With the rapid progress in telecommunications, more and more services are provided on the basis of broadband communications, such as video services and high-speed Internet. With worldwide fundamental construction of a backbone network based on optical fiber providing almost unlimited communications capability, the limited throughput of the subscriber loop becomes one of the most stringent bottlenecks.Compared to the capacity of the backbone network, which is measured by tens of gigabits per second, the throughput of the subscriber loop is much lower, only up to hundreds of megabits
per second for wired systems (including fixed wireless access). However, for mobile access the throughput is even lower, and depends on the mobility of the terminal. For example, the peak data rate is only 2 Mb/s for 3G systems.
Since there will be more and more need for mobile services, the poor throughput of mobile access not only limits user applications based on interconnection, but also wastes the capability of the backbone network. This case is quite similar to the traffic conditions shown in Fig. a, which is an image of an ultra-wide expressway with a few narrow entrances.
Since the little paths are rough,narrow, and crowded, the problems in Fig. a are:
Terminals are far away from the expressway, which will consume much power.
Too many cars converge into the same narrow paths.
Little paths converge several times before going into the expressway.
The expressway is used insufficiently, since few cars are running on it.
In telecommunications, the optical fiber network (expressway) is relatively much cheaper than the wireless spectrum (little paths), while the capability of the former is much greater than that of the later. As shown in Fig. b, besides the backbone expressway, there are some dedicated subexpressways used to provide direct entrance for distributed subscribers.The above example implies that the high-capacity wired network, being so cheap, can help us solve the problem of wireless access(too many users crowded in a very narrow bandwidth). The key issue is to provide each mobile user a direct or one-hop connection to an optical network.This structure also follows the trend in network evolution: the hierarchical or tree-like structure of traditional networks will be gradually flattened to simple single-layer ones.
The basic problem of wireless access is that the available spectrum is too limited compared to the almost unlimited service requirement, just like cars jammed in crowded narrow paths. Another basic problem is that there is great attenuation of energy. For example, the transmitter power may be 300 mW in order to transmit 2 Mb/s in a 2 GHz frequency band. Correspondingly, for a future system working on a 5 GHz band at a data rate of 100 Mb/s, we may need 30 W transmission with the same technique. This is impossible for a handset, considering the battery life and the radiation effect on the human body.
CLUES FOR SOLUTION
It seems that the only solution for the first problem is to explore the space resource. The cellular system is a successful example. With a cellular structure, the frequency can be reused as many times as needed. Also, the cellular structure reduces the maximum distance from the terminal to the nearest base station, which is also a clue to solve the second problem.
However, in a traditional cellular system, when the cell size gets smaller, capacity can be increased linearly with cell density. But this is based on the assumption of a large path loss exponent. Pathloss is the amount of loss introduced by the propagation environment between transmitter and receiver. When the cell size is small enough, the exponent gets small, which may be approximately 2; thus, the interference may be so large that the system may not work, as seen in Fig. 2.The above phenomenon indicates that the system capacity cannot be increased anymore when the density of cells reaches a certain level.
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