As bandwidth-hungry smartphones and wireless tablets become more common, scaling capacity in microwave networks becomes essential. Moving forward, operators must consider scaling options at the radio layer and at the packet layer. And they must take a network-based approach to scaling capacity; optimizing spectral efficiency on a link-by-link basis is not practical in modern microwave networks. With a network-based approach, operators can:
- Avoid optimizations that are only valid on a small scale.
- Reduce the amount of spectrum used to help save rights-of-use costs.
Capacity scaling mechanisms
This article compares 2 methods to scale capacity in modern microwave networks:
- Hierarchical quadrature amplitude modulation (HQAM), which uses higher-order modulation to maximize spectral efficiency over a microwave communications channel.
- Packet compression mechanisms, which reduce the overhead introduced by a frame or packet structure to help increase in spectral efficiency in a full packet-based environment.
HQAM formats increase the density of modulation symbols in a transmitted constellation. For example, 512-state quadrature amplitude modulation (512QAM) and 1024QAM formats provide a combined sequential gain of about 25% in useable traffic capacity compared to 256QAM. And 2048QAM and 4096QAM formats deliver an additional 15% capacity gain over 512QAM and 1024QAM. Packet compression acts on the protocol overhead portions of IP packets. Fields belonging to Ethernet, Multiprotocol Label Switching (MPLS), IP, and TCP/UDP and are compressed before transmission and rebuilt at the receiving end of a microwave link. This reduces the number of bits sent across the link, improving capacity for services and applications. The effectiveness of packet compression depends on the traffic mix and conditions, making it difficult to calculate an average figure. However, if the capacity increase for an Internet mix (IMIX) traffic profile based on IPv4 is 30% to 40%, it almost doubles with IPv6. That means scaling gains from packet compression will become even more significant as operators transition from IPv4 to IPv6.
Link versus network spectral efficiency
Spectral efficiency is often measured on isolated links — links that are not impaired by interference or disturbance from neighboring radios, and that are not themselves a source of interference. Unfortunately, this approach provides information about ideal link conditions rather than realistic network conditions, where interference may be common. Optimizing a single link is not practical network design. An optimal network design should provide the requested capacity with minimal occupied spectrum for 2 reasons:
- Spectrum is a limited resource.
- Spectrum has an associated price for its use.
As a result, using less spectrum helps operators cut near-term network operating expenditures (OPEX), and conserves spectrum for future growth. A network-based approach to scaling capacity puts more emphasis on increasing network capacity than on scaling a single link. Decisions about increasing the modulation format are made from the perspectives of end-to-end network design and the resulting interference levels. This level of analysis implies that:
- Operators should carefully evaluate whether it makes sense to use higher-order modulation formats at dense, short-haul nodal or hub points, and at locations most exposed to impairment.
- Last-mile links are less affected by impairment.
Long-haul microwave transmission is more suitable to high modulation formats: fewer links comprise a network and are less likely to converge at a single geographic point in a network, causing less interference.
A real-world network model
To better understand how higher-order modulation and packet compression methods impact scaling capacity in wireless networks, we analyzed an operational European mobile backhaul network with 890 short-haul links. The largest group of links — 146 links — falls into the 38-GHz band. It includes last-mile connections, or tails, and nodal links. The analysis was performed to:
- Define the theoretical maximum throughput possible in the network. This helps determine the maximum capacity supported by the network without touching any network components.
- Determine the limits of the network before redesign is needed to support HSPA+ and LTE services.
- Provide a guideline for adopting a technology or combination of technologies that increase network capacity.
Figure 1 shows the 38-GHz communications band and how channels are distributed across that portion of the frequency spectrum. This is the starting point for the network analysis. According to current spectrum utilization, the total throughput in the microwave network is around 1.9 Gb/s. All links use fixed modulation to support network availability equal to 99.999%, or 5 minutes of outage time per year.
From this starting point, 2 strategies — HQAM and packet compression — were analyzed for their ability to scale capacity while avoiding incremental capital expenditures (CAPEX) and OPEX.
The HQAM approach
To model the HQAM approach, the modulation index was increased from the reference modulation level to the maximum level possible to reach 99.995% uptime. Each modulation scale introduces more capacity, but not all links can reach the maximum modulation scheme due to link length and interference. This approach is a trade-off between modulation scheme scale and network spectrum efficiency. By adopting this method across the entire experimental network, the total capacity increased to 7 Gb/s, or a 4-fold improvement. Figure 2 shows the percentage of links — both last mile and nodal — for which a certain modulation index is achievable.
Figure 2 reveals 3 main points:
- At modulation rates higher than 128QAM, less than 50% of links can sustain a further increase.
- At 1024QAM, the link percentage drops to 25%.
- At rates higher than 1024QAM, the probability of supporting higher modulation keeps declining, but less steeply.
The packet compression approach
Packet compression gains are directly linked to the lengths and types of packets carried. Once operators know the packet traffic profile, they can determine the gain from packet compression mechanisms as a percentage of the capacity increase. Knowing the traffic profile is critical because capacity gain from packet compression is a function of packet length; the smaller the packet, the higher the gain. This aspect is particularly important in mobile backhaul applications where voice traffic originates in very small packets of 64 to 128 bytes. The packet-compression analysis is based on conservative assumptions:
- Traffic distribution is close to the IMIX profile.
- Traffic is carried through IPv4.
- Traffic steering is based on a virtual local area network (VLAN) with a double VLAN tag.
With these assumptions, packet compression achieves a gain of around 40%, bringing total network capacity from 1.9 Gb/s to about 2.7 Gb/s. Figure 3 contrasts packet compression with the net throughput obtained by scaling the modulation index in a 14-MHz channel. For simplicity, 1024QAM is the maximum modulation displayed. The solid red curve represents the capacity gain provided by packet compression compared to the net radio capacity.
Figure 3 reveals 2 main points:
- Given a certain capacity value — such as 100 Mb/s, represented by the dashed, dark blue line — capacity can be provided using a lower modulation index. In this example, 128QAM instead of 512QAM when packet compression is used.
- A lower modulation scheme implies less transmitted power. In this example, 5-dB less power, which is represented by the difference between the two dotted orange lines. Using less power saves energy costs. It also reduces dangerous radio frequency (RF) pollution and overall interference in the network.
Analysis informs decisions
The analysis highlights key considerations for microwave network design:
- The best potential to improve modulation is found in the tail portions of wireless networks where HQAM can be exploited, or in long-haul transmissions where there is less potential for interference.
In short-haul transmissions, HQAM becomes much less applicable beyond a modulation index of 128QAM. Even in tail parts of a network, using 1024QAM and higher-order modulation formats must be carefully considered, unless service availability is not the primary concern. This might be the case in a move from 99.995% availability to 99.99%.
It may not make sense to scale aggregation links already operating at 64QAM to 128QAM to higher-order modulation formats. Those links are generally designed to operate at 99.999% because their role is to groom traffic in the middle of the network.
- There is some uncertainty when increasing the modulation index is the primary means of scaling capacity in short-haul applications. For example, network status may make it impossible to reach the desired capacity level on a certain link.
Note - Adaptive Modulation is a valid option to increase the channel bandwidth; this has not been discussed in this article as it is an established technology applicable in the whole network.
Cross-Polar Interference Cancelation (XPIC) is an alternative to HQAM with higher feasibility. XPIC is not part of the current analysis, but it provides a 2-fold capacity increase. XPIC is more applicable in a network than HQAM, but brings the cost of new equipment. In contrast, packet compression provides only a 1.4-fold gain, but can be applied everywhere without changes to the radio environment.
- Unless network specifics constrain an operator to 1 mechanism, the best solution might be to mix technologies. For example, in network aggregation where interference might be a serious issue, combining XPIC and packet compression gives operators a 3-fold capacity increase and is applicable everywhere in the network.
HQAM and packet compression are independent technologies that can be applied at the same time to scale microwave link and network capacity. Looking ahead, packet-based scaling technologies will play an increasingly important role in microwave transmission. These technologies support capacity scaling in existing RF without impacting microwave radio-related CAPEX or OPEX. As LTE, small cells, and LTE-Advanced (LTE-A) are more widely used, microwave networks will need even more optimization technologies to sustain backhaul demands. Editor’s Note: The author would like to thank Scott Larrigan for his contribution to this article. To contact the author or request additional information, please send an e-mail to email@example.com.
- Test Methodology Journal: IMIX (Internet Mix) Journal
- Packet Microwave: Boosting Capacity For Long-Term Growth, http://resources.alcatel-lucent.com/?cid=161345&ei=1cq0ubykcsrisgb_g4dyda&usg=afqjcnh-ofj8m9hxt-v6dr388bup4cq5cq&bvm=bv.43148975,d.yms&cad=rja
- ETSI ATTMTM4(12)000056, Energy efficiency models, Alcatel-Lucent contribution to TR 103 820 Energy efficiency metrics and test procedures for point-to-point fixed radio systems.
- L. Steenkamp, Mobile capacity solution, France Telecom/Orange presentation at Packet Microwave Forum, October 2012