PaperlayerView exact revision on arXiv

Source-linked AI summary

Vehicular Communications for 5G Cooperative Small Cell Networks

Xiaohu Ge, Hui Cheng, Guoqiang Mao, Yang Yang, Song Tu

arXiv:1604.02660v1cs.NI

TL;DR

Vehicular communication in 5G small-cell networks faces coverage and frequent-handoff challenges. This paper derives distance-based cooperation and mobility measures for Poisson-distributed small-cell BSs, and reports capacity, handoff, and overhead tradeoffs that can be tuned through the cooperative threshold.

  • Problem

    Vehicles' mobility and high speed make 5G cooperative small-cell communication prone to frequent handoffs and coverage problems.

    Source: §I · pp. 1–2

  • Method

    The paper derives distance-based cooperative and coverage probabilities, then uses them to propose vehicular handoff-rate and overhead-ratio measures in Poisson-distributed small-cell networks.

    Source: §I · p. 2, §II · p. 2

  • Results

    Cooperative transmission increases vehicular communication capacity and handoff rate, and the paper reports a minimum vehicular overhead ratio across cooperative thresholds.

    Source: §I · p. 2, §VI · p. 11

  • Takeaways & Limitations

    Adjusting the cooperative threshold can be used to optimize vehicular communications by balancing capacity and overhead.

    Source: §I · p. 2

Abstract

from arXiv · show

The cooperative transmission is an effective approach for vehicular communications to improve the wireless transmission capacity and reliability in the fifth generation (5G) small cell networks. Based on distances between the vehicle and cooperative small cell BSs, the cooperative probability and the coverage probability have been derived for 5G cooperative small cell networks where small cell base stations (BSs) follow Poisson point process distributions. Furthermore, the vehicular handoff rate and the vehicular overhead ratio have been proposed to evaluate the vehicular mobility performance in 5G cooperative small cell networks. To balance the vehicular communication capacity and the vehicular handoff ratio, an optimal vehicular overhead ratio can be achieved by adjusting the cooperative threshold of 5G cooperative small cell networks.

I. INTRODUCTION

The paper addresses limited prior vehicular scenarios and rare analysis of mobility performance in cooperative small-cell networks. It models Poisson-distributed small-cell BSs and develops distance-based cooperation and mobility measures.

I. INTRODUCTION

  • Prior vehicular studies were limited to simple cooperative-cell and point-to-point scenarios, while mobility performance in cooperative MIMO small-cell networks remained rarely investigated.The paper proposes cooperative and coverage probabilities, plus handoff rate and overhead ratio, for the stated network setting.Source: §I · p. 2

II. SYSTEM MODEL

  • Macro-cell BSs carry control information, while small-cell BSs transmit user data in the two-tier network model.Small-cell BS locations follow an independent Poisson point process, and the vehicle associates with its closest BS.Source: §II · p. 2

III. COOPERATIVE PROBABILITY IN SMALL CELL NETWORKS

  • Cooperative small-cell BSs are selected according to their distances from the vehicle, making the distance distribution a basis for analyzing cooperation.The network's small-cell BSs are modeled using Poisson point processes.Source: §II · p. 2, §III · p. 3

A. Distance distribution of cooperative small cell BSs

  • The distance from a vehicle to its nth-closest point in a homogeneous two-dimensional Poisson process follows a generalized Gamma distribution.The passage also identifies the Gamma function and describes deriving the distance PDF from the CDF.Source: A. Distance distribution of… · p. 3

B. Cooperative probability

The cooperative scheme selects adjacent small-cell BSs by comparing their distances from the vehicle. The resulting cooperative probabilities are derived under stated channel assumptions.

B. Cooperative probability

  • A BS is selected for cooperation when its distance from the vehicle is no more than the cooperative threshold times the nearest-BS distance.The channel model assumes line-of-sight transmission, path loss, and Rayleigh fading, while ignoring shadowing.Source: B. Cooperative probability · p. 3
  • The cooperative probability is derived using the area between circles of radii R1 and ρR1, then extended to k adjacent small-cell BSs.The scheme is based on selecting BSs by distance from the vehicle.Source: B. Cooperative probability · pp. 3–4
  • Figure 2 shows cooperative probability with respect to the cooperative threshold, comparing cases with different numbers of cooperative small-cell base stations.Source: Figure 2 · p. 4

C. Performance analysis of cooperative probability

The cooperative probability varies with both the cooperative threshold and the number of cooperating small-cell BSs. Its response to the threshold depends on how many BSs are selected.

C. Performance analysis of cooperative probability

  • When the BS being considered is selected, its cooperative probability increases with ρ and decreases with its distance from the vehicle.For BSs ranked i = 2, 3, 4, probability approaches saturation when ρ exceeds 3.5.Source: C. Performance analysis of cooperative… · p. 4
  • Figure 3 relates cooperative probability to the number of cooperative BSs and the cooperative threshold ρ.The reported trend depends on the number of selected BSs: with one, probability decreases as ρ rises; with more than one, it first rises and then falls.Source: Figure 3 · p. 4, C. Performance analysis of cooperative… · p. 4
  • With one cooperating BS, probability decreases as ρ rises; with more than one, it first rises, peaks, then falls and saturates above ρ = 4.At a fixed threshold, probability decreases as the number of cooperating BSs increases.Source: C. Performance analysis of cooperative… · p. 4, Figure 3 · p. 4

IV. COVERAGE PROBABILITY OF COOPERATIVE SMALL CELL NETWORKS

The interference model represents cooperative and interfering small-cell transmissions in a MIMO vehicular link. It derives the received signal and SINR while accounting for adjacent-cell interference.

A. Interference Model

  • With OFDM, the model assumes no intra-cell co-channel interference and separates BSs into cooperating set C and interfering set B.The BSs in C simultaneously transmit data to the vehicle; B is drawn from the remaining small-cell BSs.Source: A. Interference Model · p. 4
  • The received vector combines desired signals from cooperative BSs, interfering signals from adjacent BSs, and additive white Gaussian noise.The model specifies small-scale fading channel matrices for both cooperative and interfering links.Source: A. Interference Model · pp. 4–5
  • For k jointly transmitting BSs, the model derives the vehicle's SINR under maximum-ratio transmission and combining, accounting for aggregate interference.The interference analysis maps the BS point process to an inhomogeneous Poisson point process and derives a Laplace transform for aggregate interference.Source: A. Interference Model · p. 5

B. Coverage probability

The coverage analysis derives an expression for cooperating small-cell BSs under simplifying assumptions, then uses a recurrence-based explicit form for evaluation. Its numerical scope includes outage threshold, path-loss coefficient, and antenna-count variations.

B. Coverage probability

  • The derivation assumes equal distances from the vehicle to the cooperating BSs and ignores noise; an analytical expression is unavailable when those distances are random.The equal-distance approximation is motivated by a vehicle at the cell edge and a limited number of adjacent cooperating BSs.Source: B. Coverage probability · p. 5
  • A linear recurrence relation and matrix form are used to obtain an explicit coverage-probability expression.The derivation uses the strictly lower triangular property of the matrix to iterate to the explicit form.Source: B. Coverage probability · p. 6
  • The numerical evaluation varies the outage threshold against path-loss coefficient and varies the number of BS transmission antennas against outage threshold.These scopes are identified in the captions for Figures 4 and 5.Source: Figure 4 · p. 6, Figure 5 · p. 6
  • The analysis specializes the expression to three cooperating BSs by specifying their distances and substituting them into the general form.The resulting three-BS coverage probability is given by expression (27).Source: B. Coverage probability · p. 6

C. Performance analysis of coverage probability

Coverage probability varies with outage threshold, path loss, antenna count, and small-cell radius. Cooperative transmission improves coverage relative to non-cooperative transmission.

C. Performance analysis of coverage probability

  • Coverage evaluations use a three-cooperative-BS model with default parameters including 4 BS antennas, 2 vehicle antennas, and a 50 m cell radius.The defaults also set the outage threshold to 0 dB and path-loss coefficient to 4.Source: C. Performance analysis of coverage… · p. 6
  • At fixed path-loss coefficient, coverage probability falls as the outage threshold increases; at fixed outage threshold and transmit power, it rises as the path-loss coefficient increases.The paper attributes the latter trend to greater attenuation of interference from more distant BSs than of the desired signal.Source: C. Performance analysis of coverage… · pp. 7–8
  • At fixed antenna count, coverage probability decreases as small-cell radius increases; at fixed radius, it increases with the number of cooperative-BS antennas.The antenna-count trend reverses with outage threshold: coverage rises with more antennas below 2.5 dB and falls at or above 2.5 dB.Source: C. Performance analysis of coverage… · p. 8, Figure 6 · p. 8
  • Cooperative transmission improves coverage probability over non-cooperative transmission, with the largest gain at an outage threshold of -1 dB.The comparison uses k = 3 cooperative small cell BSs versus k = 1 without cooperative transmission.Source: C. Performance analysis of coverage… · p. 8, Figure 7 · p. 8

A. Vehicle handoff rate of cooperative small cell networks

The handoff analysis models vehicle movement and changing distances to small-cell BSs, then evaluates handoff rate under different velocities and cooperative thresholds. Multi-cell cooperation has a higher handoff rate than fixed two-cell cooperation when the threshold differs from 1.

A. Vehicle handoff rate of cooperative small cell networks

  • The mobility model simplifies Gauss-Markov movement by setting α = 1, so vehicle velocity and direction remain constant.The vehicle's new distances to small-cell BSs are calculated from its prior position and movement.Source: A. Vehicle handoff rate of cooperative… · pp. 8–9
  • A handoff is triggered as the vehicle passes through different cooperative small cells during a slot, and the long-term handoff count sums those triggers over time T.The handoff rate is then expressed using that count and the observation duration.Source: A. Vehicle handoff rate of cooperative… · p. 9
  • When the cooperative threshold is fixed, handoff rate rises with vehicle velocity; for thresholds other than 1, the proposed multi-cell scenario has a higher rate than fixed two-cell cooperation.At ρ = 1, the two scenario curves coincide because only one small cell transmits.Source: A. Vehicle handoff rate of cooperative… · p. 9, Figure 8 · p. 9

B. Vehicle overhead ratio of cooperative small cell networks

The overhead-ratio framework measures cooperative-control traffic alongside vehicular communication capacity. It accounts for X2 control and user-plane traffic associated with cooperative-status changes and handoffs.

B. Vehicle overhead ratio of cooperative small cell networks

  • The X2-U overhead model sums handoff traffic across vehicle traffic types, assuming all handoff requests are accepted.Per-type inputs include traffic arrival rate, average session duration, flow rate, and average handoff duration.Source: B. Vehicle overhead ratio of… · p. 9
  • Total X2-link overhead combines the X2-U and X2-C components, including the expected overhead-traffic size.The X2-C component represents cooperative control traffic.Source: B. Vehicle overhead ratio of… · p. 9
  • A cooperative-status change is triggered by vehicular handoff and generates overhead traffic on X2 links.The X2-C link carries cooperative-control information, while X2-U carries handoff information.Source: B. Vehicle overhead ratio of… · p. 9
  • The overhead ratio evaluates cooperative-communication overhead against vehicular communication capacity, with capacity defined for a specified SINR outage threshold.The overhead model also uses average cooperative-control data size per small cell when a handoff is triggered.Source: B. Vehicle overhead ratio of… · pp. 9–10

C. Performance analysis

The numerical analysis reports a minimum overhead ratio for each tested cooperative threshold, while capacity and overhead vary with cell radius, threshold, and vehicle speed. Increasing the threshold raises capacity only up to a threshold-dependent limit while also increasing overhead.

C. Performance analysis

  • At fixed cell radius, capacity increases with cooperative threshold, while capacity gain is limited once the threshold reaches 1.5.At fixed threshold, capacity decreases as cell radius increases; the paper links limited gain beyond 1.5 to a declining probability of cooperation by k small cells.Source: C. Performance analysis · p. 10, Figure 9 · p. 10
  • At fixed threshold, overhead ratio increases with vehicle velocity; at fixed velocity, it increases with cooperative threshold.For fixed thresholds, overhead first decreases and then increases with cell radius, with turning points at 75, 80, and 85 m for thresholds 1.5, 1.2, and 1, respectively.Source: C. Performance analysis · p. 10, Figure 10 · p. 10, Figure 11 · p. 11
  • For different small-cell radii, the minimum vehicular overhead ratios are 4.5 × 10^-4, 3.4 × 10^-4, and 1.3 × 10^-4 at cooperative thresholds 1, 1.2, and 1.5, respectively.The minima occur at radii of 85, 80, and 75 m for thresholds 1, 1.2, and 1.5, respectively.Source: C. Performance analysis · p. 10

VI. CONCLUSIONS

The analysis finds a tradeoff between vehicular communication capacity and handoff performance in cooperative MIMO small cell networks. Evaluating overhead reveals a minimum that can guide threshold optimization.

VI. CONCLUSIONS

  • Cooperative transmission increases vehicular communication capacity and handoff rate, creating a tradeoff between capacity and the vehicular handoff ratio.The analysis considers co-channel interference in 5G cooperative MIMO small cell networks.Source: §VI · p. 11
  • Distance-based analysis derives cooperative and coverage probabilities for small cell base stations modeled by Poisson point processes.The study also proposes vehicular handoff rate and overhead ratio as mobility performance measures.Source: §VI · p. 11
  • Numerical results identify a minimum vehicular overhead ratio across cooperative thresholds, providing a guideline for optimizing vehicular communications.Source: §VI · p. 11
Loading 1604.02660v1…