Browse Prior Art Database

WLAN Wide Separation Spatial Diversity

IP.com Disclosure Number: IPCOM000008949D
Original Publication Date: 2002-Jul-25
Included in the Prior Art Database: 2002-Jul-25
Document File: 2 page(s) / 91K

Publishing Venue

Motorola

Related People

Michael H. Retzer: INVENTOR [+3]

Abstract

In packet based wireless LAN systems operating at microwave radio frequencies, the actual rate of successfully decoding any particular packet at a single receiver may be quite low. This is due to poor attenuation and multipath characteristics of the employed microwave radio channels. This performance is particularly degraded when penetrating buildings. In some areas the coverage from all available Access Points may be marginal, so handoff to better cells may not result in better performance. Also, in the case of many multimedia applications, the delays introduced by retries may be unacceptable.

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WLAN Wide Separation Spatial Diversity

By Michael H. Retzer, Randy L. Ekl & Joseph E. Phillips

 
 

In packet based wireless LAN systems operating at microwave radio frequencies, the actual rate of successfully decoding any particular packet at a single receiver may be quite low. This is due to poor attenuation and multipath characteristics of the employed microwave radio channels. This performance is particularly degraded when penetrating buildings. In some areas the coverage from all available Access Points may be marginal, so handoff to better cells may not result in better performance. Also, in the case of many multimedia applications, the delays introduced by retries may be unacceptable.

Conventionally, Wireless LAN provides increased coverage and capacity by handing off between geographically separate cells, each cell using different non-interfering channel characteristics. In conventional operation a mobile device will be communicating at any one time with one, and only one, Access Point (AP) within a fixed cell. By allowing communication with multiple cells, the probability of a dropped packet when the mobile device is within a region of marginal coverage is reduced. Suppose the probability of a successful reception to/from one cell is Pn. If the same information can be sent from a second path, separated in space so that the probability of success is uncorrelated with the first path, Pm, then the probability of successful reception of at least one copy of the information is  improved to 1- [1 – Pn][1-Pm]. For a 0.99 success rate from either cell individually, the combined success rate is 0.9999 using the wide spatial diversity.

One could imagine a mobile device communicating with non-synchronized networked cells operating on different channels, or contending on the same channel. In the first case, throughput performance would be degraded by packets missed while listening on the other cell. In the second case, more packets would collide due to the increased contention.

A way to mitigate this degraded performance is by synchronizing beacon transmissions in each adjacent cell. Each cell would then have a defined non-contention period, relative to its beacon transmission. This allows a mobile device to receive or transmit at determinable instants in adjacent cells. Establishing this synchronization when adjacent cell AP’s can hear each other’s beacon transmissions is fairly easily...