted data BCH+PCH/FACH+RACH PHY+M AC+RLC TPC +TFCI midam ble+g P The number of VoIP user can be calculated by the ratio of resource for data transmission to the total resource as shown in Eqn. (3). Protocol overhead in Eqn. (3) means the headers from each protocol layer and the resource for signaling channel, shown in Table 3. total number of ( slot code) protocol overhead N =..(3) ( slot code) per VoIP user Assuming that 3 time slots from total 15 time slots in one frame are used for signaling channels, 12 time slots can be used for transmitting user data. That is 6 time slots are used uplink and downlink transmission, so with spreading factor 16, there are 16 codes per one time slot and total 6 time slots as available for user data transmission. The resource unit is one code per one time slot, and 3 time slots are used for transmitting one VoIP packet modulated by QPSK and coded with rate 1/3. (For compressed VoIP packet, 2 time slots are used.) Therefore, one frame serves 16 VoIP users and 24 VoIP users with header compression, and 32 users and 48 users can be served during 2ms as shown in Table 6 and Table 7. C. 1xEV-DO The of 1xEV-DO system is defined as the ratio of the amount of resource for data transmission to the total amount of resource. The resource for data transmission means the difference between total number of bit of data channel and the amount of header from each protocol layer (shown in Table 4), and total resource includes not only the resource for data channel but also the resource for preamble, pilot, and MAC channel. ch. bits ch. overhead E =.(4) ( + Pilot + MAC ch.) + ch. bits

4 Table 4. 1xEV-DO Channel RLP header 22 bits Stream Layer header 2 bits MAC header 48 bits MAC trailer 2 bits PHY FCS 24 bits PHY TAIL 6 bits Fig. 9 and Fig. 1 show the portion of the amount of the resources for transmitting data and overhead and the resource remaining unused. Comparing to the other systems, we can see that the amount of unused resource is the most among four systems. D. WLAN When a packet is transmitted in WLAN system, it additionally takes the time to transmit header and packet, and the time for IFS and backoff procedure except the time to transmit data payload. So we define the efficiency of WLAN system that the ratio of time spent for data transmission to the time for total transmission procedure, as shown in Eqn. (5). T A means the time for procedure A. E = (5) T T xEV-DO (Uncom pressed VoIP) Fig. 11 and Fig. 12 show the ratio of payload to overheads as the number of user varies. We can see that the ratio of overheads is larger than that of the other systems. (Forw ard Link) Remain M AC ch. Pilot P ream ble Utilization a (U nco m pressed) Rem ain Fig. 9 Ratio of Payload and s : 1xEV-DO, Uncompressed VoIP Num ber of VoIP users Fig. 11 Ratio of Payload and s : 82.11a WLAN, Uncompressed VoIP 82.11a (C om pressed) (Forw ard Link) xEV-D O (C om pressed VoIP) rem ain VoIP data overhead MAC ch. pilot pream ble Utilization Fig. 12 Ratio of Payload and s : 82.11a WLAN, Compressed VoIP D Rem ain N um ber of VoIP users Fig. 1 Ratio of Payload and s : 1xEV-DO, Compressed VoIP Because VoIP service is symmetrical on reverse link and forward link, number of VoIP user is bound by the smaller link capacity between reverse link and forward link. Since 1xEV-DO has designed and developed for high-data-rate downlink transmission originally, the capacity of reverse link is much less compare to the forward link. So the number of VoIP user is likely to be limited by the total capacity of reverse link. In [9], by computer simulation, it is computed that the reverse link capacity of 1xEV-DO is 514.1kbps. Assuming that 28.8kbps link is served per user, about 16 VoIP users can be served. In case of using header compression, 19.2kbps link is enough to serve one VoIP user. Then the number of user increases to 26. We can see that there are amount of unused resource in forward link in Fig. 9 and Fig. 1, since the number of user is bound to the capacity of reverse link. The number of VoIP user is also calculated by the ratio of time duration that is used to calculate the efficiency. Eqn. (6) was derived from the simulation results in [12], and we calculate the number of VoIP user with parameters in Table 5. N = ( T T R AVG 1 Table 5. WLAN s MAC Header 3 bytes FCS 4 bytes 16 us 36 us 112 bits PHY 12 symbols Header 1 symbol Service Field 16 bits Tail 6 bits )..(6)

5 E. PERFORMANCE SUMMARY Assume that the VoIP users use the voice codec that generates 2 bytes voice packet, and the 4 bytes RTP/UDP/IP header are added to each packet. That means bytes packet is generated every 2ms, then OFDMA, TD- CDMA, 1xEV-DO and WLAN systems serve 1 users, 32 users, 16 users, and users respectively, and each system serves 1 users, 6 users, 6 users, and 3 users respectively per 1MHz bandwidth. In case of using header compression, the size of header from RTP/UDP/IP decreases to 2 bytes, and each system serves 156, 48, 26, and 65 users respectively and serves 15, 9, 1, 3 normalized users per 1MHz bandwidth. Table 6. Comparison : Uncompressed VoIP Measures OFDMA TD-CDMA 1xEV-DO WLAN VoIP Users VoIP Users per 1MHz (Fwd).63(Rvs).21 Table 7. Comparison Compressed VoIP Measures OFDMA TD-CDMA 1xEV-DO WLAN VoIP Users VoIP Users per 1MHz (Fwd).47(Rvs).14 Fig. 13 shows the 1xEV-DO reverse link and the TD- CDMA system to be efficient for the uncompressed VoIP, and Fig. 14 shows the OFDMA system operated efficiently for the compressed VoIP, respectively. Because the payload size of the compressed VoIP becomes shorter compared to the uncompressed VoIP, the number of users for the compressed VoIP increases. However, the efficiency of the compressed VoIP decreases compared to the uncompressed VoIP. Especially, the efficiency of compressed VoIP in the TD-CDMA system is significantly reduced compared to the uncompressed VoIP because the basic resource unit is not changed for the compressed VoIP and much resource is wasted rather than used. IV. CONCLUSIONS We analyzed four systems, such as IEEE 82.16d OFDMA, 3GPP TD-CDMA, 3GPP2 cdma2 1xEV-DO, and IEEE 82.11a WLAN, and evaluated the performance in case of supporting VoIP service. With flexible and efficient utilization of resource, 82.16d OFDMA serves the most number of VoIP user. It is shown that the efficiency of TD- CDMA and 1xEV-DO systems get lower in case of using header compression, since the size of the resource units of two system are not appropriate to VoIP service. Because of the inefficiency of multiple access scheme, WLAN system serves the less number of VoIP user even though it is operated over the widest band. (1 ) (1 ) VoIP Perform ance(uncom pressed VoIP) Fig. 13 Comparison of : Uncompressed VoIP VoIP Perform ance(c om pressed VoIP) N u m b e r o f V o IP user Fig. 14 Comparison of : Compressed VoIP REFERENCES 1xEV -D O (D) 1xEV -D O (U ) TD-C DM A xEV -D O (D ) 1xEV -D O (U ) TD-C DM A [1] IEEE P82.16-REVd/D5-24, Part 16: Air Interface for Fixed Broadband Wireless Access Systems, May 24 [2] IEEE P82.16e/D3, Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems- Amendment for Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands, Jun. 24 [3] 3GPP TS v6.1., Physical Channels and mapping of transport channels onto physical channels(tdd), Jun. 24 [4] 3GPP TS v6.., Multiplexing and channel coding(tdd), Dec. 23 [5] 3GPP TS v6.2., Medium Access Control(MAC) protocol specification, Jun. 24 [6] 3GPP TS v6.1., Radio Link Control(RLC) protocol specification, Jun. 24 [7] 3GPP2 A.S7-A v2., Interoperability Specification (IOS) for High Rate Packet (HRPD) Access Network Interfaces - Rev A, May 23 [8] 3GPP2 C.S24-A v1., cdma2 High Rate Packet Air Interface Specification, Mar. 24 [9] Qualcomm, Summary of Simulation Results for QUALCOMM s 1xEV-DO Rev. A Proposal, Sep. 23 [1] ANSI/IEEE Std 82.11, Part 11: Wireless LAN Medium Access Control(MAC) and Physical Layer(PHY) specifications, 1999 [11] IEEE Std 82.11a, Part 11: Wireless LAN Medium Access Control(MAC) and Physical Layer(PHY) specifications: High-speed Physical Layer in the 5GHZ Band, 1999 [12] S. Garg, Can I Add a VoIP Call?, Proceeding of IEEE International Conference on Communication, Jun. 23 NOWLEDGEMENT - This work was supported in part by Samsung Electronics co. Ltd.