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TDMA vs SCPC for Satellite Backhaul – The Decision is Simple

Via satellitetoday.com
By Vince Onuigbo

Wireless Access Market Drivers /Trends

Wireless access has completely changed the way we live, work, and play. Everywhere you look, people are using their mobile devices to talk, text, watch videos, and adopt new applications that seem to appear on a daily basis. As the number of people and machines to be wirelessly connected — both in urban and remote areas — grows into several billions, mobile operators struggle to provide the best user experience to their customers. Optimum service attributes for mobile operators include coverage anywhere, anytime, HD voice quality, and data throughput to support any application. In rural and remote locations, mobile operators are embracing satellite as the most cost-effective backhaul solution to provide 3G and LTE services.

Over the years, mobile operators have used Single Channel per Carrier (SCPC) or Time Division Multiplexing (TDM)/Time Division Multiple Access (TDMA) technologies to backhaul sites in remote and rural locations. This article discusses why TDMA-based solutions are becoming the preferred approach by mobile operators for sites backhauled via satellite, particularly as Radio Access Network (RAN) implementations are shifting to 3G where traffic is dominated by Internet data, which is both asymmetric and bursty.

Mobile Operator Growth and
Competitive Pressures 

In developing markets such as Asia, Africa, and Latin America, mobile operators are facing challenges on different fronts, including low average revenue per subscriber (ARPU) and highly dispersed populations that must be connected. These operators must also make a profit for their shareholders while growing their businesses, by adding new subscribers and/or by cost-effectively providing the best value and user experience to subscribers.

The use of satellite in rural and remote locations makes it possible to deploy cost effective infrastructure with low operating costs. One of the key features of satellite is its ability to provide broad coverage. Whether with the traditional Fixed Satellite Service (FSS) or High Throughput Satellite (HTS), the transponder or beam covers hundreds of miles. With the ability to very rapidly deploy satellite broadband infrastructure in hard-to-reach areas, satellite is the clear choice for interconnecting cellular base stations in rural and hard-to-reach areas.

Figure 1 - SCPC vs TDMA Bandwidth Utilization Profile

Figure 1 – SCPC vs TDMA Bandwidth Utilization Profile

Search for an Optimum Solution for Satellite Backhaul

To deliver an optimum solution based on the lowest cost and best value, mobile operators are revisiting every aspect of their network delivery. Although SCPC technology was typically deployed in the past for backhaul to remote and hard-to-serve areas, recent developments in TDMA technology demonstrate how TDMA systems offer the lowest overall cost of ownership for most networks that require satellite backhaul.

For these sites, perhaps the most significant cost is that of recurring space segment. By efficient and dynamic sharing of space segment, TDMA systems make optimum use of this scarce resource. Other key factors are latency and jitter. Latency is the time period that information travels from one point to another in a network, measured either one-way or round-trip time (RTT). Lower latency increases the efficiency and performance of the network by achieving a higher data throughput and delivering better real-time voice and video applications. Jitter is the variation in latency, which can be due to congestion or loading on the network; lower jitter increases the throughput and quality of data transmitted through the network.

SCPC solutions, on the other hand, use dedicated frequencies, particularly for return channel traffic, and are particularly well suited to constant load traffic such as that found in 2G and 2.5G implementations. TDM/TDMA systems employ a statistical multiplexing scheme to share information among multiple remotes on the outroute with the same spectrum pool, and use a demand-assigned, multi-frequency TDMA approach to allow remotes to transmit information to the core network. Figure 1 illustrates the spectrum utilization efficiency of TDMA vs SCPC; the same amount of spectrum that connects one site in an SCPC deployment is easily shared among multiple TDMA sites on as-needed basis.

Instantaneous dynamic traffic capacity variability is not possible with SCPC technology as the link is dedicated to an individual remote site. Consequently, as the traffic load within an SCPC link varies, the unused satellite spectrum is wasted.

With TDM/TDMA, however, remote base station sites share the same satellite capacity for their traffic and the capacity is dynamically assigned, as needed, to each base station in a 3G or LTE system. In a dispersed multi-site network, instantaneous traffic demand occurs at different times at different site locations. This is suitable for dynamic sharing of the same satellite capacity, a key attribute of a TDM/TDMA system, which allocates traffic to different sites and at different times on as-needed basis, in contrast to an SCPC solution in which the capacity is fixed and dedicated to a single site.

In addition, recent advances in TDM/TDMA technology, which focus on lowering network-operating costs and improving the end-user experience, have made the case for TDMA systems for backhaul even more compelling.

Advanced features of TDMA systems include:

Outroute

  • DVB-S2: the world’s most successful satellite air interface, approved by TIA, ETSI, and ITU
  • Adaptive Coding and Modulation (ACM): allows the system to dynamically vary the modulation and coding of the forward channel for each transmission
  • Generic Stream Encapsulation (GSE): reduces encapsulation overhead and provides improvement in bandwidth efficiency
  • Scheduling Bypass and Real-Time Fast Track: together these features reduce latency and jitter, preventing the queuing of real-time traffic.

Inroute 

  • Adaptive Coding: similar to ACM on the forward channel, adaptive coding on the return channel enables a remote terminal to dynamically adjust its transmissions to handle fade conditions by selecting the most efficient coding rate that enables the transmission to be received and leads to increased throughput.
  • Adaptive Low Density Parity Check (LDPC): enables greater bandwidth efficiency.
  • Just In Time (JIT) TDMA Burst Forming: ensures that data packets are transmitted at the earliest possible opportunity, reducing Real Time Transport Protocol (RTP) latency.
  • Jitter Buffer (JB): enables an operator to configure the desired jitter performance. In a mesh network configuration, JB allows the system to reduce jitter for traffic sent between mesh peers: in star configuration, JB reduces packet jitter in the inroute direction.

While there have been similar improvements in SCPC systems, such as throughput efficiency features, the central theme that operators favor the most is the ability to share a common pool of space segment  efficiently and dynamically on an as needed basis by different sites. While this is not available in SCPC systems, it is a key advantage of TDMA systems. Recently, some vendors have introduced TDM/SCPC systems on the same platform; however, these systems require reservation of bandwidth resources to be used for switchover to SCPC in the inbound. Hence, this approach is not technically efficient and is more expensive to operate. Given the combination of TDMA improvements and the advent of HTS systems which do not place limitations on throughput for TDMA systems, operators are switching their networks to TDMA systems.

 

Business Case for TDMA Systems 

Among the factors critically evaluated by operators when seeking a satellite backhaul solution are space segment efficiency, a proven solution, and ease of implementation and maintenance. Case in point: an Asian operator recently swapped its 50-site 2G and 3G network from an SCPC system to a TDMA system for the following reasons:

  1. More than 30 percent cost savings in space segment, based on dynamic sharing and statistical multiplexing of same bandwidth pool
  2. Ability to use same platform to deliver both broadband and backhaul services
  3. Lower overall equipment and operating costs
  4. Ease of network expansion

 

Conclusion

As rural and remote networks migrate to 3G-based implementations, backhaul traffic becomes dominated by Internet data, which is bursty by nature. Backhaul links to support 3G networks are best served by TDMA solutions due to their ability to statistically multiplex traffic across many sites efficiently and dynamically. TDMA satellite solutions offer better spectrum efficiency and hence opex savings over SCPC solutions. In addition, future High Throughput Satellites (HTS) with TDM/TDMA solutions will offer even higher capacity, precluding one of the key reasons legacy SCPC technology was used in the past.  With HTS lower cost per bit compared to traditional satellites, TDM/TDMA is the preferred solution for backhaul of 3G and LTE RAN that requires several Mbps per site. The bottom-line is that operators want to deliver the best user experience at the lowest cost, and when it comes to choosing SCPC or TDMA for backhaul, the decision is simple — TDMA. VS

 

Vince Onuigbo is senior marketing director of the international division at Hughes Network Systems.