# Periodic Radio Resource Allocation to Meet Latency and Reliability Requirements in 5G Networks

## Summary (3 min read)

### Introduction

- Among the URLLC services, many use cases, especially those related to factory automation, involve communications with relatively static radio conditions and a periodic generation of control or data packets.
- This large range of use cases presents different requirements in terms of latency and reliability.

### II. SYSTEM MODEL

- Wired technologies, such as Ethernet-based systems, have been used for a long time for factory automation and they are still the dominant technologies in this field because of their reliability and real-time guarantee.
- The evolution in factory automation from wired to wireless communications should take place gradually and it is commonly agreed that the wires cannot be removed everywhere.
- The authors study focuses on the wireless communication scheduling.
- On the wireless side, UEs are characterized by different radio conditions so that their resource requirements are different and depend on their long-term Signal to Noise Ratios (SNRs) and on their chosen MCSs.
- The whole system has to be designed so that the final packet loss rate is very low i.e., 10−5), with a minimal number of retransmissions in order to respect the low latency requirements (i.e., few milliseconds).

### III. RADIO RESOURCE ALLOCATION SCHEME

- The authors work targets deterministic radio resource allocation mechanism over a wireless link.
- In particular, in the time domain, they are allocated every Transmission Time Interval (TTI).
- The authors will see, in the following section, how resource allocation can be done to achieve the URLLC requirements.
- Indeed, a more robust MCS ensures fewer retransmissions, and then less reservation of resources for HARQ, but has a lower spectral efficiency leading to more reserved resources for the first transmission.
- The optimal tradeoff is studied in Section IV-C. 14G subchannels are of 180 kHz, each composed of 12 consecutive and equally spaced Orthogonal Frequency-Division Multiplexing (OFDM) subcarriers.

### A. Resource Allocation for first transmissions

- The first step is to estimate the number of RBs for transmitting one applicative packet of URLLC service.
- This number is determined by the MCS and the structure of one RB.
- Η refers to the information rate that can be transmitted over a given bandwidth and is given by: η = ηcηslog2M (1) where M denotes modulation order, ηc denotes code efficiency and ηs denotes efficiency of signaling.

### B. Resource Allocation for retransmissions

- Since the MCS stays the same in retransmission due to HARQ, the required number of RBs is exactly the same as the previous transmission for each UE.
- Therefore, the authors have as objectives are to determine the loss probability knowing the users MCS selections, #»µ , and the resources reserved for retransmissions H and to derive optimal resource allocation, as presented in the next section.

### IV. OPTIMAL JOINT MCS SELECTION AND RESOURCE ALLOCATION

- In order to obtain the optimal scheme, the authors start by computing the performance under a given users MCS selection and resource reservation.
- The authors then derive the optimal scheme under performance constraints.

### A. Distribution of the number of losses

- To calculate the probability mass function of lost RBs, every user’s transmission process should be considered.
- The authors can now compute the distribution of the overall number of retransmissions.
- The lost number of RBs is a sum of independent weighted Bernoulli trials that are not identically distributed.
- The distribution function is necessary to decide how many RBs should be reserved for HARQ for a given target reliability.
- The probability of event #»α could be expressed with error rate of each user: q ( #»α) =.

### B. Reliability computation

- To evaluate the reliability of their resource allocation mechanism, the authors have to consider two possible events for loss as follows.
- Second, even if there is enough space for a retransmissions, the retransmission may fail again.
- The authors here adopt the policy that accommodates the highest number of retransmissions.
- The MCS-dependent error rates for first and second transmissions δi and δ (1) i are calculated using link level simulations as will be explained next.

### C. Optimal scheme

- Above the authors have developed a framework for evaluating the performance (in terms of packet losses) for URLLC services using periodic scheduling.
- The performance was derived for a set of selected MCSs and for a given reservation of resources for retransmissions.
- A two stage optimization problem is proposed.
- This search is very efficient and the complexity is logarithmic in the maximum number of RBs possible.
- Once the minimal resource allocation for a given MCS selection is computed, the authors can derive the global optimal MCS for minimizing overall resource consumption: #»µ∗ = argmin #»µ.

### V. NUMERICAL RESULTS

- The authors consider a system where the machine type equipments are randomly and uniformly distributed throughout the factory.
- The environment is assumed to be controlled, including interferences, so that the average SNRs are known (or estimated on a long term).
- The authors consider a traffic with small applicative packets generated periodically, with one packet generated every 20 ms.
- The simulation parameters are listed in Table I.

### A. Link level simulations

- Optimal resource allocation analysis is performed using a link-level simulator where different modulations (64QAM, 16QAM, 4QAM), channel coding schemes (turbo, polar) and MIMO schemes are implemented, along with HARQ with Chase combining.
- Table II provides the results of the link level simulations, in terms of packet loss after the first transmission and after retransmission, for each MCS.
- As the reliability depends on the MCS combination #»µ (equation (7)), the authors define in Table III the different possible combinations, eliminating combinations for which it is obvious that the target reliability cannot be achieved.

### B. Performance evaluation

- For each MCS combination, defined in Table III, the authors compute the amount of resources needed for the first transmission and for retransmissions under constraints (8).
- Following Figure 3, the optimal combination of MCS is No.7, which corresponds to using MIMO 4×4 OFDM 4QAM for UEs with SNR 5 dB or 10 dB and Polar Alamouti OFDM 16QAM for UEs with 15 dB (see Table III).
- The trade-off between robustness of MCS and error probability can also be observed from Figure 3.
- The system is constrained with users having the lowest SNR, and an error rate lower than 10−5 is not achievable.

### VI. CONCLUSION

- The authors have developed a joint MCS selection and resource reservation scheme for reaching stringent latency and reliability targets in factory-like environments, while ensuring minimal resource consumption.
- The authors considered a system with quasi-static users and reserve resources for first transmissions and a limited set of retransmissions and formulated a twostage optimization problem for selecting the optimal MCS combination.
- The authors simulation results illustrate the optimal scheme and the impact of different reliability and latency targets on the resource reservation.

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##### Citations

149 citations

### Cites background from "Periodic Radio Resource Allocation ..."

...Industry automation (controlled indoor environment) Periodic generation of packets by a limited number of machines Persistent (cyclic) resource reservation for each UE [12]....

[...]

35 citations

### Cites background from "Periodic Radio Resource Allocation ..."

...Till recent past, the realisation of RT-IoT was not possible due to the communication delays but since the introduction of mobile edge computing and 5G technologies the end-to-end delays and latencies can be ultra-reliable which is necessary for RT-IoT systems [28,29,35]....

[...]

^{1}

21 citations

### Cites background from "Periodic Radio Resource Allocation ..."

...A joint resource allocation and Modulation and Coding Scheme (MCS) that considers reliability and latency constraints is proposed for factory automation in [11]....

[...]

16 citations

### Cites methods from "Periodic Radio Resource Allocation ..."

...In [13], a joint resource allocation and Modulation Coding Scheme (MCS) was analyzed while considering reliability and latency constraints....

[...]

11 citations

### Cites background or methods from "Periodic Radio Resource Allocation ..."

...In [18], the dedicated spectrum was allocated for periodic URLLC packets to improve the reliability performance....

[...]

...• Dedicated spectrum allocation [18]–[20]: Basically, retaining some exclusive spectrum for URLLC users can improve the URLLC performance....

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...In the literature, power control [13], [17] and spectrum management [18]–[23] are the two main technical directions to solve the packet collision issue in a spectrum sharing environment for URLLC and eMBB....

[...]

...In the literature, some spectrum allocation methods were proposed to reduce the collision of data packets in URLLC and eMBB multiplexing systems [18]–[23]....

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##### References

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210 citations

^{1}

61 citations

### "Periodic Radio Resource Allocation ..." refers background in this paper

...where Θ denotes the reliability target and ei( #» μ,H) the per user packet loss rate of as expressed in equation (7)....

[...]

...As the reliability depends on the MCS combination #» μ (equation (7)), we define in Table III the different possible combinations, eliminating combinations for which it is obvious that the target reliability...

[...]

^{1}, Eurescom

^{2}, IMDEA Networks

^{3}, Ericsson

^{4}, Telecom Italia

^{5}, University of Southampton

^{6}, Telenor

^{7}

43 citations

25 citations

### "Periodic Radio Resource Allocation ..." refers methods in this paper

...For each MCS combination, defined in Table III, we compute the amount of resources needed for the first transmission and for retransmissions under constraints (8)....

[...]

10 citations

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##### Frequently Asked Questions (2)

###### Q2. What have the authors stated for future works in "Periodic radio resource allocation to meet latency and reliability requirements in 5g networks" ?

As a future work, the authors aim at extending their model to multiple retransmissions ( within the latency budget ) and to more distributed schemes where collisions between retransmissions may occur.