3G networks 4G networks Data rate 384 kbps to 2 Mbps 20-100 Mbps Frequency band 1.8-2.4 GHz 2-8 GHz Bandwidth 5 MHz About 100 MHz Switching technique Circuit- and packet-switched Completely digital with packet voice Radio access technology WCDMA, CDMA-2000 etc. OFDMA, MC-CDMA etc. IPv4.0, IPv5.0, IPv6.0 IPv6.0 4G system must be capable of providing highly efficient and cost-effective solutions for wireless network users. Table 10.1 gives a comparison of few key features of 3G and 4G technologies. 10.2 KEY TECHNOLOGIES FOR FOURTH-GENERATION NETWORKS Although there are a few technologies vying for the top stop for fourth-generation networks, OFDM and MC-CDMA may turn out to be the key competitors for the physical interface, and All-IP and WLAN for the upper layers. This section introduces OFDM (orthogonal frequency-division multiplexing) for the air-interface and All-IP for the upper layer. Later in the chapter, an overview of WLAN systems and network planning is given. 10.2.1 Orthogonal Frequency-division Multiplexing OFDM is a frequency-division multiplexing technique that is used to transmit large amounts of data on a radio signal. Basically, a 'big' radio signal is subdivided into smaller signals and then transmitted to the receiver using different frequencies. It is thought that OFDM will be able to fulfil the three most important requirements of 4G mobile networks: higher coverage and capacity, with desired QoS at minimum cost. The biggest advantage of the OFDM technique is the mutual orthogonality of its carriers, which provides a high spectral efficiency. This is possible because there is no guard band and carriers can be packed very close together. Most of the alternative techniques require guard bands. In OFDM, even without a guard band, there is no interference because the carriers are orthogonal. The spectrum for OFDM lies between 200 MHz and about 3.5 GHz, with a spectral efficiency of about 1 bit/s/Hz. Coverage in CDMA systems is limited by the phenomenon of cell breathing (described elsewhere in this