Semandex Networks (United States)

• Resource allocation: what time-frequency bandwidth to be allocated to the selected users and what transmit power to be used.

In this chapter, we will study power and bandwidth allocation in a multi-cell scenario where inter-cell interference dominates. As in Chapter 5, we assume that the channel gain does not vary over time or frequency.

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The medium of wireless communications is the wireless radio frequency channel. We are interested in the characteristics of the wireless channel, in particular, how the channel response varies over time and frequency, as well as over the distance between a transmitter and a receiver. The variation in the channel response is usually referred to as channel fading. For a given signal, channel fading depends on the particular propagation environment, such as buildings, walls, ground, vehicles, between the transmitter and the receiver, as well as the carrier frequency of the signal. To characterize channel fading, we often use a statistical approach based on measurements made in a large variety of environments. Statistically, channel fading can be characterized by the following two different types of behaviors:

So far we have studied the system design principles of OFDMA-based mobile broadband under a conventional cellular network framework. The basic premises of the framework are: • The base stations use high transmit power, and are placed at carefully chosen locations, ideally at the vertices of regular hexagons. • A user is connected to the “best” base station. The best base station is usually the closest one that has the greatest downlink signal strength received at the user. • A base station is open to all the users within a cell by providing “unrestricted” access service. • Both the downlink and uplink communications are one-hop between the base station and the users. The users do not communicate directly even if they are nearby to each other. • The time-frequency resource is reused spatially. Among cells reusing the same resource, a signal transmitted in one cell is treated as interference/noise in another cell. • The spectrum to be used in a cell is fixed and known to both the base station and the users. In this chapter, we explore several ideas that go beyond the conventional cellular framework in pursuit of the next performance leap. The first such idea is heterogenous network topology . Inwireless, moving the transmitter and receiver close to each other increases signal strength, reduces required transmit power and thus interference to other transmissions, and allows dense spectrum reuse.

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We study the delay characteristics of voice packets that share a transmission medium with bursty data traffic. In order to model the data traffic accurately, we used real data traffic traces upon which we superimposed simulated voice sources generating real time traffic at a constant rate. Our simulation environment allows us to easily develop and compare different adaptive algorithms for the play-out delay of voice streams. We motivate and compare the performance of three algorithms based on exponential smoothing techniques. We test the algorithms in the context of a shared 10 Mbps Ethernet LAN, using the CSMA/CD protocol.

The Clara prototype architecture collocates routing and computational functionality within a network, providing a scalable, high-performance computing switch router for computational services. Multiple off-the-shelf PCs provide Clara with computational power to, for example, perform real-time transcoding of video with minimal overhead.