Performance Evaluation of Grant-Free Transmission for ... - IEEE Xplore

Performance Evaluation of Grant-free Transmission for Uplink URLLC Services Chao Wang1 , Yan Chen1 Yiqun Wu1 and Liqing Zhang2 1

Huawei Technologies, Co. Ltd., Shanghai, China Huawei Technologies Canada Co. Ltd., Ottawa, Ontario, Canada {wangchao78, bigbird.chenyan, wuyiqun, liqing.zhang}@huawei.com 2

Abstract—5G wireless networks are expected to support new applications demanding ultra high reliability and ultra low latency, such as self-driving cars, industrial control, and real-time gaming. Such services are known as ultra-reliable and low-latency communications (URLLC) targeting at 99.999% transmission correctness within 1ms delay bound. Current Long Term Evolution (LTE) system is far from supporting such services,especially for uplink. Even with around 10 times shortened transmission interval, due to the waste of time in requesting and receiving the scheduling grant, only one shot uplink transmission can be supported within 1 ms, which is very hard to meet the reliability requirement. Grant-Free transmission is thus proposed to effectively save the time of requesting/waiting for grant and thus relax the transmission interval design in frame structure. This paper presents the design of grant-free transmission for URLLC services and also the evaluation results. We shall show that the saved time can be used for more Hybrid Automatic Repeat reQuest (HARQ) retransmissions and thus further enhance the reliability. Moreover, contention based grant-free transmission with proper HARQ design can well meet the reliability requirement. Finally, the performance can be further enhanced with non-orthogonal multiple access schemes such as Sparse Code Multiple Access (SCMA) and advanced receivers such as Message Passing Algorithm (MPA) or Expectation Propagation Algorithm (EPA) receiver.

I. I NTRODUCTION One big difference between 5G and 4G is its extreme diversity in service requirement. In addition to the evolution of mobile broadband (eMBB) services, which demand very high capacity and throughput, there are emerging services such as self-driving cars, industrial control, and real-time gaming, demanding ultra low latency and ultra high reliability communications (URLLC), which has been defined by International Telecommunication Union (ITU) as one of the three application scenarios of 5G. The target latency and reliability requirement suggested by ITU for URLLC services 99.999% transmission correctness within 1ms delay budget, namely the block error rate (BLER) should be below 10−5 within total time budget of 1ms [1]. The delay budget should cover the following. ∙ ∙ ∙

Time to get transmission grant, which depends on protocol design and is constrained by hardware capability; Signal processing time at transmitter side, which is usually very short; Signal processing time at receiver side, which depends on the receiver structure and complexity;

Transmission interval, which depends on numerology and frame structure design; ∙ Hybrid Automatic Repeat reQuest (HARQ) retransmission latency, which depends on the HARQ procedure design and is also constrained by hardware capability. Current Long Term Evolution (LTE) system is thus far from supporting URLLC services, especially in uplink. In downlink, since scheduling and data transmission can happen in the same transmission interval, reducing transmission interval by enlarging the sub-carrier spacing (thus reducing the symbol length) and reducing the number of symbols in each transmission slot can help solve most of the problem. However, this is not the case in uplink. According to the current LTE protocol design, a User Equipment (UE) needs to send scheduling request to the serving base station in a dedicated and periodic resource and then wait for the scheduling grant from the base station in some slots later. Such process would take at least 10 ms before any meaningful data transmission. Grant-free transmission, a transmission scheme without scheduling request and dynamic grant, is then proposed to solve the problem especially for uplink URLLC transmission. Specifically, the arrive-to-go design target of grant-free transmission could effectively reduce the time waiting for scheduling grant and thus relax the timing design in frame structure to support traffics that demands ultra low latency, especially for uplink services. In light of this, 3GPP has agreed to consider grant-free transmission as one important candidate technology for uplink URLLC scenario [2]. Moreover, a next level benefit by saving the time for scheduling request and grant is to have longer time for data transmission, as shown in Fig. 1, which implies larger number of HARQ retransmissions would be possible and thus higher reliability within the same delay budget, compared to grant-based transmissions. The traditional semi-persistent scheduling (SPS) transmission scheme can be taken as a reduced case of grant-free transmission with no overlapping in resource reservation for different users, which is thus particularly suitable for traffic that is periodic and predictable. In the case of uplink URLLC traffic, which is believed to be sporadic small packets with near Poisson arrival patterns, it is impossible to predict the arrival time for each user. So the exclusive way of resource configuration is no longer suitable. Contention based grantfree transmission is thus the focus of this paper. The rest of this paper is organized as follows. In Section II, ∙

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we first introduce the general design framework of grant-free transmissions, followed by the elaboration on the dedicated design for URLLC scenario. Performance evaluation results based on extensive link-level simulations are given in section III under different system configuration with important observations highlighted as design guidance. Finally, Section V includes the main conclusions of the paper. II. D ESIGN FRAMEWORK OF G RANT- FREE T RANSMISSION FOR U PLINK URLLC To support and optimize grant-free transmission, many technical aspects need careful design. Fig. 2 gives the general framework of grant-free design for uplink URLLC transmission, which consists of the following 3 major components. gNB

UE Resource Configuration

Resource selection

Blind detection of UE activity & data

Collision Management & Reliability Enhancement

HARQ, link adaptation, power control

Fig. 2: General framework of grant-free design. ∙

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Resource configuration and selection, which defines at least the location and size of the physical resource (time/frequency/code resource), other related features such as modulation and coding scheme (MCS) associated with the physical resource, as well as UE identifier when the physical resource is shared among multiple UEs; Blind detection of UE activity and data decoding, which real-time detects which UEs are transmitting on each

physical resource, and then decode the data for each active UE. Multi-user detector may not be necessary if proper HARQ retransmission procedure is designed, but it can help enhance the reliability; HARQ, link adaptation, and power control, which enhances the physical layer transmission reliability by retransmission or repetition, as well as some adaptation in MCS and transmit power. Special design of HARQ procedure is needed since the traditional round-trip-time of positive/Negative ACKnowledgment (ACK/NACK) is not affordable in URLLC.

In the following, we shall elaborate on resource configuration, blind UE detection, and HARQ procedure design, which are the inevitable components that are important to URLLC performance. Better collision management and reliability enhancement by non-orthogonal multiple access (NoMA) are then discussion as extension.

A. Resource Configuration Due to lack of (dynamic) grant, at least a semi-statically configured way for the location and size of the physical resources should be enabled. Since it is impossible to predict the transmission time of the URLLC packets, the resource for grant-free transmission UE is not preferred to be UE-exclusive (such as in the traditional SPS solutions). Therefore, the same physical resource can be shared by a group of UEs. This is required especially in the scenarios when there are potentially a larger number of active users with low traffic activity than the number of orthogonal transmission resources. In the case that the same physical resource is shared by multiple UEs with random traffic arrivals, contention for resource happens when two or more UEs have data to transmit in the same slot. Fig. 3a presents the probability of number of active UEs on one resource unit given total 12 UEs with random packet arrival rate (PAR) of 1 packet/ms/UE. And Fig. 3b presents the corresponding resource allocation pattern. From Fig. 3a we can see that, with one 1 resource unit, there is a probability of 45% that contention happens, while increasing the number of resource units to 3, the probability reduces to about 10%. Note that even in the latter case, it is still (12 − 3)/12 = 75% resource saving than the traditional SPS solution with exclusive resource configuration among UEs. To take advantage of channel diversity for reliability enhancement, frequency hopping can be considered for different transmissions. For instance, some hopping patterns with different frequency locations over sub-sequential transmission slots can be pre-configured.d for each UE. In this case, different transmissions over time (e.g., initial transmission and HARQ re-transmissions) from a UE in these resource units can experience channel diversity. Note that the resource configuration and hopping design can be group-based, so that the number of potential UEs on each physical resource is reduced, which can help ease the UE detection and identification.

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Fig. 3: PMF and Resource allocation of 12 UEs random arrival on 1 and 3 resource units with PAR = 1 packet/ms*UE.

B. UE Activity Detection and Identification One big difference between grant-free and grant-based transmission is the requirement of UE identification at the receiver side. This could be done by reference signal (RS) detection if some mapping between UE identifier (UEID) and RS can be pre-configured/pre-defined. In this case, the basic channel structure with only RS and data such as in LTE Physical Uplink Shared CHannel (PUSCH) could be used for uplink grant-free transmission. For example, 12 orthogonal RSs can be assigned to 12 potential UEs, and each UE is considered to be active if the power of corresponding RS is larger than a threshold. Following the assumptions in the Table I, link-level simulation of the detection performance is shown in Fig. 4. From the figure we see that for a given 10 0

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alarmed UEs. When the SNR is larger than −5 dB, i.e. the interested SNR region, the miss detection probability can be lower than 10−5 with proper designed threshold. Moreover, by selecting the proper detection threshold, the miss detection probability can always be made at least two orders lower than the BLER of the data decoding given the same SNR. Therefore, the RS detection even with the reuse of current LTE uplink demodulation RS will not be a bottleneck issue for grant-free transmission for uplink URLLC. Detailed RS design needs further investigation and both orthogonal RS and non-orthogonal RS can be considered, depending on the potential number of URLLC UEs under service sharing the same physical resource. We thus have the following remark.

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Fig. 4: UE detection performance in the case of data collision but no RS collision. SNR, there is a tradeoff between the miss detection probability and the number of false alarmed UEs, which depends on the level of predefined threshold. With higher SNR, the miss detection probability is lower given the same number of false

HARQ is one critical technique to improve the link reliability. The traditional HARQ in uplink LTE is synchronous ACK/NACK based HARQ. In applications of URLLC services, waiting for NACK feedback and re-transmitting afterwards is not desirable for following reasons ∙ It limits the number of potential re-transmissions within the latency budget; ∙ High reliability of each NACK feedback channel needs to be maintained to ensure desired reliability and it comes at the expense of increased overhead. Instead, an alternative option to achieve high reliability with stringent latency constraints is to allow the URLLC UE to continue transmissions until an ACK is received. After the first transmission, each sub-sequent transmission can be repeated with same or different redundancy versions, transmitted on pre-configured resource units with the same or different size of the initial transmission. In other words, UE can be pre-configured with a certain maximum number of re-

transmissions (or repetitions) within the latency budget, which can be stopped if an ACK is received during the transmissions. The procedures at the receiver side for uplink grant-free re-transmissions include at least the following ∙ UE activity detection and channel estimation; ∙ Re-transmission identification and HARQ combining for data decoding. In grant-free HARQ, the base station does not know beforehand whether a transmission from a UE is a new one, or a subsequent re-transmission. And sometimes, the re-transmission index may also be needed. Therefore, certain mechanisms should be designed to support the acquisition of such information. Similar to UE detection in initial transmission, this can also be done by using pre-defined mapping between RS sequences and re-transmission indexes, without increasing any control information overhead. With the knowledge of which re-transmission(s) and initial transmission belongs to the same packet of a UE, the base station can then perform HARQ combining for joint data decoding. The way of combining can be similar to those in current systems, e.g, Chase combing (CC) or incremental redundancy (IR), etc. D. NoMA for Reliability Enhancement NoMA is a natural solution to deal with MA physical resource collision. As has been proved in [3], well designed nonorthogonal MA schemes such as Sparse Code Multiple Access (SCMA) [4] could support as large as 300% overloading1 with asymptotically the same BLER performance as no overloading case (i.e. overloading= 100%). This is mostly observed when BLER is below 10−3 . As the reliability requirement of URLLC is expected to be around 1 − 10−5 , which is already in the asymptotical region, the performance with overloading shall be expected to be very similar to that of no overloading case, i.e., single user performance. Advanced receivers such as Successive Interference Cancellation-Message Passing Algorithm (SIC-MPA) and Expectation Propagation Algorithm (EPA) are needed to facilitate the joint multi-user detection, but their complexity can be low enough for implementation [5]. In the case that NoMA is introduced to be combined with grant-free, the resource definition needs to be extended to include NoMA signatures such as codebooks, sequences, interleavers, etc. The resource configuration then needs to associate a set of NoMA signatures with each physical resource unit. Note that when the UEs sharing the same physical resource are configured to choose the NoMA signature freely from a pre-defined set, it is possible that codebook/sequence collision may happen between some UEs. However, as has been verified in [3], [6] that, as long as the RS of UEs are not in collision, the codebook/sequence collision has limited impact on the reliability (BLER) performance. This is one very good feature of NoMA for grant-free transmission. 1 Here

overloading is defined as the number of data layers (in the case of one user per layer, it is the same as the number of users) over the number of orthogonal resource elements in each spreading block.

III. E VALUATION R ESULTS In this section, we shall show that contention based grantfree transmission with the resource shared by multiple UEs can well meet the reliability requirement of URLLC. The potential data collision can be handled by proper resource configuration and HARQ re-transmissions, on top of which, resource hopping will be benefit for redundant transmissions and NoMA scheme can further enhance the performance. A. Grant-free with HARQ In this subsection, the evaluation results show the potential data collision can be handled by HARQ re-transmissions as designed in section II-C. Fig. 5 provide the evaluated grant-free OFDMA performance with HARQ operations under realistic UE detection (based on LTE uplink demodulation RS) and realistic channel estimation with and without random packet arrival. One resource unit consists of 5 resource blocks (RB) in frequency domain and 7 OFDM symbols in the time domain, and the subcarrier spacing (SCS) is 60 kHz. Each transmission slot is 0.125 ms, which is much smaller than LTE and makes it easier to meet the stringent latency requirement. The blue dash lines in Fig. 5a represent the BLER performance with maximum number of HARQ 1, 2, and 4 times when the resource unit is exclusively used by 1 user; the green dash lines in Fig. 5a represents the BLER performance with HARQ when the resource unit is shared by 4 users constantly; the red lines, in the middle of the blue and green ones, represent the BLER performance with HARQ with the 4 users sharing the same resource unit have random packet arrival rate of 1 packet/ms per UE. Fig. 5b gives the probability mass function (PMF) of number of active UEs under assumed TABLE I: simulation parameters Parameter Carrier frequency Number of RBs for Grant-free PHY packet size Latency bound Modulation and coding HARQ scheme Total number of users Channel model SNR range Subcarrier spacing TTI length OFDM symbols per TTI OFDM symbols for reference signals Number of reference signals Traffic model BS Antenna configuration UE antenna elements ACK feedback assumption Channel estimation UE detection Receiver

Value 4 GHz 5 RB for 1 resource unit 15RB for 3 resource units 32 bytes (including CRC) 1ms QPSK, Turbo CR=0.356 CC, Max number of transmissions = 4 12 TDLA, DS=30ns, 3km/h -10 dB to 10 dB 60KHz 0.125 ms 7 1 12 Constant transmitting,or Poisson arrival 4 Rx 1 Tx Ideal, in both latency and reliability Realistic Realistic, based on LTE UL DMRS MMSE-IRC

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Fig. 5: Performance of contention-free and contention-based grant-free OFDMA with HARQ operations

random arrival rate, under which contention (more than one user access the resource) happens with 15% probability. More simulation assumptions can be found in Table I. From the figure, we can observe that the reliability of grantfree transmission, both contention-free and contention-based, can be improved with increasing number of re-transmissions. For the contention-free case, i.e., blue dash line in Fig. 5a, the performance can be improved by around 3.4 dB and 6.3 dB at BLER=10−5 with 2 and 4 times of HARQ, respectively. For the constant contention case, i.e., the green dash line, the BLER performance of the initial transmission is bad due to the lack of degree of freedom and the limited capability of MMSEIRC receiver. However, when the number of retransmission increases, the BLER performance improves significantly. The gain between only one transmission and two transmissions can be larger than 3 dB thanks to the fact that some correctly decoded users will stop transmitting after receiving ACK and/or in the next round the correctly received message from some UEs can be pre-canceled before decoding. And as the number of re-transmissions continue to increase, the BLER performance is approaching the single UE performance. The reliability performance under random packet arrival of 4 users, i.e., red lines in Fig. 5a, shows the same trend and is upperbounded by that the blue dash lines and lower-bounded by green dash lines. From the evaluation above, we can make the following remark.

possible, frequency hopping can be considered in the resource configuration for different transmissions; namely, a resource unit can be configured (and can be adjusted semi-statically) in different frequency locations, following a pre-configured hopping pattern, over time slots, where different transmissions (e.g., initial and re-transmissions) from a UE in these resource units can experience channel diversity. Fig. 6 provides the evaluated reliability performance for grant-free OFDMA with and without frequency hopping, where three resource units each with 5 RB*7 OS and 60 kHz SCS are configured for a total number of 12 UEs each of them with random packet arrival rate of 1 packet/ms. The BLER curves and frequency hopping patterns are shown in Fig. 6a and 6b, respectively. In both cases, 12 UEs are assigned into 3 fixed groups, and each group with 4 UEs. When frequency hopping is not applied, the UEs always transmit on the fixed resource unit; while frequency hopping is configured, the UEs change resource unit per slot on a group based manner. From Fig. 6, we can observe that the reliability of grantfree transmission, both with and without frequency hopping, can be improved with increasing number of re-transmissions. Frequency hopping further provides around 2 dB and 3.2 dB gain at BLER=10−5 over that without frequency hopping for 2 and 4 times of HARQ, respectively. From the evaluation above, we can make the following remark.

Remark 2. Grant-free transmission with certain level of contention can still meet the reliability requirement for URLLC services with HARQ re-transmissions.

Remark 3. The grant-free transmission with frequency hopping over HARQ retransmissions can take advantage of channel diversity and user traffic disparity to further improve the transmission reliability.

B. Grant-free with HARQ and Frequency Hopping

C. Grant-free with HARQ and NoMA Enhancement

Once multiple resource units are available, UEs can be grouped into different groups for each transmission to reduce contention. To take advantage of channel diversity as much as

Fig. 7 presents the evaluated reliability performance in terms of BLER versus SNR for OFDMA based grant-free and SCMA based grant-free both with realistic UE detection and

TDLA30, 3km/h, 1T4R, 15RB, 32B PactSize, 12 UE, PAR = 1

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Fig. 6: Reliability performance and resource allocation of grant-free OFDMA with HARQ and frequency hopping

Remark 4. Performance can be further enhanced with nonorthogonal multiple access schemes such as SCMA and advanced receiver such as MPA or EPA. IV. C ONCLUSION In this paper, we discussed and verify the design and feasibility of grant-free transmission for uplink URLLC. In particular, semi-statical resource configuration with certain resource hopping, RS aided blind UE detection, and consecutive HARQ retransmissions with ACK for early termination are suggested as the major technical components and evaluated. From the analysis and simulation results we can see that contention based grant-free transmission with resource shared by multiple users is more efficient than the grant-based or traditional SPS transmission, and it can well meet the reliability requirement

TDLA30, 3km/h, 1T4R, 12RB, 32B PactSize, 12 UE, PAR = 1

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channel estimation. To guarantee the same level of diversity, UEs in grant-free OFDMA case employ the resource allocation in Fig. 3a with 3 resource units, namely randomly choosing one resource unit among the three resource units for each slot, while for SCMA, the resource allocation in Fig. 3b with 1 resource unit is applied, namely all UEs share the same resource unit all the time. Under such resource configuration, to transmit the same packet size of 32 bytes, the MCS for grant-free OFDMA is QPSK with coding rate 0.414, while for SCMA, 8-point codebook [4] with coding rate 0.395 is applied. For fair comparison, in this case, MPA receiver is applied for both cases. From the figure, we can observe that the grant-free SCMA scheme can well meet the URLLC reliability with even 1 transmission at SNR = 4 dB while grant-free OFDMA needs at least 2 times of HARQ to reach below 10−5 BLER. This is thanks to the better diversity (sharper water fall) and better coding gain provided by SCMA with sparse spreading codebook design. At BLER=10−5 , grant-free SCMA provides around 2.5 dB and 1.2 dB gain over the Grantfree OFDMA for 2 and 4 times of HARQ, respectively. From the evaluation above, we can make the following remark

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Fig. 7: Reliability performance of grant-free OFDMA and grang-free SCMA with HARQ

of URLLC within the latency bound. The data collision can be handled by the HARQ design with ACK for early termination, on top of which, frequency hopping along the retransmission slots can further exploit channel diversity and thus enhance the reliability. Moreover, the BLER performance can be further enhanced with NoMA schemes such as SCMA and advanced receiver such as MPA or EPA. R EFERENCES [1] 3GPP TR 38.913 V1.0.0, ”Study on Scenarios and Requirements for Next Generation Access Technologies.” [2] RAN1 Chairman’s notes, RAN1#86, Gothenburg, Sweden, August 22-26, 2016. [3] R1-166094, ”LLS results for UL MA schemes”, Huawei, HiSilicon, RAN1#86, Gothenburg, Sweden, Aug 22-26, 2016 [4] R1-162155, ”Sparse Code Multiple Access (SCMA) for 5G Radio Transmission”, Huawei, Busan, Korea, April, 2016 [5] X.M. Meng et al. ”Low Complexity Receiver for Uplink SCMA System via Expectation Propagation”, IEEE WCNC, March, 2017 [6] RAN2 Chairman’s notes, RAN1#85, Nanjing, China, May 23-27, 2016.