An important capability for 3G and evolved 3G systems is broadcasting and multicasting, wherein multiple users receive the same information using the same radio resource. This creates a much more efficient approach for delivering content such as video programming to which multiple users have subscriptions. In a broadcast, every subscriber unit in a service area receives the information, whereas in a multicast, only users with subscriptions receive the information. Service areas for both broadcast and multicast can span either the entire network or a specific geographical area. Because multiple users in a cell are tuned to the same content, broadcasting and multicasting result in much greater spectrum efficiency for services such as mobile TV.
3GPP defined highly-efficient broadcast/multicast capabilities for UMTS in Release 6 with MBMS. Release 7 includes optimizations through a solution called multicast/broadcast, single-frequency network operation that involves simultaneous transmission of the exact waveform across multiple cells. This enables the receiver to constructively superpose multiple MBSFN cell transmissions. The result is highly efficient, WCDMA-based broadcast transmission technology that matches the benefits of OFDMA-based broadcast approaches.
LTE will also have a broadcast/multicast capability. OFDM is particularly well-suited for broadcasting, because the mobile system can combine the signal from multiple base stations, and because of the narrowband nature of OFDM. Normally, these signals would
interfere with each other. As such, the LTE broadcast capability, as shown in Figure 64, is expected to be quite efficient.
Figure 64: OFDM Enables Efficient Broadcasting
An alternate approach for mobile TV is to use an entirely separate broadcast network with technologies such as Digital Video Broadcasting–Handheld (DVB-H), which various operators around the world have opted to do. Although this requires a separate radio in the mobile device, the networks are highly optimized for broadcast.
Despite various broadcast technologies being available, market adoption has been relatively slow. Internet trends favor unicast approaches, with users viewing videos of their selection on demand.
UMTS TDD
Most WCDMA and HSDPA deployments are based on FDD, in which the operator uses different radio bands for transmit and receive. An alternate approach is TDD, in which both transmit and receive functions alternate in time on the same radio channel. 3GPP specifications include a TDD version of UMTS, called UMTS TDD.
TDD does not provide any inherent advantage for voice functions, which need balanced links—namely, the same amount of capacity in both the uplink and the downlink. Many data applications, however, are asymmetric, often with the downlink consuming more bandwidth than the uplink, especially for applications like Web browsing or multimedia downloads. A TDD radio interface can dynamically adjust the downlink-to-uplink ratio accordingly, hence balancing both forward-link and reverse-link capacity. Note that for UMTS FDD, the higher spectral efficiency achievable in the downlink versus the uplink is critical in addressing the asymmetrical nature of most data traffic.
The UMTS TDD specification also includes the capability to use joint detection in receiver- signal processing, which offers improved performance.
One consideration, however, relates to available spectrum. Various countries around the world including those in Europe, Asia, and the Pacific region have licensed spectrum available specifically for TDD systems. For this spectrum, UMTS TDD or, in the future, LTE
in TDD mode is a good choice. It is also a good choice in any spectrum that does not provide a duplex gap between forward and reverse links.
In the United States, there is limited spectrum specifically allocated for TDD systems.129
UMTS TDD is not a good choice in FDD bands; it would not be able to operate effectively in both bands, thereby making the overall system efficiency relatively poor.
As discussed in more detail in the “WiMAX” section, TDD systems require network synchronization and careful coordination between operators or guardbands, which may be problematic in certain bands.
There has not been widespread deployment of UMTS TDD so far. Future TDD deployments of 3GPP technologies are likely to be based on LTE.
TD-SCDMA
Time Division Synchronous Code Division Multiple Access (TD-SCDMA) is one of the official 3G wireless technologies being developed, mostly for deployment in China. Specified through 3GPP as a variant of the UMTS TDD System and operating with a 1.28 megachips per second (Mcps) chip rate against 3.84 Mcps for UMTS TDD, the primary attribute of TD-SCDMA is that it is designed to support very high subscriber densities. This makes it a possible alternative for wireless local loops. TD-SCDMA uses the same core network as UMTS, and it is possible for the same core network to support both UMTS and TD-SCDMA radio-access networks.
TD-SCDMA technology is not as mature as UMTS and CDMA2000, with 2008 being the first year of limited deployments in China in time for the Olympic Games. Although there are no planned deployments in any country other than China, TD-SCDMA could theoretically be deployed anywhere unpaired spectrum is available—such as the bands licensed for UMTS TDD—assuming appropriate resolution of regulatory issues.