Now rapid growth of number of the devices communicating among themselves in LTE (Long-Term Evolution) communication networks is observed, especially growth of number of machine-to-machine (M2M) devices should be noted because the number of devices will exceed 50 billion in recent years. To simplify low-cost network connection of a set of devices, communications of machine type (Machine-type communications, MTC) evolved in low-cost MTC (LC-MTC) in 3GPP standarts. LC-MTC have to cope with intensive access through a set of narrow-band channels of random access (RACH) of LTE frequencies appropriated in the range efectively. With increase in quantity and density of placement of MTC-devices the scheme of random access for LC-MTC RACH has to be improved. In this paper we suggest to use the concept of virtual preambles in the random access procedure and we carry out the success access probability and an average delay analysis via a random access channel.
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In: K. E. Samouylov, L. A. Sevastianov, D. S. Kulyabov (eds.): Selected Papers of the VIII Conference “Information and Telecommunication Technologies and Mathematical Modeling of High-Tech Systems”, Moscow, Russia, 20-Apr-2018, published at http://ceur-ws.org
Machine-to-machine communications are extremely important for efective data
transmission from devices in a communication network for ensuring various services of
the Internet of Things, for example, such as applications for the smart house, logistic
tracking, health care, safety, observation and clever measurement [
To reduce the cost of LC-MTC equipment the 3GPP RAN working group has ofered
new structure of RACH where the LTE network appropriates several narrow-band
random access channels (NarrowBand-RACH, NB-RACH) within its bandwidth. The
LC-MTC device chooses one of available NB-RACH channels, performing a session
initiation procedure for the chosen RACH channel. Unlike the classical procedure
where the MTC device doesn’t choose the physical channel, in new structure with the
additional identifier the choice of the concrete channel can reduce the collisions arising at
session initiation procedure. Thus, introduction of additional identifiers will reduce the
consumed amount of energy [
Standard solutions on alleviation of possible overloads in RACH can be two types:
based on push and pull technology. In the decisions based on push technology, the
procedure of session initiation is started by autonomous requests from MTC devices.
For avoidance of collisions each MTC device has to generate a unique preamble for
session initiation within the restrictions [
In this paper the concept of virtual preambles for more efective recognition of LC-MTC devices and reduction of collisions is ofered.
2.
The session initiation procedure, by the so-called rule of four handshakes, assumes successful sending a preamble on the first step and successful reception of the response message of RAR (Random-Access-Response) on the second step.
The existing session initiation procedure on a radio channel of RACH means existence of preambles, physical channels PRACH (Physical Random Access Channel) for sending a preamble and PDCCH channel (Physical Downlink Control Channel) on the descending communication line for a mandrel of the response message of RAR.
The new scheme session initiation ofered for LC-MTC RACH uses new channel
EPDCCH (Enchanced Physical Downlink Control Channel) which number can be the
additional identifier in a so-called virtual preamble instead of PDCCH channel [
For LC-MTC the device can choose the EPDCCH channel number for the response
message of RAR, then for the base station such preamble will difer from other
preambles [
The number of virtual identifiers of a preamble can linearly be scaled with the number of the integrated indexes of PRACH. In the ofered LC — MTC RACH structure each EPDCCH channel can be logically connected to several PRACH channels.
In the course of session initiation the virtual preamble actually isn’t transferred, but implicitly distinguished between the base station and LC-MTC devices. Therefore the LC-MTC devices using the ofered scheme can be compatible to those MTC devices which use the outdated scheme of session initiation.
3.
We analyze the following measures: collision probability, defined as the ratio between the number of occurrences when two or more MTC devices send a random access attempt using exactly the same preamble and the overall number of opportunities (with or without access attempts) in the period; access success probability, defined as the probability to successfully complete the random access procedure within the maximum number of preamble transmissions; an average access delay, defined as the delay for each random access procedure between the first random access attempt and the completion of the random access procedure, for the successfully accessed MTC devices.
The dependence of session initiation procedure measures on a collision probability
is investigated in [
The success access probability is presented by a formula (1) [
Number — of the reserved session initiation opportunities (attempts) in a second
is presented by formula (3) [
= 1 − /
= ℎ * * 200.
(2)
(3)
Also we analyse an average session initiation delay using formula (4) [
+ + ( 1+ 2) · (1 − ) (1 − +1) (1 − +1) (︀ 1 − ( + 1) + +1)︀
+ + ( 3 + 4 − 2) · +1 (︀ 1 − ( + 1) + +1)︀ (1 − +1) (1 − +1) , (4) where = + +1(1 − ) is the probability of preamble retransmission. 4.
We analyze the dependence of a collision probability (Fig. 2)and success access probability (Fig. 3) on number of LC-MTC devices in a cell and number of EPDCCH channels on which RAR messages are transferred. Data from Table 1 is used for estimations.
With introduction of the additional identifier in the form of the EPDCCH channel number reduction of collision probability and increase in success access probability in connection for the subsequent transfer of small data is observed. Graphics in Fig. 2 and Fig. 3 ℎ = 1 show measures for the existing session initiation procedure.
All measures change their values when we apply an additional EPDCCH channel. For the case ℎ = 2 for 7 000 LC-MTC devices the collision probability of a preamble will decrease by 1.7 times. Let us show results for the case with two additional EPDCCH channels. For this case with 30 000 LC-MTC devices the collision probability falls from value 0.95 to 0.62, at the same time the success access probability rises up from value 0.45 to 0.99.
Average access delay is also decreasing with the growth of number of physical channels as shown on Fig. 4 for 30 000 LC-MTC devices average access delay with ℎ = 1 equals 142 ms, for ℎ = 2 channels the value is lower by 1.5 times and for ℎ = 3 channels the value drops by 2.15 times.
5.
The additional identifier approach allows to reduce the collision probability that influences all the indicators participating in the session initiation procedure for MTC devices on RACH radio channel. Introduction of additional identifiers increases the success access probability and reduces the average access delay. The analysis of success access probability and average delay is shown in the current paper.
As further research we plan to apply this approach for analysis of new RACH
mechanisms, for example, so called CAM RACH (critical alarm messages RACH) for
Emergency Alarm Messages, or to another preliminary measures [
Random access related system parameters
Total number of RACH opportunities Maximum number of preamble (Msg1) retransmissions Maximum number of HARQ retransmissions for Msg3 Total number of preambles in a RA slot Probability of unsuccessful transfer of the HARQ message (Msg3) Number of physical channels Time interval before sending Msg3 or preamle retransmission Time interval for Backof window Time interval between successful receiving Msg2 and sending Msg3 Time interval for sending Msg3, waiting and processing Msg4 ℎ 1 2 3 4
250 240 130 1; 2; 3
The publication has been prepared with the support of the “RUDN University Program 5-100” and funded by RFBR according to the research projects No 17-07-00845, 16-07-00766.