Enhancements for IEEE 802.11 Networks with Directional Antennas Tamer Nadeem Siemens Corporate Research Princeton, NJ 08540 tamer.nadeem@siemens.com Abstract—Directional antennas have been introduced to improve the performance of 802.11 based wireless networks. Station equipped with directional antennas can beamform data in a specific direction. However, IEEE 802.11 is developed with omni-directional antennas in mind. Thus, deploying IEEE 802.11 in a directional antenna environment leads stations to be conservative in blocking their own transmissions in favor of the ongoing transmissions. In this paper we propose two opportunistic enhancements for the IEEE 802.11 networks using directional antennas. The first enhancement is to augment the 802.11 MAC protocol with additional information (i.e., location of the stations) that gives a node the flexibility to transmit data while there are ongoing transmissions in its vicinity. The second enhancement, using the augmented MAC protocol, alters the way the 802.11 accessing its MAC data queue. Index Terms—Wireless Networks, IEEE 802.11, Medium Access Control, Carrier Sense, Directional Antenna, Performance Analysis. I. I NTRODUCTION Directional antennas have been introduced to improve the performance of wireless networks [1]–[5]. A station equipped with directional antennas can beamform data in a specific direction. The transmitter beamforms the data in the direction of the receiver with diminished interference in the remaining directions. Thus, the network capacity is increased as a consequence of spatial spectrum reuse. The IEEE 802.11 [6], and carrier sensing protocols in general, was developed with omni-directional antennas in mind. It assumes that all packets (RTS/CTS/DATA/ACK packets) are transmitted as omni-directional signals and are received by all nearby nodes. Deploying IEEE 802.11 in a directional antenna environment does not exploit fully the directional antennas characteristics. The main reason is that 802.11 stations are conservative in blocking their own transmissions in favor of the ongoing transmissions, although their transmissions will not result in interferences with other transmissions. Thus, many modifications (e.g., [5], [7]–[9]) were introduced to allow 802.11 based protocols to exploit the intrinsic features of directional antennas to increase throughput. In this paper, we propose two opportunistic enhancements for IEEE 802.11 networks to increase the number of simultaneous data transmissions, and thus, improve the overall wireless network throughput. The term opportunistic refers to D B E C A Fig. 1. Opportunistic MAC example mechanisms that exploit the directional antennas characteris- tics by taking immediate advantage of any circumstances of possible benefit. II. PROBLEM FORMULATION Our proposed enhancements to the IEEE 802.11 aim to decrease the number of unnecessary blocking. The first enhancement we propose is to augment the MAC protocol with additional information (e.g., locations of the sender and the receiver) that gives a station the flexibility to transmit data in the presence of ongoing transmissions in its vicinity. In the original 802.11 protocol, a station blocks its transmission when it senses a busy carrier. But, if the direction of transmission does not interfere with the ongoing transmissions, this blocking is unnecessary. One way for a station to determine whether its transmission will interfere with these ongoing transmissions is by collecting the locations of transmitters and receivers. Several methods have been proposed in literature allowing a station to acquire its own location, such as using GPS [10], or using RF based localization method [11], [12]. Consider node A that is engaged in a beamforming transmission to the direction of node B as in Figure 1. If Node C wants to beamform data to node D, the running IEEE 802.11 carrier sense mechanism at node C will block this transmission because of the ongoing transmission between A and B. However, since the ongoing transmission is not destined to C and since the C-D transmission direction would not interfere with A-B transmission as shown in the figure, node C should not block. To achieve this, we developed a scheme, called OPP CS scheme, where a station can determine more flexibly, based on directional sensing and locations of the stations, whether to block its transmission or not. Network wide, more concurrent transmissions are permitted by OPP CS and the overall network throughput can be improved.