Random Access Technologies in Next Generation Cellular M2M Networks

Random Access Technologies  in Next-Generation Cellular M2M Networks                 (차세대 셀룰러 M2M 네트워크에서의 랜덤 액세스 기술)

With the advent of machine-to-machine(M2M) communications in the existing human-centric cellular communications, people and objects(machine nodes) coexist, and in particular, an environment where tens of thousands of machine terminals exist within a single base station(eNode B) coverage area is a new situation that has not yet been experienced. Random access in a massive number of machine nodes may cause many new problems, such as collision problems between preambles, depletion of radio resources to accommodate a massive number of machine nodes, and access delay problems. We propose new random access  technologies which can solve or greatly alleviate  the above mentioned related problems, and the proposed random access schemes include a root-index based prioritized random access scheme, a spatial group based random access scheme,  an early preamble collision detection scheme, a preamble collision resolution mechanism, a spatial group based reusable preamble allocation scheme, a non-orthogonal resource allocation scheme,  a non-orthogonal resource allocation scheme, a message-embedded random access scheme, etc.  

Figure 1 shows a typical random access procedure which consists of 4 steps:  preamble transmission (Step 1); random access response (Step 2); scheduled message transmission (step 3); and acknowledgement (Step 4). The step 1 procedure in the conventional random access procedure may experience a preamble collision if preamble resources are assigned to different nodes and it does not have such functions as access priorities and simultanous transmissions of preamble and data. Since the step 2 procedure requires a single  RAR(random access response) message per detected preamble, we may need a large number of RAR messages for multiple preamble transmissions from a massive number of machine nodes. The step 3 may experience a lack of control resources due to a large number of scheduled message transmissions from a large number of machine nodes.

Figure 1 A Typical Random Access Procedure with 4 Steps

We proposed a new random access framework in future cellular M2M networks considering the following features:

•   Access priorities

•   Preamble collision mitigation/detection/resolution

•   PUSCH (physical uplink shared channel) resource reuse

•   Preamble transmission with message-embedding

(1) How to Give Access Priorities in Random Access Procedure in Future Cellular M2M Networks?

We can support different QoSs for various service applications from the access phase by using the concept of access priority. (See Figure 2)

Figure 2 A Prioritized Random Access Scheme

We proposed a root-index based prioritized random access (RIPRA) scheme that implicitly embeds the access priority in the root index of the RA preambles. Figure 3 shows a root-index based prioritized preamble transmission and a random access response procedure.  If three machine nodes utilize different root indices which embed different access priorities, the eNodeB can respond to the detected prioritized preambles according to the embedded access priorities.  Through the proposed RIPRA scheme,

•   Each node can indicate its access priority during the RA procedure

•   The eNodeB can respond to corresponding prioritized preambles according to the access priority

•   Access priority between machine nodes and UEs can be supported

•   Access priority among machine nodes can be also supported

•   Access priority among UEs can be also supported

Figure 3  A root-Index Based Prioritized Preamble Transmission and a Random Access Response (RAR) Procedure

(2) How to Resolve The Preamble Collision Problem in The Random Access Networks?

We proposed a spatial group based random access (SGRA) scheme to effectively increase the number of available preambles through spatial grouping in LTE cellular networks. Figure 4 shows a spatial group based random access scheme in which  a basestation coverage area is divided into multiple sub-areas and a different root index is assigned to each sub-area. We canexpect to accommodate 57,000 nodes (one random access per 5 minutes) with a preamble collision probability of 3% at a cell radious of 2km. This indicates approximately  5 times higher capacity compared with the result of  the conventional scheme. Through the proposed scheme, we can expect lower random access(RA) collision probability and RA delay at additional costs of broadcast of spatial group information and group identification of machine nodes.

Figure 4   A Spatial Group Based Random Access (SGRA) Scheme

(3) How to Resolve the Preamble (PA) Collision Problem in the Random Access Networks?

We proposed an early PA collision detection scheme based on tagged PA transmissions in order to detect PA collisions and notify these at the first and second step of RA procedure. Through the proposed scheme, we can achieve approximately 100% PUSCH resource efficiency at the third step of the RA (random access) procedure. In other words, we can save the unnecessary resource waste. We also reduce the random access delay by approximately a half (60 ms vs. 110ms (conventional scheme) at 20 attempts per PRACH).

We proposed a preamble collision resolution mechanism based on multiple-captured timing alignment values in order for even nodes transmitting collided preambles to continue successful random accesses. Through the proposed mechanism, we can obtain a significantly higher RA success probability of 95% (vs. 63% (conventional)) and reduce random access delay. We also can capture multiple timing alignment values based on tag detection. Here, we propose a mechanism to capture multiple TA (timing alignment) values for a single detected Preamble(PA).

(4) How to Increase the Resource Efficiency  in PUSCH Resources?

We proposed a non-orthogonal resource allocation (NORA) scheme combined with the spatial group based random access scheme in order to resolve PUSCH resource shortage problem and enhance the PUSCH resource efficiency. We here first propose a PUSCH resource reuse mechanism in the RA procedure. Since one resource is allocated to many preambles(PAs) at the second step of the RA procedure, we can increase the resource efficiency.  We can reduce preamble collision probability usiing the proposed SGRA scheme and increase the resource efficiency of PUSCH resource using the NORA scheme, resulting in higher success rates of random access.The proposed  scheme is highly compatible with the current LTE random access system.

We also proposed a PUSCH resource reuse mechanism based on RA load monitoring scheme in order to resolve PUSCH resource shortage problem and enhance the PUSCH resource efficiency. We can expect to achieve higher random access success probability and shorter random access delay. IN addition, we can obtain better PUSCH resource utilization. As additional costs of the proposed mechanism, we need to perform successive interference cancellation (SIC) techniques and data decoding failures may occur due to insuficient SNR difference among multiple received data.

(5) How to Simultaneously Send Small-Sized Messages with Preamble Transmissions?

We proposed a message (MSG)-embedded random access (MERA) scheme in order to transmit a short-MSG during a preamble transmission without scheduling and PUSCH resource.  Here, we provide a novel concept of message embedding with preamble transmission. In this scheme, we need  to detect messages after detection of preambles.  These messages may contain scheduling messages and/or connection-less user short messages. The expected benefits include loe-latency connection-less data transmission; no control and signaling overhead due to resource scheduling; and resource saving on physical shared uplink channel (PUSCH, data channel).

(6) How to Suppress the  Noise Rise under the Environment of Multiple Root Sequences? 

We proposed an enhanced PRACH preamble detector.

<Selected Journal Papers>

• T. Kim, B.C. Jung, and D.K. Sung, “An Enhanced Random Access with Distributed Pilot Orthogonalization for Cellular IoT Networks,” IEEE Trans. On Vehicular Technology, vol. 69, no. 1, pp.1152-1156, Jan. 2020.

• H.S. Jang, B.C. Jung, and D.K. Sung, “Dynamic Access Control with Resource Limitation for Group Paging Based Cellular IoT Systems,” IEEE Internet of Things Journal, vol. 5, no. 6, pp.5065-5075, Dec. 2018.

• H.S. Jang, S.M. Kim, H.S. Park, and D.K. Sung, “A Preamble Collision Resolution Scheme Based on Captured Multiple Timing Advance Values for Cellular IoT/M2M Communications,” IEEE Trans. On Vehicular Technology, vol. 67, no. 2, pp. 1825-1829, Feb. 2018.

• T. Kim, I.K. Bang, D.K. Sung, “An Enhanced PRACH Detector for Cellular IoT Communications,” IEEE Communications Letters, vol. 21, no. 12, pp.2678-2681, December 2017.

• T. Kim, D.M. Kim, N.K. Pratas, P. Popovski, and D.K. Sung, “ An Enhanced Access Reservation Protocol with a Partial Preamble Transmission Mechanism in NB-IoT Systems,” IEEE Communications Letters, vol.21, no.10, pp.2270-2273, Oct. 2017.

• H.S. Jang, S.M. Kim, H.S. Park, and D.K. Sung, “An Early Preamble Collision Detection Scheme Based on Tagged Preambles for Cellular M2M Random Access,” IEEE Trans. On Vehicular Technology, vol. 66, no. 7, pp. 5974-5984, July 2017

• H.S. Jang, H.S. Park, and D.K. Sung, “A Non-Orthogonal Resource Allocation Scheme in Spatial Group based Random Access for Cellular M2M Communications,” IEEE Trans. On Vehicular Technology, vol. 66, no. 5, pp. 4496-4500, May 2017.

• H.S. Jang, S.M. Kim, H.S. Park, and D.K. Sung, “Message-Embedded Random Access for Cellular M2M Communications,” IEEE Communications Letters, vol. 20, no. 5, pp.902-905, May 2016.

• T.H. Kim, H.S. Jang, and D.K. Sung, “A Novel Root-Index Based Prioritized Random Access Scheme for 5G Cellular Networks,” ICT Express, Dec. 2015.

• T.H. Kim, H.S. Jang, and D.K. Sung, “An Enhanced Random Access with Spatial Group Based reusable Preamble Allocation in Cellular M2M Network,” IEEE Communications Letters, vol. 19, no. 10, pp. 1714-1717, Oct. 2015.

• H.S. Jang, S.M. Kim, K.S. Ko, J.Y. Cha, and D.K. Sung,” Spatial Group based Random Access for M2M Communications,” IEEE Communications Letters, vol.18, no.6, pp.961-964, June 2014.

• K.S. Ko, M.J. Kim, K.Y. Bae, D.K. Sung, J.H. Kim, and J.Y. Ahn, “A Novel Random Access  for Fixed-Location Machine-to-Machine Communications in OFDMA based Systems,” IEEE Communications Letters, vol. 16, no. 9, Sep. 2012.

<Conference Papers>

• T.H. Kim, K.S. Ko, and D.K. Sung, “Transmit Power Optimization for Prioritized Random Access in OFDMA Based Systems,” ICC 2016 May 2016.

• T. Kim, K.S. Ko, and D.K. Sung, “Prioritized Random Access for Accommodating M2M and H2H Communications in Cellular Networks,” IEEE Globecom Workshop 2015, Dec. 2015.

• H.S. Jang, S.M. Kim, H.S. Park, D.K. Sung, “Enhanced Spatial Group Based Random Access for Cellular M2M Communications,” IEEE ICC 2015 Workshop on Massive Uncoordinated Access Protocols, June 2015.

• T. Kim, K.S. Ko, and D.K. Sung, “Prioritized Random Access for Machine-to-Machine Communications in OFDMA Based Systems,” IEEE ICC 2015, June 2015.

• T. Kim, K.S. Ko, I.K. Bang, and D.K. Sung, “A Random Access Scheme Based on a Special Preamble for Supporting Emergency Alarms,” IEEE WCNC 2014, Istanbul, April 2014.