MAC LAYER RETRANSMISSIONS IN 1XTREME
Mainak Chatterjee , Giridhar D. Mandyam and Sajal K. Das Center for Research in Wireless Mobility and Networking (CReWMaN) Department of Computer Science and Engineering University of Texas at Arlington Arlington, TX-76019-0015 E-mail: chat,das @cse.uta.edu Nokia Research Center 6000 Connection Drive Irving, TX-75039 E-mail:
[email protected]
Abstract - In this paper, we study the joint reliability as offered by the medium access control (MAC) and the radio link protocol (RLP) in CDMA systems, particularly for 1XTREME. The retransmission mechanism in the RLP has considerable amount of delay associated with it which might not be able to sustain real-time communications with strict delay requirements. Retransmissions done at a lower layer, such as the MAC, enhances the performance of the system. We show how the performance varies with respect to delay and throughput as observed from the RLP, for finite number of retransmissions at the MAC. We find that there is a substantial gain in the performance with the incorporation of fast retransmissions at the MAC. Synthetically generated HTTP traffic was used as the application, the objects of which were fragmented into equal sized RLP frames. We consider a 1-path Rayleigh fading channel with correlated errors. We also consider that soft packet combining at the receiver is supported, which effectively lowers the frame error rate (FER). Simulation experiments are conducted to verify the performance as offered by the MAC and the RLP. Keywords - CDMA networks, radio link protocol, medium access control, retransmission, packet combing, 1XTREME. I. I NTRODUCTION Transport Control Protocol (TCP) is still the major suite for the Internet Protocol (IP) and provides reliable end-toend transmission. All the emerging wireless data technologies today rely on the IP-based network because IP is the most dominant inter-networking protocol. As the World Wide Web (WWW) traffic continues to grow, it is important that these kinds of services are also supported in the wireless domain. The design of TCP has been such that it performs well in wireline networks where the channel error rates are extremely low and whatever congestion occurs is due to loss of packets. However, when TCP is used in the wireless domain which is characterized by high bit error rate, the performance of TCP severely degrades [3]. Any packet loss at the wireless link is interpreted as congestion by TCP which responds to it by reducing the transmission window size, initiating the congestion control mechanism and resetting the retransmission time. The congestion control mechanism designed for wireline networks causes an unnecessary reduction in the TCP throughput.
0-7803-7589-0/02/$17.00 ©2002 IEEE
Several schemes have been proposed to alleviate the effects of non-congestion related losses over wireless links [2], [4], [7]. Radio Link Protocol (RLP) is one such scheme which shields the effect of the loss over wireless links from the TCP layer [3], [6]. The RLP is generally employed between the physical layer and the TCP layer. It breaks down an upper layer packet (a TCP segment in this case) into several RLP frames before transmitting over the wireless channel as shown in Figure 1. A physical layer header is added to the RLP frame before it is mapped on to a physical layer frame for transmission. The fragmentation is done to reduce the granularity of the transmission, i.e, in case of any error, an RLP frame which is of a smaller size is affected rather than the whole TCP segment. In case of an RLP frame loss during transmission, the RLP uses an Automatic Repeat reQuest (ARQ) error recovery mechanism to retrieve the lost RLP frame. The process for recovery of erroneous frames is initiated by the receiver by requesting retransmission of the erroneous frames. Recovery of the erroneous frames should be done before the TCP timer expires for the TCP throughput to remain unaffected. Radio link protocols are usually sufficient to shield the physical layer impairment from the TCP, but might fail to do so if the application has very strict delay requirements. The delay associated with retransmissions at the RLP might not be small enough to sustain a real-time application. Similar problems will arise if we deal with interactive real-time traffic. In such scenarios, the TCP timer might time-out before the RLP recovers a missing frame. To tackle this problem, another fast retransmission mechanism must be incorporated below the RLP layer. This can be done by an ARQ mechanism at the MAC layer, thus providing two layers of retransmission reliability. Our main motivation behind this work is to study the reliability offered jointly by the RLP and the MAC layers of CDMA systems (particularly for 1XTREME) in supporting HTTP traffic. We analyze the delay and throughput as experienced by the MAC and the RLP for finite number of retransmissions. We model the wireless channel as 1-path Rayleigh fading with correlated errors and consider soft packet combining at the receiver which effectively lowers the frame error rate (FER). Simulation experiments are also conducted to validate the correctness of the model. It is observed that there is tremendous
PIMRC 2002
gain if the MAC offers at least one retransmission, however, the gain does not increase substantially if the number of retransmissions is made more than two. The recovery rate of missing frames at the RLP is prominent at higher FER and also when the misinterpretations of acknowledgements are high. TCP Segment / IP Packet
header
Payload
header
RLP frame No. 1
header
Payload
header
RLP frame No. 2
Payload
RLP frame No. L
RLP frame
Physical Layer Packet
Fig. 1. Fragmentation of TCP segments II. R ETRANSMISSIONS
AT
RLP
AND
MAC L AYERS
In this section, we will briefly introduce the necessary background on 1XTREME required for this study. We will mainly discuss the retransmission mechanism at the RLP and the fast ARQ mechanism at the MAC layer. A. 1XTREME 1XTREME (1X ThiRd-generation Enhanced Modulation and Encoding) is an IS-2000 compatible standard which supports both voice and data on the same CDMA channels. It is one of the active projects of the 3GPP2 standards body wherein the modifications and enhancements to the IS-2000 CDMA standard are dealt with. This evolution of IS-2000 tries to meet the increase in demand for both voice (real-time) and data (best-effort) services. 1XTREME is one such system which supports a mix of both best-effort packet data services and realtime voice services on the same carrier. Some of the features of 1XTREME are multicode channel aggregation, higher order modulation, link adaptation and hybrid ARQ. More detailed discussion on 1XTREME can be found in [1], [8]. B. Retransmissions at RLP There are various ways in which the RLP acknowledges the correctly received packets. The IS-95 and its evolutions, IS-95B and cdma2000, use NACK based RLP with a finite number of retransmissions. When the RLP finds a missing frame or a frame in error, then it sends a NACK requesting for a retransmission. If the requested frame is not received within a stipulated time (given by a timer), then another NACK is sent to the transmitter and the timer is reset. Each NACK received at the transmitter triggers a retransmission of the requested frame. Since the number of allowable repeated requests for a frame is finite, the RLP cannot completely eliminate detectable errors. After the maximum number of retransmission attempts have been made without success, the RLP aborts its recovery process and passes on the responsibility to the higher layer (TCP).
Then TCP uses its own ARQ mechanism to recover the missing frames. In a selective NACK based scheme, NACKs are sent out only on the detection of missing frames. Therefore, the transmitter takes no action unless and until a NACK is received. Conversely, in an ACK based scheme, the transmitter waits for the ACKs. The optimization problem here is to find the optimal timer value i.e., the time a transmitter has to wait before it can retransmit. If the timer is set to a low value then retransmissions will be triggered even for correctly received frames, thus lowering the throughput. On the other hand, if the timer is set to a high value then the delay of sequencing and reassembling the packets would be high. So, an optimal value of the timer is desirable and should be customized for a particular kind of channel. To avoid the delay associated with retransmissions at the RLP, a faster lower layer MAC-ARQ can be used. The faster retransmissions can provide a better round trip time for realtime applications. Since the number of transmissions allowed at the MAC layer is finite, it does not completely eliminate the possibility of having missing or damaged frames. If the MACARQ fails to deliver a frame correctly even after retransmitting the maximum allowed number of times, then responsibility is passed on to the RLP to retrieve the frame. Thus we deal with two layers of retransmission reliability as shown in Figure 2. From application
To application Air interface
RLP
RLP
RLP header added header
RLP NACK
Payload
header
MAC
Payload
MAC
MAC header added header
RLP header removed
Payload
Transmitter side
ACK/NACK Packets transmmitted
MAC header removed header
Payload
Receiver side
Fig. 2. Two layer reliability C. Fast MAC-ARQ The fast ARQ mechanism (also called MAC-ARQ) in 1XTREME ensures that some performance loss can be recovered. There are two reasons why RLP cannot provide the functionality needed for MAC-ARQ in 1XTREME. First, in the process of selecting the base station with the strongest signal for the cell selection, RLP terminates at the last network element (for example, base station) resulting in network delays in servicing retransmission requests at the RLP layer. Second, the forward shared control channel might contain several protocol data units (PDU’s), not all of which come from RLP. As a result, the MAC-ARQ provides retransmissions quickly in 1XTREME which employs a stop-and-wait hybrid ARQ method. In this method each packet received by the receiver must be acknowledged on a dedicated feedback channel to the transmitter. This dedicated feedback takes the form
of the reverse acknowledgment indicator and takes values for an ACK or for a NACK. However, the receiver does not discard the received soft-information associated with the incorrectly-received packet. Rather, it buffers the data and coherently combines the buffered data with the received soft information of the retransmission of the bad packet [9]. This type of packet combining provides increased reliability in CDMA systems. 1XTREME uses -phase stop-and-wait MAC-ARQ. By “ phase” it is meant that multiple ARQ instances are employed in consecutive time slots (i.e., 5-ms frame durations). For instance, assume 3 ARQ channels are used. Then in time slot , the receiver will receive a packet corresponding to phase 1. In time slot , the receiver will receive a packet corresponding to phase 2. In time slot , the receiver will receive a packet corresponding to phase 3. In time slot , the receiver will receive a packet corresponding to phase 1 again, and so on. The receiver must keep separate packets received for different phases for packet combining and packet acknowledgements; however, once any packet for any phase is received correctly, the receiver may deliver the packets to the higher layers (e.g. RLP). The timing diagram for this scheme is explained later in Section IV-B.
D. Packet Combining In order to ensure that the receiver does not try to combine packets from one ARQ phase with another, outband signaling is sent on the forward shared control channel concurrently with each frame that the receiver receives on the forward shared channel. It can be noted that the FER experienced by a packet during its 1st transmission is not the same as that during the 2nd transmission. It is less in the latter case because there is at ) due least a 3-db gain in the bit energy-to-noise ratio ( to packet combining.
III. P ERFORMANCE M ODELING We will mainly concentrate on the delay and throughput as experienced by the RLP in 1XTREME. We do not consider the time taken for a packet to get scheduled for transmission at the RLP. The delay considered is the delay suffered by a packet before it is received correctly by the RLP at the receiver and delivered to the upper layer. We define throughput as the ratio of the number of successful packets received to the total number of packets undergoing transmission. RLP recovery is defined as the fraction of the packets recovered by the RLP when the MAC fails to deliver a packet correctly. We assume that there is no transmission delay, i,e., if a packet is successfully transmitted at the first trial then the delay incurred is zero. This assumption can be taken care by adding an offset (with a statistical fluctuation) to the delay value. It can ms, where is the number be noted that it takes about of ARQ phases and is the frame duration, for the ACK to reach the transmitter. It is only then that the transmitter can remove the packet from its transmission buffer. If a packet does not successfully reach the receiver at the first trial, then on the receipt of a NACK, it will undergo retransmission. Some delay
will be incurred while the MAC tries its fast ARQ mechanism to retransmit and recover the missing packet. The maximum number of retransmissions allowed is given by the system parameter MAXRETRANS. If the packet is still not recovered, then the RLP triggers its own retransmission mechanism. Thus, the total delay ( ) experienced by a packet to successfully reach the receiver can potentially have two components: ) due to the MAC-ARQ mechanism and the the delay ( delay ( ) due to the RLP retransmissions. Hence the total delay can be defined as
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A. Channel Characteristics In a wireless environment, the characteristics of the radio channel is very important since the channel conditions vary with time and space. There are several negative factors that affect the link attenuation. The hindrance may be due to multipath fading, shadowing or any other noise source. The most commonly used AWGN (additive white Gaussian noise) channels do not represent the channel conditions accurately. A more realistic view of the channel is necessary for any kind of analysis or experimentation. Moreover, the channel model should include the correlated errors, which is most often the case. Therefore, we consider 1-path Rayleigh model for the losses which generally has the worst behavior amongst multipath Rayleigh models for CDMA systems. B. Effective FER Whenever a packet is not correctly decoded at the receiver, the packet is not discarded but stored in the buffer. It has been observed that there is a gain of 3-db in the value if the retransmitted packet is combined with the stored packet. If the packet is not correctly decoded even after the packet combination, the packet is retransmitted for the second time (if ). With the second retransmitted packet MAXRETRANS the gain in would be 6-db. That is, with every retransmission there is a gain of 3-db. If we consider 1-path Rayleigh channel with rate 8-PSK modulation, then according to simulations the effective FER would approximately fall to onethird its value for every transmission. The FER for successive retransmissions (RET) is shown is Table 1. These values were obtained through simulation of the said channel. Although, the decrease in the effective FER could not be generalized, it works well with Rayleigh fading channels. For example, if the FER is 0.3 for the initial transmission (Tx), then on successive retransmissions the effective FER would be 0.095, 0.031, 0.0095 and so on, decreasing by approximately one-third every time. C. Delay and RLP Recovery Analysis Let us now derive the expressions for mean delay and throughput for MAXRETRANS = . We assume that the raw FER offered by the physical channel is . When a packet is transmitted, the MAC usually waits for certain number of ARQ phases (NARQP = ) for the ACK. We will derive the expressions for and first and then derive the general expression.
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