The upgrade to LTE/LTE-Advanced is relatively straightforward, with new LTE infrastructure having the ability to reuse a significant amount of the UMTS- HSPA cell site and base station including using the same shelter, tower, antennas, power supply and climate control. Different vendors have different, so- called “zero-footprint” solutions, allowing operators to use empty space to enable re-use of existing sites without the need for any new floor space.
An operator can add LTE capability simply by adding an LTE baseband card. New multi-standard radio units (HSPA and LTE), as well as LTE-only baseband cards, are mechanically compatible with older building practices, so that operators can use empty space in an old base station for LTE baseband cards, thus enabling re-use of existing sites without the need for any new floor space, as mentioned previously.
Base station equipment is available for many bands including the 1.7/2.1 GHz AWS band and the 700 MHz bands in the U.S.A. (for more details about other frequency bands of LTE / LTE-Advanced and their spectrum management see the following section 1.8). On the device side, multi-mode chipsets will enable devices to easily operate across UMTS and LTE networks.
There are many different scenarios that operators will use to migrate from their current networks to future technologies such as LTE/LTE-Advanced. Figure 1.22 presents various scenarios including operators who today are using CDMA2000, UMTS, GSM and WiMAX. For example, as shown in the first bar, a CMDA2000 operator in scenario A could defer LTE deployment to the longer term. In scenario B, in the medium term, the operator could deploy a combination of 1xRTT, EV-DO Rev A/B and LTE and, in the long term, could migrate EV-DO data traffic to LTE. In scenario C, a CDMA2000 operator with just 1xRTT could introduce LTE as a broadband service and, in the long term, could migrate 1xRTT users to LTE including voice service.
3GPP and 3GPP2 both have specified detailed migration options to LTE. One option for GSM operators that have not yet committed to UMTS, and do not have an immediate pressing need to do so, is to migrate directly from GSM/EDGE or Evolved EDGE to LTE with networks and devices supporting dual-mode GSM-EDGE/LTE operation.
Moreover, in order to achieve effective performance and broadband mobility, a careful radio planning needs to be performed. Since LTE and LTE- Advanced are very flexible, i.e. they can be deployed in various frequency bands using a mixture do channel bandwidths, the actual planning decision is based on various factors, some of which are illustrated in the Figure 1.23.
Figure 1.22. Different Deployment Scenarios for LTE.
Figure 1.23. Factors influencing LTE cell planning.
The typical deployment will be based on a three sector site. This is apparent due to historic frequency planning methods, vendor implementation and also the fact that allocation of the LTE PCI (Physical Cell Identifier) includes a CellID1 (Cell Identity Group Number) and CellID2 (Cell Identity Number), the latter
is encoded as 0, 1, or 2 to reflect one of the three sectors. There are also various scenarios when a two sectored site or an omni directional site would be implemented.
In addition to standard frequency reuse, LTE/LTE-Advanced radio planning can also employ SFR (Soft Frequency reuse). To explain the concept of SFR, it is first best to describe FFR (Fractional Frequency Reuse) and PFR (Partial Frequency Reuse) schemes. In this two network technologies, OFDMA (Orthogonal Frequency Division Multiple Access) and SC-FDMA (Single Carrier - Frequency Division Multiple Access) are defined. These both utilize 15KHz subcarriers which are then grouped into PRB (Physical Resource Blocks), each containing 12 subcarriers equating to 180KHz of spectrum. The figure 1.24 presents this concept.
Figure 1.24. The utilization of 15KHz subcarriers.
There are various options how these Physical Resource Blocks can be allocated, as well as implemented for FFR, PFR and SFR. Fractional and partial frequency reuse schemes are both fundamentally based on allocating a number of these PRBs in a sector. The main issue with these is that they limit the maximum throughput available to a user - since they are not able to allocate the full bandwidth.
In comparison, the concept of Soft Frequency Reuse enables the system to maximize the capacity of the network by enabling each sector to utilize the full bandwidth. To do this, SFR adjusts the power allocated to certain PRB’s in order to mitigate ICI (Inter Cell Interference). It also enables the eNB to allocate the full bandwidth (all PRBs at a lower power) to users close to the cell, thereby achieving higher peak rates. This process is shown in Figure 1.25.
Figure 1.25. The Soft frequency reuse.
In addition, the LTE/LTE-Advanced system includes ICIC (Inter-cell Interference Coordination) techniques which enable the eNB (Evolved Node B),
via the X2 interface (eNB to eNB), to pass overload and high interference information. This in turn may be used by the eNB to dynamically adjust the number and power of PRBs allocated in a cell. In the following figure 1.26 the above techniques are illustrated.
Figure 1.26. Inter-cell Interference Coordination technique.
Another capability being planned for LTE-Advanced is relays as shown in Figure 1.27. The idea is to relay frames at an intermediate node, resulting in much better in-building penetration, and with better signal quality, user rates will be much improved. Relay nodes can also improve cell-edge performance by making it easier to add picocells at strategic locations. Relays provide a means for lowering deployment costs in initial deployments in which usage is relatively low. As usage increases and spectrum needs to be allocated to access only, operators can then employ alternate backhaul schemes.
Figure 1.27. LTE-Advanced Relay.
The final phase of the Radio Frequency planning and deployment process involves continuous optimisation of the Radio Frequent plan to accommodate for changes in the environment or additional service requirements (e.g. additional coverage or capacity). This phase starts from initial network deployment and involves collecting measurement data on a regular basis that could be via drive testing or centralised collection. The data is then used to plan new sites or to optimize the parameter settings (e.g. antenna orientation, downtilting, frequency plan) of existing sites.