Network planning and testing for LTE-Advanced

& -November 14, 2012

Streaming video, gaming, advanced applications, and more are putting demands on today’s wireless networks and increasing the need for capacity and tower density.  In response, carriers are looking at options, such as Wi-Fi underlay and backhaul, to limit the load on networks.

Simultaneously, various carriers are conducting trials and looking to introduce LTE-Advanced to the masses in 2013, which promises to make a performance leap by bringing more low-powered nodes closer to the user.  However, issues around standards and how each carrier will make network handovers complicate things when deploying heterogeneous network components such, as smaller cell sites (e.g., picocells, femtocells, etc.).

Why LTE-Advanced?
LTE-Advanced (LTE-A) is a 3rd Generation Partnership Project (3GPP) specification in response to International Telecommunication Union (ITU) requirements for International Mobile Telecommunications-Advanced (IMT-Advanced) systems.  These requirements define what fully compliant 4th generation cell phone mobile communications system needs to satisfy, most importantly:
•    Peak speed requirements at 100 Mb/s for high mobility communication (such as from trains and cars) and;
•    Peak speed requirements at 1 Gb/s for low mobility communication (such as pedestrians and stationary users).

Even though Mobile WiMAX and LTE don’t meet these objectives, they are considered “4G” technologies since they are significantly better performing and capable then initial 3rd generation systems and are early versions of fully IMT-Advance compliant Mobile WiMAX Release 2 and LTE-Advanced.

For the LTE-Advanced system, 3GPP has further required the following:
•    Higher spectral efficiency (from a maximum of 16 bps/Hz in LTE to 30 bps/Hz in LTE-A);
•    Improved performance at cell edges (e.g. for downlink 2x2 MIMO at least 2.40 bps/Hz/cell).

What is new in LTE-A
To meet these requirements, LTE-A systems have some improvements compared to LTE, namely, carrier aggregation (CA), improved multi-antenna techniques and support for relay nodes (RN).  We will look at each one.

Carrier Aggregation
LTE-A systems need considerably more signal bandwidth to meet the requirement of the significant throughput increase compared to 3G and LTE.  Since LTE-A also needs to maintain backward compatibility with the LTE terminals, the LTE-A bandwidth increase is performed by aggregating multiple LTE carriers into a single LTE-A signal.  Each LTE carrier that comprises the LTE-A signal is called component carrier (CC).  A single LTE-A signal can consist of component carriers with different bandwidths as defined in the LTE specification.  This allows for effective utilization of the available spectrum.

Figure 1. The LTE-A component carriers can be of different bandwidth.  The downlink (DL) can have extra carriers compared to the uplink (UL).

Component carriers can be aggregated contiguously, with spectrum gaps between them (non-contiguously) or even across multiple bands (inter-band).  The current standard limits aggregation across no more than any three bands within 0.3 - 6.0 GHz.  In the future, each CC could be located in a separate band.  This again allows for effective utilization of the available spectrum.

Figure 2. The LTE-A Component Carriers can be discontinuous and even in different bands.

LTE terminals utilize only one of these carriers while the LTE-A terminals can utilize up to five CC.  Per the LTE specification, the maximum bandwidth of a single CC is 20 MHz.  Therefore, the maximum LTE-A bandwidth achievable when aggregating five CCs is 100 MHz.  It is also important to note that the downlink (DL) and uplink (UL) do not need to be symmetrical, and the downlink can have the same number or more CCs than in the uplink transmission direction.  This again allows for effective utilization of the available spectrum, and optimization of the channel based on the throughput required by the user, which is often also asymmetrical.

Figure 3. The LTE-A component carriers (transmitted from the same tower) may form the independent cells with different footprints.

LTE-A radio resource control (RRC) is handled by one of the CC.  This CC is called LTE-A Primary CC (PCC).  Other CCs are called LTE-A Secondary CCs (SCC).  Some additional RRC messages are introduced in LTE-A to support this division.  Each CC, however, forms an independent cell with potentially different coverage.  This is due to the freedom to adjust transmitting power for each CC but also due to different band and antennas potentially being used for different CCs.  The positive aspect of this is that a heterogeneous network can be formed this way (as we will see later), but the downside is that the LTE-A terminal might not always be able to aggregate all the CCs.

Improved multi-antenna techniques
To meet the requirement of increased spectral efficiency (throughput per bandwidth), LTE-A had to build on LTE multi-antenna techniques.  In high signal-to-noise environments, LTE uses a spatial multiplexing technique called multiple input multiple output (MIMO).  MIMO allows higher throughput communication by using two or more transmit (Tx) streams received (Rx) by two or more antennas at the same time while occupying the same bandwidth.  Each transmit antenna uses a different reference signal which allows separation of the signals by the receiver.  If these antennas are appropriately spaced on the tower and on the terminal, then propagation paths between the transmitting and receiving antennas can be spatially sufficiently different to provide higher throughput with same time/frequency resources.

LTE-A increases the maximum number of the DL antennas from four present in LTE to eight and the maximum number of the UL antennas from the two present in LTE to four.  This results in almost double the spectral efficiency in high signal-to-noise environments.

>>Support for Relay Nodes
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