MA) is a technique which aims to multiply the data throughput of a wireless network by using spatial position as an additional parameter to control user access to the transmission medium. As a simple example, if a base station is equipped with sector antennas with a 30° horizontal beamwidth, it can separate users into twelve spatial segments or channels depending on their location around the base station. (Figure 3-17 shows the beam pattern for a base station with 6 such elements.) This arrangement would enable the network to achieve a potential twelve-fold increase in data capacity compared with a base station using a single isotropic antenna. As well as simple sector antennas, smart antenna systems are being developed which combine an array of antennas with digital signal processing capabilities in order to achieve spatial control of transmission and reception. Smart antenna systems can adapt their directional characteristics in response to the signal environment and system demands, and can provide the basis for SDMA. Generally a second multiple access technique, such as TDMA or CDMA, is also used in combination with SDMA in order to allow multiple user access within a single spatial segment. Space division multiplexing (SDM), as opposed to SDMA, is based on the use of multiple propagation paths to simultaneously transmit multiple data channels using the same RF spectrum. This is the basis of MIMO radio (see the Section "MIMO Radio, p. 124") which is specified in the IEEE 802.11n standard. Code Division Multiple Access CDMA is closely related to DSSS, where a pseudo-noise code is used to spread a data signal over a wide bandwidth in order to increase its immunity to interference. As noted above, if two or more transmitters use different, orthogonal pseudo-noise (PN) codes in DS spread spectrum, Radio Communication Basics Figure 4-20: Construction of the Walsh Codes they can operate on the same frequency band and in the same physical area without interfering. This is because a correlator using one PN code will n