In recent years, research has been performed in the area of multihop cellular networks due to their potential for higher capacity, lower energy consumption, and wider coverage. The architecture of a multihop cellular network is similar to that of a traditional cellular network but differs in one significant way. In a multihop cellular network, the physical layer of mobile terminals is modified so that they can transmit not only to base stations but also directly to other mobile terminals. A mobile terminal can then reduce its transmission power and use other terminals as relays to enable its traffic to reach a base station. For this reason, the architecture of the multihop cellular
network is similar to the ad hoc network in that transmissions from a source to a destination may require multiple hops.
Although more transmissions are required for the traffic to reach its destination, the reduction of path loss allows us to reduce the transmit power sufficiently so that the average power consumption per terminal is lower. The majority of the intercellular interference at a base station is caused by transmissions of mobile terminals that are near the boundaries of neighboring cells. Because the transmission power of terminals at the cell edge of neighboring cells is reduced in a multihop cellular network, the intercellular interference may also be lower. The decrease in intercellular interference in a multihop cellular network is what gives them the potential of higher Signal to Interference and Noise Ratios SINR than traditional cellular networks[3].
Brief Description:
Multihop cellular networks were envisioned to solve many of the limitations that are characteristic of ad hoc networks while maintaining their advantages. By allowing multihop Communications on top of a cellular infrastructure, many of the drawbacks of traditional cellular networks can also be mitigated. Multihop cellular networks have been shown to increase SINR and coverage as compared to a traditional cellular network with an overall decrease in power consumption by mobile terminals. Mobile terminals in a multi- hop cellular network transmit over smaller distances than in a traditional cellular network which makes higher bit rates possible while maintaining a low EIRP transmission power. Multihop cellular networks can also provide load-balancing among base stations and help the network recover from base station failures. Multihop cellular networks also solve the ad hoc network's problems of inefficient channel assignment and routing due to the presence of a centralized base station. The presence of base stations allows multi-hop cellular networks to be used in networks where a large coverage area is required as opposed to ad hoc networks that can only cover a limited area.
Proposed Scheme:
Multihop cellular networks attempt to benefit from the advantages of traditional cellular networks and ad hoc networks by utilizing properties from each of these networks. In this hybrid network, transmissions between mobile terminals and base stations are performed using multiple hops. Mobile terminals (or dedicated relay stations) are required to relay traffic for other terminals to the base station. Because mobile terminals and base stations are no longer required to be in direct transmission range of one another, their transmission power can be reduced and they can be designed to have lower power transmitters. A cell in a multihop cellular network is defined differently than in a traditional cellular network because transmissions from a mobile terminal can no longer necessarily reach the base station in a single hop. In a multihop cellular network, a cell is defined as one base station and all of the mobile terminals that transmit to that base station using multiple hops if necessary. The reverse link is defined as the transmissions emanating from a mobile terminal having the base station as their final destination. The forward link is defined as the transmissions emanating from a base station having a mobile terminal as their final destination. The The transmission range of each mobile terminal is decreased in a multihop cellular network and therefore traffic cannot necessarily be transmitted from a mobile terminal to a base station in a single hop. The impact of fading and other impairments in wireless channels can be counteracted by leveraging communication diversity and introducing cooperative paradigms, where third-party nodes contribute to assist the communication[4].Fig.1 Example of Routing in a Multihop Cellular Network
Comparison among,
1. Traditional Network
2. Ad-Hoc Network
3. Co-operative Diversity
(Multi-Hoc Network)
1. Traditional Network:
A base station is located at the center of each cell and is responsible for handling the traffic of all mobile terminals within the cell. Base stations have a wire line connection to the Public Switched Telephone Network (PSTN) through the Mobile Switching Center (MSC) which is connected to the Base Station Controller (BSC) as shown in Figure 2.A fraction of the overall radio spectrum is allocated to each cell. Spectrum can be reused for cells that are far apart due to the power attenuation of radio signals that occurs with distance. This allows the network operator to service a greater number of customers using the same spectrum.
Smaller cells also mean that base stations and mobile terminals will require lower power transmitters and smaller batteries. Because users are mobile, a handoff mechanism is necessary when a user moves from one cell's coverage area into another. Efficient handoff schemes have been developed so that mobile coverage is ubiquitous across cells and handoff occurs without the user's knowledge, thus allowing calls to be uninterrupted.
Fig.2 Cluster of Cells in a Tradional Cellular Network
2. Ad-Hoc Network:
An ad hoc network is a multihop wireless network where a collection of mobile terminals form a temporary network without any centralized infrastructure or wire line backbone.
The main feature that distinguishes these networks from the conventional wireless cellular network is that an ad hoc network does not rely on an expensive centralized base station. For this reason, ad hoc networks are rapidly deployable and self-organizing.
Transmissions between terminals in an ad hoc network may require multiple hops and therefore all terminals work together to ensure successful packet delivery from the source to the destination. Terminals in an ad hoc network may be either stationary or mobile resulting in a dynamic topology requiring frequent routing path updates. Initial research on ad hoc networks was primarily conducted for military applications where rapid network creation and survivability are of the utmost importance. Relaying on a centralized base station is impractical and often impossible in a hostile environment.
Conclusion:
In this paper we have provided an overview of Multihop cellular networks and their advantages over traditional cellular networks.we have compared all types of wireless networks with Multihop cellular networks which provides a better performance over conventional cellular networks.
