Operators are currently focusing on deploying Long-Term Evolution (LTE), and the first LTE handsets are emerging in the marketplace. Yet basestation manufacturers, as well as quite a few handset manufacturers, are already working on the next evolution of the technology, LTE-Advanced, which will bring true 4G to customers.
Today, the industry is raving about data rates of 150 Mbits/s. But LTE-Advanced, which promises 1 Gbit/s (over six times the current data rate) and 100-MHz bandwidth, is set to unleash the content customers are craving.
“At home, consumers get a maximum of 3 or 5 Mbits/s from their cable service currently. With 300 Mbits/s, the average user will be able to get 30 Mbits/s, which would be 10 times what they get at home from their cable modem,” highlights Amir Ghanouni, director of wireless sales at Aeroflex (see the figure).
At the same time, these significant leaps in data rate and bandwidth translate into significant challenges for the design engineer. Among those, higher-order multiple-input multiple-output (MIMO), carrier aggregation, and clustered SC-FDMA are perhaps the most important.
MIMO
The multiplication of antennas with LTE-Advanced is probably the greatest challenge ahead for engineers. With LTE, 2x2 MIMO has been deployed, meaning that basestations’ transmitters and receivers have two antennas on each side. Release 8 is expected to move to 4x4 MIMO while Release 10, LTE-Advanced, is expected to provide 8x8 MIMO. Eight different antennas will be required for every transmission, translating into a significant design challenge due to interference and the ability to split the signal.
Moreover, contrarily to LTE, LTE-Advanced handsets will also have at least two antennas (maybe four). For MIMO to work, signals will have to come into the phone differently so it will be able to pull the data from them and give users the high data rate they are seeking.
“The closer the signals physically, the harder it becomes to de-correlate those signals, and handset manufacturers will have to design multiband MIMO antennas with good de-correlation within the space constraints of the handset,” explains Jan Whitacre, LTE program manager for Agilent Technologies.
Whether this will work or not is still up in the air at the moment. In addition to the technical issues, testing MIMO will require a lot of equipment, making it an expensive testing proposition for handset manufacturers (see “Multi-Antenna Designs Turn MIMO Testing Into Heavy Work”).
Carrier Aggregation
To increase the data rate to 1 Gbit/s, LTE-Advanced requires a high bandwidth of 100 MHz. To reach such bandwidth, engineers will leverage carrier aggregation, enabling them to pull different bandwidth of signals from up to five carriers.
However, those bands do not have to be continuous, which can lead to the mixing of the signals, causing interference. Engineers will have to spend more energy to make sure that the transmitter from the basestation does not create interference problems when it sends out these signals. The transmitter for the basestation will hence become a key design area for them.
Clustered SC-FDMA
In LTE, a different technology, SC-FDMA, is used for the uplink (handset to basestation) than for the downlink ( basestation to the handset) due to its lower peak to average power ratio. A high peak to average power ratio makes it more difficult for engineers to design amplifiers and make them work in the right range.
However, LTE-Advanced uses clustered SC-FDMA, which adds more signals, increasing back the average peak to average power ratio. This is expected to make it harder for engineers to design amplifiers, as they will have to worry about the peak to average power ratio for that amplifier both within the basestation and the handset.
Solutions
While it won’t be easy to make LTE-Advanced work, it certainly is doable. Agilent Technologies and Aeroflex are two companies helping engineers overcome the challenges brought by the technology.
Agilent’s LTE-Advanced product portfolio encompasses three key products for signal generation, signal analysis, and design and simulation that are used to address all of the test challenges mentioned above. The software for signal analysis allows engineers to look at each of the carriers simultaneously. This is critical to ensure that they are functioning correctly as they are working together.
Aeroflex has full end-to-end test capability for LTE and is developing a corresponding range of solutions for LTE-Advanced. The first product for LTE-Advanced will be the TM500 Test Mobile, which will simulate single or multiple Rel-10 mobile user terminals to test new eNodeB designs. The company recently announced single- and dual-layer beamforming and 4x2 MIMO capability for the TM500, and carrier aggregation will be released shortly.
The Future Is Now
MIMO, carrier aggregation, and clustered SC-FDMA are only some of the key challenges engineers will have to overcome to bring LTE-Advanced to life. But no matter how significant those challenges are, they are not insurmountable and research predicts deployments of LTE-Advanced to start in 2012/2013.
Three months ago, NEC announced it was putting a test network together for LTE-Advanced, while others such as Qualcomm already have such a network in place. Deployment of LTE-Advanced will ultimately depend on the demands on the network. However, with the rollout of LTE devices, the demand for higher data rates is bound to explode.