Operators nowadays are faced with a daunting task of finding ways on how to be able to cope up with the ever changing technologies and balancing that with customer satisfaction without having to invest heavily in technologies and infrastructure.
During the early days these investments were essential because most of them were just starting up and rolling out their 2G offerings. However, today the greatest challenge lies on how to be able to utilize these available infrastructures/technologies and make it co-exist with the new ones. But co-existence is just part of the story; making both technologies work towards the evolving future technologies and making it scalable is one overwhelming task. And as operators continuously find ways to better their current services with customers, at the back of their mind they are already thinking ahead on where they would invest next to have the continuity of the best experience they are currently offering their end users.
The MIMO Technology
As we move towards the next generation of mobile computing different technologies and ways are being looked upon to meet the challenges and more so the increasing demand. One of these is the MIMO (Multiple Input and Multiple Output) technology.
MIMO was standardized by the 3GPP in 2008 but it was only delivered to the market by mid 2009. Because that time MIMO in HSPA networks was not fully deployed because there were few available devices that were compatible to it and it was then perceived as added cost to upgrade the base station hardware and last but not one of the least considerations was that making MIMO work alongside to the legacy HSDPA users.
But as the demand for data continues to explode, operators continue to seek new solutions and even reexamine old ones on how they would be able to address and if not overcome such great demand and they are looking for opportunities to cost-effectively increase their capacity.
MIMO is poised to be able to be one of the antidotes to be able to address it, since it can significantly improves data rates, user experience and capacity within existing spectrum and through an upgrade to existing infrastructure. MIMO, along with Beamforming/Transmit Diversity and others, is one of a slew of advanced antenna techniques in the HSPA evolution roadmap. MIMO, in particular, plays a central role in delivering the successively higher peak data rates in each iteration of HSPA+, from 28 Mbps in Rel. 7 to 168 Mbps in Rel. 10 and even higher in Rel. 11 and beyond, also called as HSPA+ Advanced.
MIMO involves multiple transmission and receiving antennas available at the radio base station and thus it multiplies the capacity to devices and it increase data rates, overall capacity and improve user experience. Essentially, the MIMO system uses the antennas and it processed transmitter and receiver to create multiple uncorrelated fading characteristics streams between the transmitter and receiver. These streams use the same time and frequency as their resources, thus enabling capacity to be increased without an increase in spectrum. The increase in antenna would allow more streams in between thereby would result to more information to be transferred and a higher data rate. While all of this is happening it uses the same amount of spectrum and transmission power. Every order of MIMO increased and if not doubles the peak rates. Based on Qualcomm simulation, MIMO provides at least 20% increase in the overall capacity of the network.
Is it More Affordable?
Today it is more affordable to put MIMO into place. Although this is one of the drawbacks when it was earlier introduced, but because of the current telecommunication ecosystem, MIMO will just fit in perfectly to the networks. To highlight this here are some of the break down regarding the major cost considerations for operators to introduce MIMO:
- Antenna system: Most telecom operators use mainly dual-polarized antennas, or have two antenna arrays for the Rx diversity of the Uplink. These antenna configurations and the relevant RF feeders do not need to be transformed to support MIMO. This cost is therefore zero.
- RF system and radio modules: Previously telecom operators were severely concern with the impact to its RF modules in the base stations. Many RF deployed modules in existing networks support only one transmit channel, so such RF modules must be duplicated and interconnected to support MIMO. In comparison with legacy RF modules supporting only one transmit channel, MIMO-ready RF modules normally cost up to 30% more. This means a delta cost at base station level of only up to 10%, as compared to its legacy counterpart.
Legacy Issues
Another key challenge is to be able to deploy it and to ensure that MIMO can co-exist with legacy HSPA terminals. This co-existence challenge dates back to 2007 when early HSPA terminals with advanced receivers were first deployed. An HSPA terminal can be categorized by the performance level of its receivers.
Devices equipped with advanced receivers have all been rolled out commercially. However, some commercial devices have been equipped with an equalizer that will fall back to the lower performance RAKE receiver functionality, if the serving cell is in STTD (Space Time Transmit Diversity) mode. MIMO can be operated in one of two modes. In STTD mode, the base station transmits primary CPICH (P-CPICH) and diversity CPICH, while, in Primary-Secondary-Pilot (PSP) mode, the base station transmits in both primary and secondary CPICH (S-CPICH).
In theory, STTD mode provides the largest capacity gain, since both MIMO users and non-MIMO users can benefit from transmit diversity. However, this mode will result in a net degradation of performance when a large proportion of user devices have equalizers that fall back to RAKE receiver functionality in STTD mode. In such circumstances, the PSP mode is preferable. However, when PSP mode is used for MIMO, the base station needs to be provisioned with certain capabilities to achieve optimal performance. These capabilities are Virtual Antenna Mapping (VAM), PCI codebook restrictions, and unequal setting of P-CPICH and S-CPICH transmit power. Each of these is described below.
When the base stations transmit in S-CPICH, the transmit power of the two antennas may differ, which can lead to the non-optimum use of power amplifier (PA) resources. To overcome this problem, Virtual Antenna Mapping, or Common Pre-coder power balancing, is used. VAM introduces a power balancing network to enable base stations with P-CPICH and S-CPICH to fully exploit the power of the two PAs when transmitting to non-MIMO devices. The use of VAM has no impact on legacy devices' performance. However, in the case of single-stream MIMO transmission, the use of VAM may result in some PA power imbalance and hence PCI restrictions need to be in place.
However, in a test done by Huawei Technologies, it used a new interference cancellation technique along with a Virtual Antenna Mapping (VAM) solution, to reduce the interference caused by the secondary antenna and reduce the loss in legacy HSDPA throughput.
Moving Towards MIMO
Perhaps MIMO is one of those solutions that were born ahead of its time. It has been around for years but due to some considerations, particularly cost of deployment, it was seen then as unworthy evolution. But as the telecommunication technologies evolved and the ecosystem improved MIMO is slowly creeping back as one of the solutions that need to be taken into consideration. In fact it was seen as one as one of the "must have" changes that networks need to put into place to address changes in the network as well as to be able to address the growing demand for data over the network. And as HSDPA and LTE are being rolled out, MIMO will surely be a part of the network solution that ensures us that end user experiences are as seamless as possible.
© Telecom Review 2011
