ple, Barker Code is used in 802.11, Complementary Code Keying (CCK) is used in 802.11b, and 64-bit Walsh Code is used in CDMA cellular. Figure 13.9 shows an example of DSSS that uses Barker Code, which is an 11-bit chipping code. The 1 bits are encoded as a particular sequence of 1s and Os, and the 0 bits are the inverse of that sequence. In the course of a transmission, if a bit is affected by noise and several chips get knocked out, the receiver can determine from the pattern of remaining bits whether a 1 or a 0 got knocked out and recover that information accurately. This is why DSSS provides such good performance and resistance to interference and noise. DSSS also provides great reliability because the DSSS operation generates more bits per second than it starts with, and the resulting signals spread over a wide range of frequencies when it is transmitted, minimizing the impact of inter- ference and multipath fading. Chapter 13 Wireless Communications Basics Consider an 11-bit chipping code, as follows: 1=11100010010 (Note: a Barker Code) 0=00011101101 (the inverse of the 1 bits) Input data: 0110 Results in: 00011101101 11100010010 11100010010 00011101101 Figure 13.9 An example of DSSS OFDM OFDM is the newest spread spectrum technique, and its main purpose is to resolve the problems that result from multipath distortion. OFDM has some key advantages over CDMA, which is used in many of today's 3G cellular networks. First, it is more robust, which means it provides better performance in multipath situations. It also allows for simpler receivers. Perhaps most importantly, OFDM is more amenable to MIMO technologies than are the other spread spectrum tech- niques. A Nortel (www.nortel.com) trial conducted in early 2005 provides a good example: Nortel was able to demonstrate peak data rates of 37Mbps over a stan- dard 5MHz PCS mobility band. Using OFDM and MIMO, wireless subscribers could download a 128MB file in approximately 30 seconds, 4 to 10 times faster than today's 3G networks can support (see www.nortel.com