ADAPTIVE, ASYNCHRONOUS INCREMENTAL REDUNDANCY (A2IR) WITH FIXED TRANSMISSION TIME INTERVALS (TTI) FOR HSDPA Arnab Das, Farooq Khan, Ashwin Sampath and Hsuan-Jung Su Bell Labs, Lucent Technologies Holmdel, NJ 07733, USA
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[email protected] Abstract – In anticipation of the high demand for wireless data services, several wireless standards are evolving to support packed data more efficiently. In UMTS, a new, shared channel called the High Speed Downlink Shared Channel (HS-DSCH) that multiplexes packet data users has been defined. Several performance-enhancing technologies are included in High Speed Downlink Packet Access (HSDPA): Adaptive modulation and coding (AMC), Hybrid ARQ (HARQ), scheduling etc. HARQ algorithm plays a critical role in HSDPA performance. HARQ techniques are classified in terms of the combining method (Chase or Incremental Redundancy, IR), protocol timing (synchronous or asynchronous) and adaptivity (if modulation/code-rate can be changed for retransmission or not). In previous work ([6] and [7]), an adaptive, asynchronous, incremental redundancy (A2IR) scheme was proposed and shown to provide substantial gains over other HARQ options. In this paper, the design and performance aspects of A2IR in the code domain (as against the time domain in [6] and [7]) are considered. Due to the higher bandwidth of UMTS channels compared to cdma2000 or HDR, it is expected that code-division multiplexing (CDM) of users will be needed to “fill the data pipe” instead of pure time-division multiplexing. The paper also analyzes tradeoffs that are unique to implementing A2IR with CDM. A2IR with a fixed TTI is now part of UMTS Release 5.
between HARQ, AMC and scheduling are important to consider. In this paper we provide results for an adaptive, asynchronous, incremental redundancy (A2IR) scheme (originally proposed in [6]) for the case of fixed transmission time interval (TTI) and code division multiplexing of users. Adaptivity in HARQ retransmissions is provided by varying the number of codes rather than varying the TTI as done in [6]. Keeping the TTI fixed may be desirable from backward compatibility and User Equipment (UE) complexity points of view. Detailed design aspects of A2IR in the code domain are covered along with performance analyses that contrast the performance of A2IR with other HARQ combining methods such as Chase combining and non-adaptive Incremental Redundancy (NAIR). Also, operational tradeoffs unique to HARQ in the code domain, such as whether to allow parallel simultaneous transmissions to a UE (mobile) within a TTI or not, are also covered. A2IR with a fixed TTI has been adopted for UMTS Release 5. The paper is organized as follows. In Section II an overview of different HARQ schemes and their key attributes is provided. Detailed design and performance of A2IR are covered in Section III, while concluding remarks are made in Section IV.
I. Introduction Several performance-enhancing technologies are included in the UMTS High-Speed Downlink Packet access (HSDPA) specifcation to ensure high packet data rates while supporting circuit switched voice and packet data on the same spectrum [1]. These techniques include Adaptive Modulation and Coding (AMC), Hybrid ARQ (HARQ), scheduling etc (see [8] for an overview of adaptation techniques for wireless data). Scheduling treats all available resources for HS-DSCH collectively and selects user(s) to assign the resources to, typically based on channel quality information (CQI) feedback from the UE. Typically, a CQI sensitive scheduler that picks users with “good” channel quality would be employed. AMC selects the modulation and coding scheme (MCS) to suit the selected user’s current CQI. AMC can still result in errors due to feedback delays, noisy measurements and channel variability during the packet transmission time. To partly mitigate this problem, hybrid ARQ is proposed for HSDPA. The attributes of the HARQ protocol used play a significant role in HSDPA system performance and capacity. Also, the interaction
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II.HARQ Schemes for HSDPA – an overview The HARQ schemes considered in this paper are all StopAnd-Wait (SAW) schemes wherein a new (re) transmission on a SAW channel may be done only when an acknowledgement (ACK) (negative acknowledgement (NACK)) for the previous transmission is received [3]. For the purpose of this study, HARQ schemes may be classified based on three characteristics: combining method, timing and adaptive operation. A.Combining Three types of HARQ schemes are defined in literature – Types I, II and III (see [5] and references cited therein). In Type-I no combining of transmissions is done. HARQ-Type I when used in conjunction with AMC, is just link adaptation (LA) with retransmission. In Type II schemes erroneous transmissions of a packet are combined with each retransmission. Type-III is a variant of Type-II wherein each retransmission is self-decodable. A few definitions are
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needed: a code block is a set of information bits, N, (including a CRC) to be transmitted in a TTI. Each code block is encoded using a channel encoder, typically a Turbo encoder, as shown in Figure 1. If the encoder base rate is r (e.g. r=1/3), then N/r coded bits are produced by the encoder. Coded sub-blocks are, in general, formed by puncturing the coded bits in some manner. For example, Figure 1 shows the formation of four coded sub-blocks from the coded bits. Even though the figure shows a case where the sub-blocks are all disjoint, this need not be the case at all. In general a subset of the coded bits may be selected for first transmission of a packet depending on the data rate selected by AMC. One example of HARQ operation would be that sub block A11 is sent for the first (new) transmission and, if in error (i.e. NACK reception at transmitter), each time one or more of the other sub-blocks is transmitted. Code Block A1
Coded Sub- Blocks Channel Encoder e.g Turbo encoder
NEW Sub-block
A11
A12
A13
A14
RETX Sub-block
Figure 1. Code block and sub-block formation.
The two main combining schemes considered for HSDPA are - Chase combining [4] (Type III with one redundancy version) and Incremental Redundancy (IR) (either Type II or III). Chase combining involves the retransmission of the same coded sub-block in each retransmission. In IR, instead of simple repetitions, additional redundancy (parity bits) information may be transmitted with each retransmission until the base code rate is reached. Thus, one main difference between Chase combining and IR is that, with IR, the code rate will decrease in general, with each retransmission (consequently providing coding gain) while for Chase combining the code rate stays fixed at the initially selected value. From a pure link-level perspective, the performance of IR is thus lower bounded by the performance of Chase combining. However, IR introduces additional memory requirements at the receiver compared to Chase combining. B.Timing When the timing relationship between the first transmission and retransmissions is fixed, HARQ operation is deemed synchronous. If retransmissions may be scheduled at any time after receiving a NACK, it is termed asynchronous. In synchronous operation, the transmitter has flexibility in choosing time only for the first transmission of a code block (i.e. transmission of the “NEW” sub-block as shown in Figure 1). Subsequent retransmissions, if needed, can take place at fixed time intervals relative to the first transmission. For example, with (N-1)-slots feedback delay, if the first
transmission took place in SLOT#i, then a retransmission must take place in SLOT number (i+N). If that retransmission fails, then the next retransmission will take place in SLOT number (i+2N) and so on. An example of synchronous SAW scheme is in Figure 2. A11
A12 NACK(A)
A13
A14
NACK(A)
NACK(A)
A21 ACK(A)
Figure 2. Synchronous stop-and-wait IR
Synchronous HARQ has the advantage that UE and SAW channel identification do not have to be sent to the UE at the time of retransmission. Thus, synchronous HARQ requires quite low control information overhead. However, synchronous operation has several disadvantages but mainly the loss of multi-user diversity, since scheduling is constrained. Multi-user diversity can be fully exploited only by allowing fully asynchronous operation (see Figure 3) wherein transmissions/retransmissions to any user on any SAW channel can be done at any time. In typical operation, the extra control overhead compared to synchronous HARQ is usually well justified. The retransmission to user A can now be performed in the 6th SLOT even though the interval between transmissions is not an integer multiple of N. 1
2
3
4
A11
5
6
7
8
B11 A12 NACK (A)
NACK (B)
9
10
11
12
B12
13
14
15
16
B21
B13 NACK (B)
17
ACK (B)
Figure 3 An example of fully asynchronous operation. A and B represent two different users.
C.Adaptivity An HARQ scheme is called adaptive if retransmissions may be performed using a different modulation scheme compared to the original transmission. Adaptive operation is desirable because the channel conditions and/or available resources (power and/or channelization codes) at the time of retransmission may be different from the time of the original transmission. Furthermore, with an estimate of the aggregated energy for an outstanding code block from previous transmissions, the rate (MCS and number of codes) at retransmission can be selected so that just the required residual energy for “successful” decoding is supplied. Both Chase combining and IR can be operated in an adaptive mode.
III.Adaptive, Asynchronous, Incremental Redundancy (A2IR) – Design and Performance A.Adaptive Operation Overview As the name suggests A2IR is an HARQ scheme based on IR and is both asynchronous and adaptive. In this study, MCS selection at retransmission is based both on channel conditions and an estimate of the residual energy required for the packet to succeed. Retransmissions that do not take into account the aggregated energy of previous transmissions of a code block would often provide too much energy at the receiver. For example, consider a non-adaptive scheme and assume that for a particular MCS an Eb/N0 of 1(=0 dB) is necessary for successful decoding (i.e. very low block error probability). Suppose the first transmission resulted in an Eb/N0 of 9/10 (=-0.5 dB). If at the time of retransmission the channel is unchanged then retransmission with a nonadaptive scheme will deliver at least 3 dB of extra Eb/N0 to the receiver, which in this case is clearly excessive. The A2IR scheme will deliver just the right amount of energy (within the coarseness provided by the rate selection table) for the packet to succeed with high probability. In the case of variable TTI [6], the resultant higher rate selected would shorten the TTI for retransmission and free up time slots for scheduling other users. With fixed TTI considered here the resultant higher rate selected would free up codes for assignment to other users. The adaptive scheme uses link quality feedback valid during previous transmissions of a frame to obtain an estimate of the aggregated energy for that frame at the receiver. That information is used in conjunction with the most recent link quality feedback to determine the MCS and the appropriate number of codes required for the frame to be successful. B.Design The rate table corresponding to the design of the A2IR scheme is shown in Table 1. The Transmission Time Interval (TTI) is fixed to be 3 slots (i.e. 2 ms, for a UMTS slot of 0.667ms). Different code block sizes are allowed and the selected code block size for transmission determines the data rate. The data rate and the MCS can be selected (number of codes is then implicit) to best match the channel conditions for the user. The rate table assumes that a maximum of 10 channelization codes of Spreading Factor (SF) = 16 are available for HSDPA. The rate table provides flexibility in matching the user’s backlog as best as possible (via column selection) and the sustainable modulation/code rate (using channel quality information), based on row selection. While multiple ways for MCS/code block selection from the table are possible, the following method is used in this paper. Based on the number of codes available and the data backlog, the best MCS that can be supported is selected. User’s backlog may be rounded up to the nearest code block size via padding or data may be segmented.
Retransmissions are performed by selecting within the column corresponding to the code block. The number of codes required (and corresponding modulation and code rate) are selected to deliver the required residual energy to the receiver. The entries marked “X” in the table correspond to non self-decodable transmissions (i.e. code rate>1 even with the highest modulation level) and may only be used for retransmissions with A2IR. Table 1: Data Rate and MCS table. Q=QPSK, 16Q=16QAM. Code s
10 8 6 4 2
MCS, Code Block Size and Data Rates 1280 bits
2560 bits
3840 bits
5120 bits
7680 bits
11520 bits
15360 bits
640 Kbps
1280 Kbps
1920 Kbps
2560 Kbps
3840 Kbps
5760 Kbps
7680 Kbps
Q, 0.13 Q, 0.17 Q, 0.22 Q, 0.33 Q, 0.67
Q, 0.27 Q, 0.33 Q, 0.44 Q, 0.67 16Q, 0.67
Q, 0.4 Q, 0.5 Q, 0.67 16Q, 0.5 X
Q, 0.53 Q, 0.67 16Q, 0.44 16Q, 0.67 X
Q, 0.8 16Q, 0.5 16Q, 0.67 X
16Q, 0.6 16Q, 0.75 X
16Q, 0.8 X
X
X
X
X
X
X
C.Traffic Model and Simulation Assumptions The traffic model used in the HSDPA study consists of a fixed number of UEs, each in a Web-browsing session. The web-browsing session comprises of several packet calls (web page downloads) followed by a “think time” interval to view the downloaded contents. At the end of the “think time” interval, the UE downloads another web page and so on. A packet call completion corresponds to web page downloaded completely at the UE. Each packet call is characterized by its size in bytes based on the content of the web page and arrives at the base station as segmented Internet Protocol (IP) packets of a fixed size. Since this study is mainly to compare HARQ schemes and to determine the achievable throughputs when advanced technologies are employed, only an open-loop traffic model is considered here and therefore, the effects of protocols such as TCP are not accounted for. The traffic model used in the HSDPA study (see [2]) is shown in Table 2. Other simulation parameters used are as in Table 1 in [2] and are not repeated. Additional assumptions: -
70% of cell’s power is used for HSDPA
-
CQI feedback delay is 6 slots (4ms) and ACK/NACK delay is 3 slots (2ms)
-
Neighbor cells are assumed to be fully loaded and transmitting at constant (maximum) power
-
ACK/NACK is assumed to be error free. CQI is assumed to be unquantized and error free as well.
-
Only one transmission per UE per TTI, unless indicated otherwise.
-
Maximum C/I scheduler is used.
Process
Random Variable
Packet Calls Size
Pareto with cutoff
Parameters Α=1.1, min=4.5 KB, max=2 MB, mean = 25 KB
Time Between
Geometric
mean = 5 seconds
Fixed
1500 bytes
Geometric
mean = 6 ms
Packet Calls Packet Size Packet Inter-arrival Time (open loop)
Throughput metrics used are Over-The-Air (OTA) Throughput, Service Throughput and Packet Call Throughput (PCT) as defined below Total good bits (Total slots with transmissions)×Slot duration Total good bits Service throughput = Total slots ×Slot duration Num_pkt_calls 1 Bits in pkt call j Packet Call Throughput= ∑ j=1 Num. pkt calls Duration of pkt call j OTA=
.
Utilization is defined as Utilization =
Total slots with transmissions Total slots
.
In addition, the cumulative distribution function (CDF) of the user average packet call throughput is also provided as a measure of quality of service (QoS). Note that none of the average throughput metrics defined above capture the perceived per user throughput; service throughput and OTA are based on the total transmissions on the air-interface while packet call throughput is an average over all the packet calls in the system, not over the users. The PCT CDF is the only per user metric and hence important to consider as a QoS and system capacity metric. D.Results The HARQ schemes compared in this study are Chase combining, Non-Adaptive IR and A2IR. Results with pure link adaptation with retransmissions (i.e. no combining) are also shown. At a UE speed of 3km/hr, A2IR provides the best performance although the gains from Hybrid ARQ itself are small. The three HARQ schemes, Chase combining, NAIR and A2IR, are all quite close to the performance of pure link adaptation (LA). This result is not surprising because, at 3km/hr, the channel changes slowly (relative to CQI feedback delay) and AMC itself is quite accurate. However, if measurement and quantization errors in CQI are considered, then HARQ combining can provide sizeable
gains even at slow speeds. Also, in some cases when the data rate granularity provided by the rate table is low, A2IR can provide substantial gains at slow speeds. At a UE speed of 30 km/hr, the feedback delay between the time of channel quality measurement and the time at which the Node B receives the CQI report is large enough for the channel to have changed somewhat in the interim. It is precisely in such situations that AMC requires the additional robustness to MCS selection provided by Hybrid ARQ. The user packet call throughput CDFs shown in Figure 4 shows that even Chase combining greatly outperforms pure link adaptation, indicating that in this case HARQ combining is necessary. Furthermore the two IR schemes, NAIR and A2IR, perform considerably better than Chase combining, indicating that the coding gain achieved with retransmissions using IR does provide gain. Finally, comparing the two IR schemes, A2IR outperforms NAIR substantially. This illustrates the fact that adaptivity via rate selection using the residual energy is able to exploit multi-user diversity to a greater degree. If we keep increasing the number of UEs for A2IR, we find that the cdf of A2IR with 70 UEs is close to that of NAIR with 56 UEs. Thus, A2IR yields a gain of around 33% in system capacity over NAIR. The A2IR scheme more than doubles the capacity obtained with link adaptation and provides better than 50% improvement in system capacity as compared to Chase combining. Similar gains are observed when we compare service throughputs for A2IR with 70 UEs (85 UEs) with NAIR (Chase) 56 UEs. Also note the importance of comparing packet call throughput CDFs because if we just compare average throughputs for A2IR with 56 UEs with either Chase combining or NAIR with 56 UEs, the gain is less than 10% (see Table 3). Thus the average throughputs, by the nature of their definitions tend to hide the true gains obtained with A2IR over other HARQ combining methods. (Note that, for all HARQ schemes, PCT decreases with increasing load due to greater queuing delay). 100 90
%UE with PCT < abscissa
Table 2: Traffic Model Parameters (Source: Table 2 in [2]).
A2IR – 85 UEs
80
Chase
70
LA
60 50
A2IR
40 30
A2IR – 70 UEs
20 10
NAIR
0 0
500
1000
Kbps
1500
2000
2500
Figure 4: Comparison of packet call throughput CDFs for the different HARQ schemes at UE speed of 30 km/hr. Results are for 56 UEs unless otherwise labeled.
Table 3: Average throughput metrics at 30km/hr. Results are for 56 UEs unless otherwise stated.
HARQ Scheme
OTA
Pkt Call Throughput
Service Throughput
Utilization
LA Chase NAIR A2IR A2IR – 70 UEs A2IR – 85 UEs
1369 1756 1957 2129 2375
775 855 1004 1075 982
1369 1724 1851 1960 2310
1.00 0.98 0.95 0.92 0.97
2629
929
2621
1.00
IV.Conclusions Advanced technologies such as Adaptive Modulation and Coding (AMC), Hybrid ARQ (HARQ) and scheduling will be part of UMTS evolution to HSDPA. Hybrid ARQ combining, timing and adaptivity are important considerations for performance in this context An adaptive, asynchronous version of incremental redundancy (A2IR) was proposed (see [6]-[7]) and shown to provide considerable gains over Chase combining and synchronous HARQ schemes. While the original version of A2IR assumed a variable TTI as the enabler of adaptivity, this paper outlines a design where adaptivity can be supported even with a fixed TTI (version is part of HSDPA standard), via code division multiplexing. This paper outlines the design and performance aspects of A2IR with a fixed TTI and demonstrates that A2IR with a fixed TTI retains its substantial performance advantage over Chase combining (up to 50% gain) and over Non-Adaptive IR (up to 25% gain). An important question with code division multiplexing is whether to support parallel simultaneous transmissions to the same UE in an HSDPA TTI or not. Simulation results indicate that, at 3km/hr speed and medium loads, up to 50% gain in system capacity is achievable when parallel transmissions to the same UE are allowed. While the UMTS Release 5 version of HSDPA does not allow multiple simultaneous transmissions to a UE, future evolutions may allow this feature. Technologies such as fast cell selection (FCS) and multi-input multi-output (MIMO) antenna processing are also being considered for HSDPA evolution.
While the previous results for A2IR applied to HSDPA [7] assumed a purely TDM system (one user at a time) with a variable TTI, the A2IR design in this paper uses a fixed TTI (of 2ms) with the possibility for parallel simultaneous transmissions to UEs via code division multiplexing. An important question in this regard is whether parallel simultaneous transmissions to the same UE on different Stop and Wait (SAW) HARQ channels should be allowed or not. For example, in a TTI, one may schedule a UE to receive a retransmission on HARQ SAW channel #1 using 4 codes and a new transmission on HARQ SAW channel #2 using 6 codes. The advantage of this feature is better exploitation of multi-user diversity since we transmit as much data as possible to the best user at the time. Also, the scheduler operates in an unrestricted manner and is hence less complex. The disadvantage of this feature is that UEs may have to simultaneously decode parallel transmissions and associated control channel signaling on multiple HARQ SAW channels, thus slightly increasing their complexity. Figure 5 shows that the cdf of 56 UEs without restriction closely matches the cdf of 37 UEs with restriction at 3km/hr for A2IR. This represents a better than 50% improvement in capacity. The gains extend to higher loads and also to higher UE speeds, but are generally smaller (10-15% range).
References [1] 3GPP TS 25.308, “High Speed Downlink Packet Access (HSDPA); Overall Description” V5.2.0, March 2002. [2] Ericsson, Motorola, Nokia, “Common HSDPA System Simulation Assumptions” 3GPP TSG RAN WG1, TSGR1#15(00)1094, Meeting 15, Berlin, Germany, Aug.2000.
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[3] R. Gallagher and D. Berteskas, Data Networks, Prentice Hall, 2nd Edition, 1992.
% UE w PCT
