# Reliability evaluation of 5G C/U-plane decoupled architecture for high-speed railway

- Li Yan†
^{1}Email author and - Xuming Fang†
^{1}

**2014**:127

https://doi.org/10.1186/1687-1499-2014-127

© Yan and Fang; licensee Springer. 2014

**Received: **4 April 2014

**Accepted: **27 July 2014

**Published: **9 August 2014

## Abstract

To facilitate the mobility of heterogeneous networks, control plane (C-plane) and user plane (U-plane) decoupled architecture is being considered by the fifth generation (5G) wireless communication network, in which relatively crucial C-plane is expanded and kept at dependable lower frequency bands to guarantee transmission reliability and the corresponding U-plane is moved to available higher frequency bands to boost capacity. Moreover, we apply this architecture to future professional high-speed railway wireless communication system to fulfill the wireless access desire of train passengers. However, for such emerging architecture, there still exist many problems to be solved to guarantee the reliable transmission. In this article, the problem of how to appropriately evaluate the transmission reliability of C/U-plane decoupled architecture is investigated. Due to the lack of ability to reflect the importance of C-plane, conventional outage probability cannot properly indicate the transmission reliability of C/U-plane decoupled architecture whose primary design consideration is that C-plane more heavily affects the transmission reliability thereby being kept at dependable lower frequency bands. Based on this, a novel indicator named unreliability factor (URF) is proposed. Theoretical analysis and simulation results demonstrate that URF can exactly highlight the effects of C-plane on the entire transmission process. Hence, it is more appropriate to employ URF as the indicator to evaluate the transmission reliability of C/U-plane decoupled architecture.

## Keywords

## Introduction

To cater to the exponentially increasing traffic volume requirements in public mobile network, higher frequency bands with wider spectrum are exploited to further extend the capacity of Long-Term Evolution (LTE) network. Unfortunately, compared with lower frequency bands, higher frequency bands suffer from severer propagation loss which seriously limits the coverage. Hence, cells working at higher frequency bands are called small cells. On the account of mobility performance, small cells are usually overlaid on the coverage of macro cells that use lower frequency bands, which forms heterogeneous network. However, as deployment gets increasingly dense, the huge redundant control signaling interaction caused by frequent handovers between small and macro cells reduces the efficiency of heterogeneous networks. In order to mitigate this situation, C/U-plane decoupled architecture is proposed for upcoming 5G wireless communication network [1, 2]. In this architecture, the relatively important C-plane is extended and kept at lower frequency bands of macro cells. In addition, the main capacity demander U-plane is moved to the small cells using available higher frequency bands with wider spectrum. With considerable coverage of macro cells, much fewer handovers happen to the C-plane compared to conventional coupled architecture of heterogeneous networks. Therefore, under a macro cell, the handover process is just simplified to the U-plane handover, which saves lots of control signaling interaction.

For clarity, the booming LTE network is employed as the benchmark system for following analysis. In LTE network, C-plane is responsible for essential control operations such as broadcasting system information, network attaches, paging,and mobility management [8]. Moreover, the functionality of U-plane is to forward user data flow. Definitely, without a reliable C-plane, the U-plane cannot work properly. From the perspective of air interfaces, without reliably transmitted Physical Downlink Control Channel (PDCCH) that accommodates control information to indicate ‘who’ the data are for, ‘what’ data are sent, and ‘how’ the data are sent on Physical Downlink Shared Channel (PDSCH), the user data cannot be correctly decoded. Considering the above mentioned in [9, 10], the crucial C-plane instead of U-plane is kept at dependable lower frequency bands so that the transmission reliability of C/U-plane decoupled architecture could be well guaranteed. However, the performance evaluation of this architecture has not been well studied. Actually, for such emerging network architecture, there still exist many problems to be solved to guarantee the reliable transmission. For instance, Doppler effect is always a severe challenge for high-speed railway scenario. While in the C/U-plane decoupled architecture, C-plane and U-plane of the same user are transmitted at different frequencies thereby facing different Doppler shifts and Doppler spreads. Hence, the Doppler effect may get even worse. Fortunately, railways are mostly built in wide suburban or viaduct environment where multi-paths can be neglected and the wireless channel can be regarded as LOS [4]. Thus, there almost exists no Doppler spread in high-speed railway scenario. According to [4, 6], with a known train’s location and velocity supplied by the communication-based train control (CBTC) system, it can be assumed that Doppler shifts are separately compensated for C-plane and U-plane. Hence, how to appropriately evaluate the transmission reliability of this decoupled architecture becomes an urgent problem, which will greatly impact subsequent research directions on performance enhancement.

In wireless networks, the outage probability defined as the probability that the received signal quality is lower than some threshold is a popularly used indicator to reflect the transmission reliability [11, 12]. Under this definition, the outage probability of C/U-plane decoupled architecture can be expressed as the complementary probability of an event that both signal qualities of C-plane and U-plane are larger than the outage threshold. Obviously, from the view of air interface reliability, the effects of C-plane and U-plane on outage probability are virtually equal. That is to say, due to the lack of ability to reflect the importance of C-plane, the conventional outage probability cannot properly indicate the transmission reliability of C/U-plane decoupled architecture whose primary design consideration is that C-plane more heavily affects the transmission reliability thereby being kept at dependable lower frequency bands.

To facilitate the presentation, except for special behaviors such as paging and handover, we only take the general communication process as an example to qualitatively analyze the reliability relationship between C-plane and U-plane. In terms of theoretical analysis based on signal quality of air interface, the analysis procedures and results of uplink and downlink are the same. Hence, for simplicity, the following analysis is just on the basis of downlink. In the general communication process of C/U-plane decoupled architecture, PDSCH served by the small cell carries user date and PDCCH provided by the macro transmits corresponding control information such as transmission format to help receiver correctly decode the data on PDSCH. For PDCCH, its symbol error rate (SER) is directly caused by its poorly received signal quality. Nevertheless, these errors of PDCCH will badly affect the decoding correctness of data on PDSCH [13]. As a consequence for PDSCH, its errors result from two aspects, some of which are caused by its own poor signal quality and others are induced from the errors of PDCCH. In practice, if SER of PDCCH exceeds some threshold, then no matter how well the signal quality of PDSCH is, the receiver cannot correctly decode the data on PDSCH. Based on this, a more appropriate indicator named unreliability factor (URF) which can highlight the importance of C-plane is proposed to evaluate the transmission reliability of C/U-plane decoupled architecture.

The rest of this article is arranged as follows. ‘Radio propagation model’ section gives the propagation model for C/U-plane decoupled architecture. ‘System outage probability of C/U-plane decoupled architecture’ section proves that conventional outage probability cannot properly indicate the transmission reliability of this architecture. ‘System reliability-based reliability evaluation method’ section describes the proposed indicator URF and its appropriateness in evaluating the transmission reliability of this architecture. Finally, ‘Conclusions’ section concludes the whole article.

## Radio propagation model

*SIR* model

*D*to the rail as shown in Figure 2. Suppose the base stations’ radiation is omnidirectional, and the coverage radiuses of macro and small cell are

*R*

_{C}and

*R*

_{U}respectively. Besides, the abscissa axis coinciding with the driving direction is set to facilitate the expression of train travel distance

*d*. Without loss of generality, two macro cells are considered in the following analysis, i.e., the analysis scope of

*d*is from 0 to 4

*R*

_{C}- 2

*a*

_{C}, where

*a*

_{C}is the overlapping area distance of two macro cells and as shown in Figure 2, the overlapping area distance of two small cells is denoted by

*a*

_{U}.

*d*, then the C-plane signal propagation distance between the train and macro cell is

where ⌊·⌋ denotes rounding down operation.

*P*

_{t}, the received signal-to-interference ratio (SIR) can be expressed as

*x*) is the path loss with propagation distance of

*x*;

*ε*is the decibel attenuation due to shadow fading with zero mean and standard variance

*σ*; and the co-channel interference

*I*is given by

where *N*_{eNB} represents the number of co-channel eNodeBs.

For clarity, subscripts of parameters that relate to C-plane are set to C. And for U-plane they are set to U. However, in fact *P*_{t}, PL(*x*), *ε*, and *I* are determined by current serving base station while not the plane, that is, *P*_{t,C}, PL_{C}(*x*), *ε*_{C} and *I*_{C} are the properties of the macro cell which serves the C-plane. With subscript U, they are the properties of the small cell which provides U-plane. The above also applies to the following expressions.

### Cross-correlated shadow fading

*W*,

*W*

_{C}, and

*W*

_{U}are independent Gaussian random variables with zero mean and standard variances

*a*,

*b*, and

*c*, respectively, and they satisfy

*γ*is referred to as a parameter determined by the size and height of terrain and the height of base station and is generally set to 0.3 [17];

*Θ*

^{T}corresponds to the angle threshold that depends on the serial de-correlation distance

*d*

_{cor}and is defined as

*Θ*of (9) is the angle between the propagation paths of C-plane and U-plane signals as shown in Figure 3. As the site-to-site distance between macro and small cells can be calculated via the following equation

*Θ*can be derived as

Then, substitute (12) into (9), *ρ*_{(C,U)} is obtained, with which the standard variances, *a*(*d*), *b*(*d*), and *C*(*d*) in (8) can be worked out.

## System outage probability of C/U-plane decoupled architecture

_{C}

^{th}and SIR

_{U}

^{th}are the decibel outage thresholds of C-plane and U-plane, respectively, and

**Simulation parameters**[18]

Parameters | Values |
---|---|

Frequency of macro cell | 2 GHz |

Frequency of small cell | 5 GHz |

Path loss model of macro cell PL | Hata |

Path loss model of small cell PL | M.2135 |

Transmit power of macro cell | 43 dBm |

Transmit power of small cell | 33 dBm |

Radius of macro cell | 1 km |

Radius of small cell | 0.25 km |

Overlapping area distance of macro cells | 0.2 km |

Overlapping area distance of small cells | 0.05 km |

Distance between base station and rail | 30 m |

C-plane SER threshold th | 10 |

U-plane SER threshold th | 10 |

| 10 |

Correlation distance | 100 m [19] |

Modulation scheme of U-plane | 16QAM |

Standard variance of C-plane shadowing | 6 dB |

Standard variance of U-plane shadowing | 8 dB |

## System reliability-based reliability evaluation method

### Reliability relationship between C-plane and U-plane

_{C}. However, for PDSCH there are two aspects resulting in data errors. As shown in Figure 6, some errors of PDSCH are due to the poor received signal quality, while others are induced from the errors of PDCCH. Maybe, these parts of symbols are of high signal quality, but they cannot be correctly decoded because of the inaccurate transmission format indication on PDCCH. Hence, it is reasonable to believe that if SER of PDCCH exceeds some tolerable value, then no matter how well the signal quality of PDSCH is, the data cannot be correctly received. This exactly reveals why C-plane is regarded more crucial than U-plane. Although how the errors of PDCCH affects the data receiving on PDSCH is beyond the scope of this study, a mapping function is required to describe the relationship between SERC and SERU/C that is

where *α* function is supposed to be monotone increasing with definition field of SER_{C} from 0 to 1 and range of SER_{U/C} from 0 to 1 as well. However, the exact expression of *α* function depends on the system settings and is out of our study scope.

### Unreliability factor

It is obvious that URF has the ability to reflect the importance of C-plane. When the SER of crucial C-plane is beyond the threshold th_{C}, in spite of the transmission performance of PDSCH, URF is directly set to 1. This exactly conforms to the previous analysis result that if SER of PDCCH exceeds some tolerable value, then no matter how well the signal quality of PDSCH is, the data cannot be correctly received. While if C-plane is reliably transmitted, the value of URF will depend on the SER outage probability of PDSCH which is defined as the probability that the SER is higher than some threshold [20]. Practically, the SER outage probability of PDSCH is much lower than 1. Hence, at the point of SER_{C}=th_{C}, URF is not rightly continuous thereby not a probability function. As a matter of fact, URF can be interpreted as a kind of indicator, which equals to a complex and reasonable probability value.

_{C}and SIR

_{U}are not absolutely independent. Then, it seems that SER

_{U/C}and ${\text{SER}}_{{\text{U/SIR}}_{\mathrm{U}}}$ are correlated. Fortunately, there exists the principle of conditional independence that two random variables

*X*and

*Y*are conditionally independent if given

*Z*as shown in Figure 7[21], that is, with given

*Z*, for any real number

*x*,

*y*, and

*z*, the following equation is satisfied:

_{C}and SER

_{C}is known, SIR

_{C}and SER

_{U/C}are conditionally independent. Therefore, SER

_{U/C}and ${\text{SER}}_{{\text{U/SIR}}_{\mathrm{U}}}$ are conditionally independent and the total SER of U-plane can be derived as

*Q*function is defined by

where *Q*^{−1} is the inverse function of *Q* function, and *C*(*x*_{C}(*d*)) is defined to simplify the following expressions.

*M*-QAM, ${\text{SER}}_{{\text{U/SIR}}_{\mathrm{U}}}$ is given by [22]

_{U}<1, then

where *U*(*x*_{U}(*d*)) is defined to simplify the following expressions.

## Conclusions

For upcoming 5G wireless communication system, C/U-plane decoupled architecture is a potential way to not only expand capacity but also to prevent unnecessary control signaling overhead. In addition, we apply this architecture to future professional high-speed railway wireless communication system to fulfill the wireless access desire of train passengers during long-distance journey. However, how to properly evaluate the system transmission reliability of C/U-plane decoupled architecture becomes an urgent problem, which will impact the subsequent research direction on performance enhancement. It has been proved that the conventional outage probability cannot convey the primary design consideration of this decoupled architecture that C-plane more heavily affects the entire transmission reliability than U-plane thereby being kept at lower frequency bands. Based on this, a novel indicator URF is proposed. The theoretical analysis and numerical simulation results have confirmed that URF performs more properly in evaluating the entire system transmission reliability of C/U-plane decoupled architecture.

## Notes

## Declarations

### Acknowledgements

The work of the authors was supported partially by the 973 Program under Grant 2012CB316100, NSFC under Grant 61032002, and the Program for Development of Science and Technology of China Railway Corporation under Grant 2013X016-A.

## Authors’ Affiliations

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This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.