If you are reading this post, then you probably have heard about “4G”, the 4th generation cellular network. For a cell phone user, 4G means improved data speeds that allow faster delivery of multimedia-based applications, see our previous post, What is 4G?, for more details. On the other hand, the network operator desires to spend a minimum on upgrading network infrastructure and prefers to buy a backhaul solution that supports current and near future capacity demands of a cellular network.
Thus, it is important to improve the capacity of wireless backhaul links. To increase transmission capacity, wider channel spacing can be used. However the wireless spectrum is expensive and may not be available in some countries. Using transmission in high frequency bands, such as 60 GHz and above, provides the bandwidth needed to increase capacity. However, very high radio frequencies increase the cost of radio components. In addition, 60 GHz links limit transmission range due to high absorption of radio waves by the atmosphere, making this solution somewhat cost inefficient. One efficient way of improving the capacity of a communication link is to increase the order of the digital communication modulation scheme used for transmission.
In simple terms, digital modulation is the process of mapping a group of data bits into an information symbol that gets transmitted, after up-conversion to the radio frequency (RF) of the link. The most popular digital modulation scheme used in wireless radios is known as quadrature amplitude modulation (QAM). For a given symbol rate, increasing the modulation order, or equivalently packing more bits per symbol, would be an effective way to increase the capacity of a microwave link. For example, each symbol in a 64-QAM signal represents 6 data bits, while for 256-QAM and 1024-QAM signals it represents 8 and 10 data bits, respectively. Therefore, 1024-QAM provides (theoretically) a 25 percent increase in capacity over 256-QAM and an impressive 67 percent increase in capacity compared to 64-QAM.
The price paid for achieving such an increase in capacity is more complex signal processing algorithms and stricter requirements for channel quality, e.g. higher signal-to-noise ratio (SNR) at the receiver is required. In that case, increasing the modulation order for some networks under normal operating conditions can have a diminishing return on throughput. This is due to the fact that the required SNR for an acceptable receiver performance rarely can be met.
Why this is the case? Let us briefly discuss the challenges in increasing the modulation order. Higher modulation order results in larger pool of symbols available for transmission. For example, for 64-QAM, there exists 64 symbols in a 2D grid (known as constellation points) compared to 1,024 symbols for 1024-QAM for the same grid size. Clearly, increasing the number of symbols (assuming fixed power) makes the symbols closer to each other in this 2D grid. Thus, data detection at the receiver becomes more susceptible to errors due to impairment.
In practical terms, receiver circuits are affected by thermal noise, clipping and non-linearity of power amplifiers, phase noise and many other distortions that are beyond the scope of this post. It is worth mentioning that increasing the signal power beyond some limits results in actually decreasing the received SNR since many of these distortions associated with RF circuits are dependent on the transmitted power. Rather, the way to increase the modulation order is to improve the detection schemes and build circuits that are less susceptible to power-related distortions, along with improving the correction mechanisms at the receiver for phase noise and other impairments.
At Aviat Networks, we have the expertise and knowledge to build the highest quality microwave radios that can work at cutting edge signaling schemes. We will make sure that our customers see a sizable return—not a diminishing one from increasing the modulation order. Our pledge is that microwave backhaul will always exceed the capacity requirements of our customers.
Ramy Abdallah,
Senior Signal Processing Engineer, Aviat Networks
Joint Application Note with Symmetricom and Aviat Networks.
Mobile Backhaul Networks are evolving to packet, driven by 4G evolution, requiring high data and video traffic and growing number of apps, users, smartphones and tablet devices. 1588v2 microwave are a perfect match for Mobile Backhaul evolution. Paper covers 1588v2 overview, unique considerations for microwave and typical deployment scenarios (multi-hop, ring).
The beauty of IEEE 1588v2 (i.e., Precision Time Protocol) synchronization is that it is a bookended solution. In theory, there is no need to worry about what is in between or underneath—from a Layer 1 transport perspective. While in principle this is accurate, there are a couple “unique” aspects of running 1588v2 over a microwave network that should be carefully considered in your deployment plans.
First, the infamous “last mile” is in reality typically many miles across multiple microwave radio hops—which may consist of a mix of linear, ring and hub-and-spoke configurations. Unfortunately, more hops introduce more packet transmission delay and delay variation over the backhaul—a potentially lethal mix for sync transport—the amount of which is proportional to the number of microwave hops. Careful design and engineering are required. On a bright note, Aviat Networks and Symmetricom recently validated <1.5ms delay could be achieved across 10 hops—well within the requirements for mobile backhaul.
Second, most advanced microwave systems now support Adaptive Coding and Modulation (ACM), a key benefit for microwave transport that allows the effective throughput of the microwave link to be dynamically changed to accommodate for radio path fading, typically due to changes in the weather. If bandwidth is reduced as a result of an ACM change, it is critical that advanced traffic and QoS management techniques be applied in the microwave systems to ensure that 1588v2 traffic (packets carrying timestamps) are given the highest/strict priority for transmission, and are not subject to delay or discard. On a brighter note, Aviat Networks and Symmetricom recently validated that 1588v2 could operate over a highly loaded (approaching 100 percent) microwave network running ACM.
In a nutshell, there are some unique considerations for running 1588v2 over microwave – but the outcome can be predictably bright with proper engineering.
Check out the Aviat Networks application note for more information on the Aviat Networks/Symmetricom partnership and 1588v2 network synchronization over microwave backhaul.
Errol Binda
Senior Solutions Marketing Manager, Aviat Networks
