Multihop Cellular Networks

Introduction

Multihop cellular network can potentially enhance coverage, data rate, QoS performance in terms of call blocking probability, bit error rate.
This article aims to present an overview of resource allocation framework for out-of-band relaying and the throughput enhancement.
The conventional approach to increase network capacity is to install more base stations (BSs) to exploit spatial reuse. This solution is not very efficient because the cost of the BS transceiver is quite high.
An alternative approach is to employ relay stations (RSs) as intermediate nodes to establish multihop communication paths between mobile hosts (MHs) and their corresponding BSs.

Relaying for Load Balancing

To balance traffic load among highly loaded cell and lightly loaded, the author proposed primary relaying and secondary relaying schemes.
Each cell has a finite number of channels and preinstalled RSs. Each RS is equiped with two air interface, a C (cellular) interface for communications with a BS and and R (Relaying) interface for communications with MHs or other RSs.
Mobile hosts also have a C interface to communicate with a BS and R interface to communicate with RSs. relay.jpg

In conventional System, if an MH wishes to establish a new call and cannot find an available channel in its home, it is blocked.
In an MCN using primary relaying, this MH switches to its R interface and establishes multihop communication with a neighboring BS through multiple RSs.
If primary relaying is not possible, the secondary relay will be activated. A new call inisiated by MH2 could not be accomodated By BS A. so ongoing call from MH 1 may be diverted to BS B by using multihop connection through RS 1 and RS 2. The channel for MH1 is released and re-allocated to MH 2

In-Band Relaying VS Out-Band Relaying

Relaying can be used to assist communications between MHs BS.
The capacity advantage of multihop relaying comes from the reduction of path loss..
In fact, it shown that relaying not always beneficial, especially if the target MH is close to the BS.

In ODMA proposed in-band-relaying method, MHs can serve as RSs if they are not transmitting their own data
The advantage is no modification of MHs.
In future, MHs are likely to be equipped with multiple interfaces to communicate with different wireless system on different frequency band,it called out-of-band relaying,.

Fixed VS Mobile Relay

Fixed RSs can be much cheaper than normal BSs because their function is just to decode received packets, then re-encode and forward them to the next station.
For mobile RSs, a significant performance gain can be achieved free of charge in low traffic load and high node density because many idle MHs are available to relay data from active MHs.

Relaying VS Cooperative Transmission

Cooperative diversity has emerged as an efficient way to achieve diversity gain through forming a virtual antenna array.
The advantage of this type of cooperation is that each node (i.e., MH) need only one antenna and a virtual antenna array is formed through multiple nodes in the network.
the most popular cooperative strategies are amplify-and-forward (AF) and decode-and-forward (DF).

The cooperative transmission concept

  • single-state :transmitting node employs several RSs to assist its transmission.
  • multiple-state : through multiple clusters to reach the receiving node

The are two popular approaches to modeling interference in an MCN :

  • Signal to Interference and Noise Ratio (SINR) [4] : In that paper the authors first obtained the minimum path loss routing solution.
  • graph-theoretic approach : graph-theoretic approach.
    In this approach transmission links that interfere with each other are assumed to be known.
    Only links that do not interfere with one another are allowed to be active at the same time.

System Model

Each MH has two Interface. one is used to communicate with the BS using the cellular frequency band and another is for relaying using the ad hoc frequency band.
MH can serve as an RS for another MH and forward data to MH.

Resource Allocation Algorithm

Relay selection aims at improving the data rate transmitted by the BS to an MH by relaying traffic via an idle MH.
With Minimal Power (MinP) algorithm requires O(M2B2K) feasibility check and power calculation.
To reduce the computational complexity, author modify with algorithm Maximum Channel Gain and Transmission Rate Ration (MGR). This algorithm requires O(MBK) feasibility checks and reduce power calculation.

Performance Evaluation

We evaluate throughput performance for the resource allocation framework with and without relaying.
Average data rates achieved by each MH for both MinP and MGR algorithms with and without relaying.

significant throughput enhancement can be achieved by multihop transmission scheme compared to direct transmission.
Relative throughput enhancement due to multihop implementation increases with the number of users.
Although the data rate achieved by each MH decreases because the network more congested.

When cell size increases, the throughput achieved by non-relaying schemes decrease significantly.
When relaying is employed, throughput remains stable for both MinP and MGR algorithms for different values of cell size.

Conclusions

Cooperative diversity technique can be exploited in multihop cellular networks.
Significant throughput gain can be achieved through multihop implementation.

Source : IEEE Communications Magazine – September 2007 by Long Le, University of Waterloo and Ekram Hossain, University of Manitoba

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