For those who are too picky on the definition of "true 4G," hold your breath...4G version of LTE is taking shape as LTE-Advanced!  LTE-Advanced is a Release 10 feature of 3GPP.  Recall that Release 8 defined LTE.  Release 9 makes some enhancements to LTE such as support for emergency calls using IMS. We'll summarize below motivating factors behind LTE-Advanced and briefly introduce main features of LTE-Advanced.

ITU (International Telecommunication Union) has defined requirements for IMT-Advanced (just a prettier name for 4G?).  LTE-Advanced aims to meet and often exceed IMT-Advanced requirements.  Remember that LTE is a Lon...g Tem Employment (LTE!).  Hence, LTE-Advanced needs to aim for an even longer term employment!  IMT-Advanced requires the support for 100 Mbps for high mobility and 1 Gbps for low mobility.  While Release 8 can already meet 100 Mbps peak rate requirement, enhancements are needed in Release 10 to support 1 Gbps.  Peak spectral efficiency targets in IMT-Advanced are 15 bps/Hz and 6.75 bps/Hz for DL (downlink) and UL (uplink), respectively.  LTE-Advanced aims to achieve the peak spectral efficiency of 30 bps/Hz and 15 bps/Hz in DL and UL, respectively.  IMT-Advanced needs to support wider bandwidths such as 40 or 100 MHz.  LTE-Advanced allows 100 MHz bandwidth.  While IMT-Advanced specifies the delay of 100 ms for an idle mode to connected mode transition, LTE-Advanced intends to achieve a 50 ms delay.  Recall that even Release 8 LTE can meet the 100 ms transition delay requirement.  In addition to IMT-Advanced requirements, LTE-Advanced attempts to achieve higher performance with reduced cost.  Furthermore, LTE-Advanced facilitates meeting future operator and user needs.  Note that wireless data traffic has been experiencing explosive growth.  Of course, LTE-Advanced cannot forget the competition...LTE-Advanced needs to be prepared for its arch rival- WiMAX!  In summary, IMT-Advanced requirements, lower cost per bit, and competition are major motivating factors driving the design of LTE-Advanced.

Let's turn our attention to the main features of LTE-Advanced.  LTE-Advanced is primarily an air interface enhancement, and, hence the features that we would briefly highlight below are related to the radio link between the UE and the eNodeB.  Main features of LTE-Advanced are Carrier Aggregation, DL MIMO enhancements, UL SU-MIMO, CoMP (Coordinated Multipoint), and Relay.  Main benefits of these features are higher peak and average cell and user throughput and lower cost per bit. 

Carrier Aggregation means that multiple carrier frequencies are aggregated to increase the overall bandwidth and hence data rates.  These carrier frequencies are called component carriers (CCs).  The component carriers may or may not be contiguous in frequency domain.  A CC could have any of the channel bandwidths defined for Release 8, which ranges from 1.4 MHz to 20 MHz.  The eNodeB and the UEs may be capable of transmitting/receiving one or more CCs.  The standard aims to support the total channel bandwidth of up to 100 MHz.  Depending upon the UE capabilities, the eNodeB may allocate multiple CCs to the UE for the DL and the UL. 

While Release 8 already supports (4x4) DL SU-MIMO, LTE-Advanced further increases it to (8x8) SU-MIMO in the downlink.  Additionally, beamforming is enhanced in the downlink using enhanced reference signals to improve MU-MIMO performance.  Release 8 does not support SU-MIMO in the uplink.  However, Release 10 extends SU-MIMO to the UL with the support for up to four layers of spatial multiplexing. 

If you were missing soft handoff/handover and fast cell (or sector) switching in LTE, we have good news for you!   CoMP introduced by LTE-Advanced works similar to soft handover/handoff.  CoMP feature is available for the DL and the UL.  In the DL, it is CoMP Transmission, while it is CoMP reception in the UL.  Multiple cells could now be involved in communication with the UE.  Two main CoMP transmission approaches are Joint Processing (JP) and coordinated scheduling and beamforming (CS/BF).  The JP transmission approach offers two methods: (i) The eNodeBs transmit the same information from two cells in the "joint transmission" method.  The UE then combines these two signals similar to a UMTS UE combining signals in soft handover.  (ii) In the "dynamic cell selection" method, one cell among a set of cells is dynamically chosen for the DL transmission to the UE.  In the CS/BF approach of CoMP transmission, beams are formed in individual cells while reusing the subcarriers near cell edge.  Scheduling in different cells would need coordination to realize such beamforming.  The CoMP reception in the UL involves reception of the UE signal at more than one cells.  One of the cells would be a "central" cell responsible to combine signals received at multiple cells. 

A relay can be thought of as an enhanced repeater, where the cell coverage (and hence cell-edge throughput) can be extended/improved.  A new interface, Un interface, exists between the traditional eNodeB and a relay node.  An example of relaying in LTE-Advanced is Layer 3 relaying with self backhauling.  The eNodeB uses the help of a relay node that takes care of some users.  Such users would be outside the coverage area of the eNodeB but inside the coverage area of the relay node.  LTE-based air interface can indeed be used as the wireless backhaul between the eNodeB and the relay node.  Such backhaul could share the same bandwidth with users or could use different spectrum bandwidth. 

LTE-Advanced is fully backward-compatible with Release 8 LTE.  A Release 8 UE can work with a Release 10 E-UTRAN, and a Release 10 UE can work with a Release 8 E-UTRAN.  LTE-Advanced would support interworking with legacy technologies such as UMTS and 1xEV-DO.

We hope that you have digested LTE-Advanced in 5 minutes.  If you finished up digesting LTE-Advanced in less than 5 minutes, congratulations!  If you took longer than 5 minutes, well...what can we say...you are a slow reader and you need to work on your reading skills!  "See" you next time!