LTE is sometimes called a 4G standard, which is actually not correct as it merely satisfies the 3GPP requirements for a 3G standard. In reality, international standardization bodies, such as 3GPP and ITU, have already set high-level requirements for 4G technologies, and those will be met by LTE-Advanced.
The goal for LTE-Advanced is to provide "peak data rates equal to or greater than those of wired networks, e.g., FTTH or VDSL2. It will also maintain an equivalent quality of service compared to wired networks and compatibility with current 3GPP systems, but with a reduced total cost of ownership."
The target peak rates for downlink and uplink speeds are 1 Gbps and 500 Mbps, respectively. These are obviously significant and challenging. However, peak rates are deceiving in real networks as it is rare that individual users can enjoy those speeds, and the reality is often one-tenth of the peak. That is why for operators and end-users the more important figures for performance are rates at the cell edge, average user throughput and spectrum efficiency.
In that regard, the following performance targets have been set in comparing LTE-Advanced to LTE:
- Cell edge user throughput two times higher than that in LTE
- Average user throughput three times higher than that in LTE
- Capacity (spectrum efficiency) three times higher than that in LTE.
Should these be met, then it will ensure that not only users near the antenna benefit from the technology, but also the average user, located at different distances from the base station.
To meet these goals, the industry must develop many new innovative technologies and successfully integrate those to form cost-competitive solutions. As the LTE-Advanced standardization is still in its early stages, candidate technologies remain to be selected, but include relay nodes, UE dual TX antenna solutions for SU-MIMO and diversity MIMO, scalable system bandwidth exceeding 20 MHz, up to 100MHz, flexible spectrum usage, cognitive radio, interference management and suppression, asymmetric bandwidth assignment for FDD, hybrid OFDMA and SC-FDMA in uplink, and uplink/downlink inter eNode-B coordinated MIMO.
Some of these will make it into the final standard and some will not. Nevertheless, one thing that is sure is that LTE-Advanced will bring a new radio network and the implied costs and deployment issues that come with any new system - right on the heels of LTE. So, the timing of its arrival is by no means certain. On the other hand, competition forces operators and vendors to keep investing in new technologies to provide a better user experience for customers at lower costs, which is what LTE-Advanced will do.
Perhaps the most interesting and likely to be approved enhancements will be support of wider transmission bandwidths, advanced multi-cell transmission/reception technologies and the concept of relay nodes.
Wider bandwidths up to 100 MHz will be supported with either continuous or discontinuous spectrum. Continuous spectrum usage will require frequency allocations in new bands, such as 3.4 to 3.8 GHz. Wider spectrum is required to increase the bits-per-Hz performance and achieve the desired peak data rate of 1 Gbps on the downlink. Discontinuous bandwidth can be implemented within current spectrum allocations, but it will require several receivers to be built in to the UE. This will have to be balanced against increases in UE size, cost, and power consumption.
Coordinated multipoint transmission/reception can be used to increase frequency efficiency and cell edge user throughput. One proposed technology to realize this is use of RRE (remote radio equipment), this would enhance ICI (inter-cell-interface) management and handover performance.
Various concepts of relay nodes are also being proposed, and are beneficial for coverage extensions. L1, L2 and L3 relay nodes are proposed. L1 relay nodes are like traditional repeaters as we see in 2G and 3G networks, whereas L2 and L3 relay nodes use different frequency/time resources and can achieve wider coverage extensions via increases in signal to noise ratio.
LTE and LTE-Advanced are becoming the de facto standards for future telecommunication. As such, LTE looks like it will enjoy a robust and rich ecosystem. Thus an almost inevitable path to LTE-Advanced has already been set.
Juha Ristimaki is with the Asia-Pacific Technology Group of Nokia Siemens Networks