3G Evolution. HSPA and LTE for Mobile Broadband

f3

f1 f2 f3

Figure 20.8 Fractional frequency reuse. In this example there is a reuse of 3 for users at the cell edge, while users closer to the BSs have a single frequency reuse.

up to 250 km/h and spectral efficiency on par with mobile systems is part of the MBWA system’s scope, making it potentially more than a MAN system.

The work on the 802.20 standard was initiated in 2002, and has since then moved to become a high data rate, flexible bandwidth system that is quite similar to the other technologies discussed in this book, such as LTE, CDMA2000 Rev C, and IEEE 802.16. The main proposals under discussion today are MBFDD and MBTDD (Mobile Broadband FDD and TDD) as described in [3].

Some key features of the proposal in [3] are listed below. Note that there are some concepts very similar to CDMA2000 Rev C:

•The radio interface has OFDM data transmission with 9.6 kHz subcarrier spacing and an FFT size of 512, 1024, and 2048, supporting operation bandwidths of 5, 10, and 20 MHz, respectively. The cyclic prefix is configurable from 6% to 23% of the OFDM symbol duration. Most of the uplink control channels are transmitted with CDMA on a contiguous set of OFDM subcarriers.

•Hopping of subcarriers is possible at symbol level, occurring every two symbol intervals (to allow for Alamouti coding), or at a block level. Blocks are about the same size as a resource block in LTE.

•There is one frame structure for FDD and one for TDD where guard times are added between upand downlink parts.

•There is support of QPSK, 8PSK, 16QAM, and 64QAM modulations.

•Convolutional coding is used for small packets (mainly signaling) and Turbo codes for larger packets. HARQ is used with a modulation step down for retransmissions.

•Fast closed-loop uplink power control is used for control channel power. Traffic channel power is set relative to control channels. Uplink interference from neighboring Access Points (AP) is controlled through an Other Sector Indication Channel (F-OSICH) that signals a load indication to the mobiles.

•Handovers are mobile station initiated. The mobile station makes SINR measurements on candidate AP pilots and keeps an active set of up to 8 APs. All have allocated MAC IDs and control resources, but only one is the serving AP. Handover can be disjoint (independent) between uplink and downlink.

•The uplink can be quasi-orthogonal where multiple mobiles are assigned the same bandwidth resources.

•A fractional frequency reuse scheme enables mobiles in different channel conditions to have different frequency reuse.

•Space Time Transmit Diversity (STTD) is supported.

•There is support for downlink MIMO with single codeword and multiple codeword designs.

•Eigen-beam-forming is supported using feedback from the mobile station. A special beam-forming mode is supported for TDD operation.

•There is possibility of embedding other physical layers such as single frequency network technologies for broadcast services.

•Scalable bandwidth options are supported so that mobiles capable of only receiving on a lower carrier bandwidth (say 5 MHz) can still be operated on a 20 MHz carrier being transmitted by the base station.

20.6Summary

The IMT-2000 technologies and the other technologies introduced above are developed in different standardization bodies, but all show a lot of commonalities. The reason is that they target the same type of application and operate under similar conditions and scenarios. The fundamental constraints for achieving high data rates, good quality of service, and system performance will require that a set of efficient tools are applied to reach those targets.

The technologies described in Part II of this book form a set of such tools and it turns out that many of those are applied across most of the technologies discussed here. To cater for high data rates, different ways to transmit over wider bandwidth is employed, such as singleand multi-carrier transmission including OFDM, often with the addition of higher-order modulation. Multi-antenna techniques are employed to exploit the ‘spatial domain,’ using techniques such as receive and transmit diversity, beam-forming and multi-layer transmission. Most of the schemes also employ dynamic link adaptation and scheduling to exploit variations in channel conditions. In addition, coding schemes such as Turbo codes are combined with advanced retransmission schemes such as HARQ.

As mentioned above, one reason that solutions become similar between systems is that they target similar problems for the systems. It is also to some extent true that some technologies and their corresponding acronyms go in and out of ‘fashion.’

Most 2G systems were developed using TDMA, while many 3G systems are based on CDMA and the 3G evolution steps taken now are based on OFDM. Another reason for this step-wise shift of technologies is of course that as technology develops, more complex implementations are made possible. A closer look at many of the evolved wireless communication systems of today also show that they often combine multiple techniques from previous steps, and are built on a mix of TDMA, OFDM, and spread spectrum components.

Another major activity within ITU-R concerning IMT-Advanced is the preparation for WRC’07, the upcoming World Radio Congress. One target is to identify spectrum that is suitable for IMT-Advanced and that is available globally, since adequate spectrum availability and globally harmonized spectrum are identified as essential for IMT-Advanced [43]. In preparation for WRC’07, a range of sharing studies between IMT-Advanced and other technologies in the different candidate bands have been produced within WP8F.

21.2 The research community

In the research community, several research projects are run in the area of IMTAdvanced and the next generation of radio access. One example is the Winner project, which is partly funded by the European Union. The Winner concept has many components that are very close to LTE. However, Winner is targeting higher data rates than LTE and is therefore designed for a wider bandwidth than 20 MHz. Another key difference is that the Winner concept will work with relaying and multi-hop modes. For further details, see the Winner homepage [118].

Other regions are running research projects similar to the European ones, such as the Future project in China, all with the goal of making an IMT-Advanced radio interface proposal. In the end however, these research communities will not make the final inputs of IMT-Advanced concepts. It is expected that the established standards developing organizations (ETSI, ARIB, CWTS, etc.) will do down selections of proposals from their respective regions. A global standard body such as 3GPP will most likely also have a role to play in harmonizing proposals across the different regions and standards bodies.

21.3Standardization bodies

Although 3GPP currently does not perform any direct work towards IMTAdvanced, 3GPP will most likely make a proposal to ITU-R. IEEE802.16 is also on the move improving its concept and is targeting a proposal for IMT-Advanced in its work on 802.16 m. Similarly 3GPP2 and the community around cdma2000 are expected to come with an IMT-Advanced proposal.

21.4Concluding remarks

While LTE standardization is not yet completely finalized, the race for the next radio interface is already ongoing. Many activities are initiated in the research community and some standardization developing organizations are about to start (or has just started) their work towards IMT-Advanced. However, IMT-Advanced is still

several years away whereas HSPA Evolution and LTE are just around the corner. HSPA Evolution is built as a continuing evolution of the existing WCDMA/HSPA technology whereas LTE is a new radio access optimized purely for IP based traffic. These two technologies promise to give more services, capabilities, and performance to the end users than any other radio interface technology has been able to do to date.

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