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Jul 24 2014

Boosting microwave capacity for LTE-Advanced

Customers will be using LTE-Advanced Category 6 user devices within the next year[1]. Capable of mobile downlink speeds of 300 Mb/s and uplink speeds to 100 Mb/s, these devices threaten to overwhelm cellular backhaul networks. But capacity-enhancing features for packet microwave systems can help you meet this need for speed.

LTE-Advanced and the push for faster mobile speeds

LTE-Advanced is a 3rd Generation Partnership Project (3GPP) initiative to provide faster mobile connections. It uses advanced technologies such as carrier aggregation, coordinated multipoint operation, and enhanced interference control to provide higher peak downlink and uplink speeds.

The LTE Category 4 user devices sold today make downloading content faster than previous technology, allowing peak downlink speeds of 150 Mb/s and peak uplink speeds of 50 Mb/s. LTE-Advanced Category 6 user devices will push these speeds even higher, supporting 300 Mb/s downlink and 100 Mb/s uplink.

Mobile service providers are concerned about the impact this bandwidth will have on their backhaul networks. Many have deployed or are planning to deploy wireless transmission/microwave to support their small cells, macro cells, cell site aggregation and long-haul transport (Figure 1). Over half of cell sites use microwave links for their backhaul connections, and upgrading many of these connections to fiber optic cable would be too expensive. For LTE and LTE-Advanced to succeed, increasing microwave capacity so mobile backhaul networks can handle increased radio access network (RAN) traffic is critical.

How much capacity is enough?

Capacity estimates are provided here by analyzing a simple model (Figure 2). The network is structured in layers: small cell, macro cell, second-level aggregation and third-level aggregation. Capacity shown at the macro cell is as proposed by the Next Generation Mobile Networks (NGMN) Alliance in its Guidelines for LTE Backhaul Traffic Estimation. The capacities for second and third stages of aggregation are based on established traffic engineering assumptions.

Satisfying LTE-Advanced backhaul demands

Fortunately, packet microwave systems are also continuously evolving their capabilities to handle increased 4G/LTE, and LTE-Advanced RAN capacities in mobile backhaul networks. Available advanced features include:

  • Cross-polarization interference cancelation (XPIC) to double frequency capacities
  • Adaptive modulation-aware high-order quadrature amplitude modulation (H-QAM)
  • Advanced packet compression to increase packet microwave link throughput
  • Reliably bonding channels together into higher-capacity microwave links
  • Standards based (ITU-T G.8032v2) networking to double microwave network capacity

Increasing capacity for existing links

Existing backhaul links are generally limited in how they can use allocated microwave spectrum:

  • Standard frequency bands of 6 GHz to 38 GHz are normally used
  • Spectrum allocation may vary according to region.
  • Links are operated under licenses that force service providers to follow specific design rules

Service providers must make both operational and design decisions when determining how capacity can be increased on existing links, as changes may cause service outages or require a costly site visit. With that in mind, a realistic strategy to increase capacity on existing links can include the following steps:

  1. Enable packet compression, which does not impact radio setup or link design
  2. Enable adaptive modulation (AM) together with higher-order modulation schemes
  3. Deploy an XPIC option that uses both horizontal and vertical frequency polarizations to double frequency capacity, and use multichannel 2+0 radio configuration to optimally balance load over the two XPIC channels

The total increase in capacity available from using these techniques depends on the specific configuration. Figure 3 depicts results for the following base case scenario:

  • 28 MHz channel operating at 128 QAM fixed modulation
  • 50% of the carried IP traffic consists of smaller sized packets
  • Net capacity of around 150Mb/s to 170 Mb/s

These techniques increase capacity from 150 Mb/s to around 650 Mb/s as follows:

  • Packet compression provides about a 40% gain (conservatively)
  • 1024 QAM-capable adaptive modulation adds another 20% to 25%
  • XPIC together with 2+0 doubles throughput

Ultimately, deploying a second XPIC radio unit doubles channel capacity. While this doesn’t meet the 950 Mb/s macro cell backhaul capacity that will be needed by 2017, it does represent a significant microwave link gain. Service providers can meet the 950 Mb/s mark using the following capacity enhancements:

  • More efficient packet compression, which occurs naturally when moving from IPv4 to IPv6
  • Moving to 4+0 multichannel arrangements to reach capacities beyond 1 Gb/s
  • Reorganizing spectrum and adopting wider channels. Using two 56 MHz channels increase capacity to approximately 1.3 Gb/s, which would accommodate RAN capacity advancements beyond 2017

Increasing capacity for new links

The design of new links can use all available mechanisms to scale capacity, including these 3 main applications:

  1. Short-haul urban links using traditional microwave frequency bands (13 GHz to 38 GHz)
  2. Short-haul urban links using millimeter-wave frequency bands
  3. Aggregation links using traditional longer-reach microwave frequency bands

Typical methods of scaling short-haul urban link capacity include using:

  • Wider channels (e.g., 56 MHz)
  • Adaptive modulation combined with H-QAM (e.g., 1024 QAM)
  • Header compression to increase throughput capacity
  • Multichannel and XPIC to double frequency capacities

Figure 4 compares the effect of incrementally applying these methods to a base case using 256 QAM. This example shows a multichannel 2+0 XPIC arrangement using two 56 MHz channels: AM and 1024 QAM. Packet compression can provide bandwidth as high as 1.3 Gb/s, well beyond the forecasted traffic requirements of an LTE-Advanced macro cell in 2017.

Service providers can use one of two methods to handle second-stage aggregation:

  1. Deploying four 56 MHz channels using a 4+0 multichannel configuration to obtain 2.5 Gb/s capacity
  2. Using two 112 MHz channels, in a 2+0 multichannel configuration to deliver 5 Gb/s capacity (If further spectrum grooming is possible)

Short-haul urban links can use millimeter-wave E-band frequencies (e.g., 70/80 GHz). Millimeter-wave propagation characteristics can support 2.5 Gb/s over a single 500 MHz channel operating at 16 or 64 QAM.  Figure 5 compares a 56 MHz channel working at 256 QAM in fixed modulation to a 500 MHz E-band channel at 16 QAM.

E-band offers more capacity, but can only be considered for links less than 3 km, due to millimeter-wave propagation characteristics. This means E-band can only be used for the first two stages of aggregation in urban or semi-urban environments. Key requirements for long-haul aggregation microwave links are:

  • High capacity and high availability
  • Support for advanced network topologies

Long-haul aggregation nodes typically act as hubs for multiple links, and connect to the rest of the network through a ring topology. New advanced Carrier Ethernet ring protocols, such as ITU-T G.8032v2, provide benefits over traditional SDH ring mechanisms. Optimal exploitation of available ring bandwidth (both east and west directions carry full traffic simultaneously) requires fewer radio units than standard linear connectivity. Newly designed long-haul microwave networks can use the following mechanisms to scale capacity:

  • Wider channels (e.g. 56 MHz)
  • Adaptive modulation combined with HQAM (e.g. 1024 QAM)
  • Header compression to increase throughput capacity
  • Multichannel and XPIC to double frequency capacities
  • New ring topologies based on ITU-T 8032v2

Figure 6 shows the effect of incrementally applying these technologies, with the base case using 256 QAM.

Each microwave link direction of a ring aggregation site can be set up with a 4+0 multichannel configuration that uses 2 frequencies, with 2 polarizations per frequency.  This delivers a total link capacity of 2.5 Gb/s per ring direction using only two frequencies.

Using these 4+0 virtual links in a Carrier Ethernet ring topology effectively doubles this capacity to 5 Gb/s, since both directions around the ring can carry traffic. This is enough to support the aggregation of three branches consisting of 4 macro cells and 9 small cells each, for a total of 12 LTE-Advanced macro cell sites with 27 subtended small cells sites, as described in Figure 2.

Modern microwave systems also support adapting traditional 2G TDM to packet. This allows them to support a common backhaul system for 2G and 3G, as well as LTE/LTE-Advanced.  Supporting these types of deployments doesn’t generally impact the capacity requirements, as they are addressed with the aforementioned capacity scaling mechanisms.

In summary, by using the capacity scaling techniques described above packet microwave networks are well positioned to address the backhaul needs of LTE-Advanced mobile networks.

Related Material

Microwave for Ultra-Broadband Era e-book

Footnote

  1. [1] REPORT:  Status  of  the  LTE  Ecosystem”, GSA (Global mobile Suppliers Association), July 2014.

To contact the author or request additional information, please send an email to networks.nokia_news@nokia.com.

About Paolo Volpato
Paolo has been a Product Strategy Manager at Alcatel-Lucent since 2008. In this role, he deals with evolution strategy and positioning for Alcatel-Lucent microwave products. Prior to joining Alcatel-Lucent, Paolo worked for Wind, Infostrada and Italtel. Paolo has a degree in Electronic Engineering from the Polytechnic of Milan and a masters degree in Marketing and Communications. He is currently involved in Next Generation Mobile Networks (NGMN ). Under the framework of the LTE backhauling workgroup, he co-edited two technical papers (“LTE backhauling architectures” and “LTE backhaul security”). At present he contributes to the design of backhaul and fronthaul architectures for Hetnets and LTE-Advanced.