[Paper Review] Cell planning for mobility management in heterogeneous cellular networks

Arpan Chattopadhyay, Bartłomiej Błaszczyszyn+1 more|arXiv (Cornell University)|May 24, 2016

Advanced MIMO Systems Optimization19 references3 citations

TL;DR

This paper proposes a heterogeneous two-tier cellular network where mobile users are served exclusively by macro base stations to reduce handoff frequency and data outage, while static users are served by both macro and micro base stations. Using stochastic geometry, the authors derive optimal power levels and macro base station fractions that maximize throughput for both user types under spectral efficiency and power constraints, showing that homogeneous network designs are optimal under certain conditions.

ABSTRACT

In small cell networks, high mobility of users results in frequent handoff and thus severely restricts the data rate for mobile users. To alleviate this problem, we propose to use heterogeneous, two-tier network structure where static users are served by both macro and micro base stations, whereas the mobile (i.e., moving) users are served only by macro base stations having larger cells; the idea is to prevent frequent data outage for mobile users due to handoff. We use the classical two-tier Poisson network model with different transmit powers (cf [1]), assume independent Poisson process of static users and doubly stochastic Poisson process of mobile users moving at a constant speed along infinite straight lines generated by a Poisson line process. Using stochastic geometry, we calculate the average downlink data rate of the typical static and mobile (i.e., moving) users, the latter accounted for handoff outage periods. We consider also the average throughput of these two types of users defined as their average data rates divided by the mean total number of users co-served by the same base station. We find that if the density of a homogeneous network and/or the speed of mobile users is high, it is advantageous to let the mobile users connect only to some optimal fraction of BSs to reduce the frequency of handoffs during which the connection is not assured. If a heterogeneous structure of the network is allowed, one can further jointly optimize the mean throughput of mobile and static users by appropriately tuning the powers of micro and macro base stations subject to some aggregate power constraint ensuring unchanged mean data rates of static users via the network equivalence property (see [2]).

Motivation & Objective

• To address the severe throughput degradation experienced by mobile users due to frequent handoffs in small cell networks.
• To design a heterogeneous network architecture where mobile users are served only by macro base stations to reduce handoff frequency and data outage.
• To jointly optimize the transmit power of macro and micro base stations and the fraction of macro base stations to maximize the average throughput of both mobile and static users.
• To ensure the mean data rate of static users remains unchanged via network equivalence, under a total power constraint.
• To provide a stochastic geometry-based analytical framework for evaluating user performance in mobility-aware cell planning.

Proposed method

• Models the network as a two-tier Poisson process with macro and micro base stations having different transmit powers.
• Assumes static users follow a homogeneous Poisson point process, while mobile users move along random straight lines generated by a Poisson line process at constant speed.
• Uses stochastic geometry to compute the average downlink data rate for typical mobile and static users, accounting for handoff outage periods.
• Defines user throughput as data rate divided by the mean number of co-served users per base station.
• Applies a constrained optimization framework to jointly tune macro base station fraction and transmit powers under an aggregate power constraint.
• Employs the network equivalence property to maintain static user data rates while optimizing for mobile user performance.

Experimental results

Research questions

• RQ1 What is the optimal fraction of macro base stations that minimizes handoff-related data outage for mobile users in a heterogeneous network?
• RQ2 How do user mobility speed and network density affect the trade-off between handoff frequency and data rate for mobile users?
• RQ3 Can a heterogeneous network architecture with macro-only serving for mobile users improve overall system throughput compared to homogeneous small cell deployments?
• RQ4 Under what conditions does a homogeneous network design (with equal power levels) become optimal for balancing mobile and static user throughput?
• RQ5 How does the choice of user velocity distribution and base station density influence the optimal cell planning strategy?

Key findings

• When the product of mobile user speed, handoff time, and the square root of network density is large, it is optimal to restrict mobile users to only a fraction of base stations to reduce handoff frequency and outage.
• For high-speed mobile users or dense networks, serving mobile users via only macro base stations significantly improves their average data rate by minimizing handoff-related outages.
• The optimal design often results in a homogeneous network (equal macro and micro base station powers) when the weight on static user throughput is high, due to sensitivity of mobile user rate to power and fraction parameters.
• The network equivalence property allows maintaining static user data rates while optimizing for mobile user performance through power and base station fraction tuning.
• Numerical results show that macro base stations can improve mobile user throughput despite larger cell sizes, due to reduced handoff signaling and outage.
• The optimal solution depends on user density, mobility speed, and system constraints such as maximum transmit power and base station availability.