Distributed Medium Access Control for Next Generation

Distributed Medium Access Control for Next Generation

CDMA Wireless Networks

Generic Distributed Network Model

We focus on a generic network model where a number of wireless mobile nodes are organized in a flat fashion, communicating with each other via one-hop or multiple-hop connections without any central control. Connections to outside correspondence nodes are supported through a gateway (e.g., in the wireless backbone) or an access point (e.g.,in ad hoc networks or WPAN).

In the past decade, code-division multiple access (CDMA) technology has been developed and deployed popularly with great success in the second-generation (2G) cellular systems (e.g., IS-95). It also been selected as the major multiple access technology for the third-generation (3G) systems. CDMA has many promising merits such as universal frequency reuse, soft handoff, and high spectrum efficiency.

Desired Features of Distributed MAC:

AMAC scheme is to coordinate the mobile nodes access to the shared medium such that efficiency can be achieved. Although MAC schemes in traditional cellular networks and WLAN have been extensively studied in the literature, more efforts are still needed for distributed MAC supporting multimedia traffic in the network without a central controller.

In a centralized network such as the cellular network, the central controller makes decisions on when and how the mobile nodes should access the medium. However, in a network without a central controller, MAC is carried out in a collectively coordinated manner, where a mobile node determines its access behavior according to its local observation.

Features Of Distributed MAC:

Quality of service (QOS) support

Service differentiation

Low overhead

Bandwidth Efficiency

Distributed Code Assignment

Generally, before a transmission, the transmitter and receiver need to know the code (sequence) to be used. In cellular networks, the BS collects information from all the nodes, and informs them what codes are used in upcoming transmissions. However, as our network model dose not have central authority, distributed code assignment is desired.

There are three kinds of codes that can be used

v Common code

v Receiver based code

v Transmitter based code

Distributed Interference Control

By proper code assignment to the CDMA transmissions, collisions among them can be avoided. However the interference among simultaneous transmissions with unique codes should also be managed Generally, power control can be applied to manage the interference levels in the network, targeting at guaranteed transmission accuracy of each link. Power control can be executed in two manners: global or incremental.

A global power control approach re-assigns the power levels of all the links in the network at any time when there is a change in link activities, e.g., when a new link is admitted or an existing link is completed.

In the incremental power control, it is important for a candidate transmitter to know the interference tolerance level of active links.

The Proposed Distributed MAC

In our network model, the MAC is to coordinate the one-hop transmission from a transmitter to one of its neighbors. Each node has its own transmitting code and receiving code, the information of which is available to its neighbors through some information exchanges in the routing protocol.

I am discussed the limitations of existing distributed CDMA-based MAC schemes when applied to our generic network model with distributed control. By letting active receivers estimate potential increase in the interference level, our proposed MAC scheme can achieve bit-level QoS, low overhead, accurate channel and interference estimation, and high bandwidth efficiency.