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Performance Challenge of 3G over Satellite
Methods for Increasing Revenue & Quality of Experience
May 2016
Performance Challenge of 3G over Satellite
May 2016
Mobile telephony has been in existence for over 40 years. The first generation (1G) developed in the
1970s was based on analog standards, including Advanced Mobile Phone System (AMPS) and Nordic
Mobile Telephone (NMT). In the early 1980s, standardization of a second generation (2G) digital circuit
switched mobile telephone system based on the Global System for Mobile Communications (GSM)
standard began. The first GSM-based voice call was made in 1991.
With the advent of the Internet, the International Telecommunications Union (ITU) set out to standardize a
third generation (3G) mobile service under something called the IMT-2000 specification. 3G standards
are required to meet the technical requirements of IMT-2000 for speed and reliability. The most common
3G standard today is called Universal Mobile Telecommunications System (UMTS) and is based on reusing most of the core network elements of GSM while introducing a new radio interface based on
Wideband Code Division Multiple Access (WCDMA). The first UMTS network was launched in Japan by
NTT DoCoMo in 2001. Since then, the standard has evolved and introduced additional functionality
primarily enabling higher data rate services also known as High Speed Packet Access (HSPA). The
WCDMA standard has been a big success and is set to overtake 2G as the largest mobile standard within
the next two years.
Figure 1: Subscribers per Wireless Technology
Source: Ovum
On request by Mobile Network Operators (MNOs) wanting to recover significant investments made in 2G
during the 1990s, UMTS is re-using many core network nodes from the GSM standard; the 3G standard
share the same hierarchical architecture as shown in Figure 2:
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Performance Challenge of 3G over Satellite
May 2016
Figure 2: 2G/3G Network Architecture
In the 3G standard, Radio Base Stations called NodeBs connect to a Radio Network Controller (RNC)
over an interface called IuB. The RNC further connects to the Core Network (CN) through interfaces
called IuCS and IuPS. Handsets, also called User Equipment (UE) connect to NodeBs over a radio
interface called Uu.
As mentioned above, 3G and 2G have a lot of nodes in the Core Network in common. The two standards
also share a common design philosophy to keep most of the network intelligence and management
centralized to a few key nodes rather than distributing it out to the edge as in a flatter network architecture
like 4G LTE.
The RNC plays a very important role in the UMTS Radio Access Network (UTRAN) as it is responsible for
controlling all the NodeBs and UEs connected to it. The RNC carries out the radio link management
setting up the Radio Access Bearers (RAB) used to connect UEs over the air interface, as well as
performing Radio Link Supervision, Power Control, Encryption, Cell synchronization and other key tasks
all aimed at using the UTRAN and the limited radio spectrum resources available as efficiently as
possible. This is by no means an easy feat and the standard governing the IuB interface between the
NodeB and RNC is called TS 25.433 and is no less than 1378 pages long!
When the IuB interface was first drafted, it was decided that transmission delay between a NodeB and
RNC should be kept to less than 30 ms. Among other things, this was to ensure that key protocols like
Radio Link Control (RLC) and Media Access Control (MAC) from the UE would arrive over the radio
interface via the NodeB and be managed at the RNC in timely and precise manner to make the best use
of the limited spectrum resources in the UTRAN. Needless to say no one from the satellite industry was
present during that meeting. As the UMTS standard has evolved over time and new RABs supporting
even higher modulations and data rates have been introduced, the requirements on the RLC and MAC
layer has increased even further. The constant quest to achieve higher and higher data rates and a more
efficient use of spectrum makes a stable, error and jitter free IuB connection between a NodeB and RNC
even more important.
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Performance Challenge of 3G over Satellite
May 2016
So where does that leave satellite backhaul when used in 3G? Well, all the major UTRAN vendors have
been able to implement proprietary adaptations to their respective IuB interface that goes beyond the
limits set by the IuB standard and can now support most 3G services also when used together with
satellite backhaul. However, it has come at the cost of less efficient resource management and an
increase in synchronization/framing errors on RLC/MAC layer compared to when using terrestrial
transmission. This causes re-transmissions to occur more frequently between NodeB and RNC as the
UTRAN tries to interpret the information from the UE. This can, in a long delay backhaul scenario such as
when using satellite, make up a significant part of the available backhaul. Furthermore re-transmissions
on the RLC/MAC layer has an adverse effect on applications further up in the protocol stack running TCP,
which can cause even more re-transmissions to occur. This almost perfect storm of issues occurring in
the IuB protocol stack is especially common in a shared resource backhaul scenario, like in a Time
Division Multiple Access (TDMA) environment where bandwidth is on-demand and delay and jitter is far
greater than in a Single Channel per Carrier (SCPC) environment.
TDMA systems offer a shared outbound channel received by all remotes and bandwidth on demand on
shared aggregate return carriers. The key drawback of TDMA in a 3G backhaul scenario is that with a
TDMA access scheme comes significant delay and jitter on the return link. Because of this,
measurements from a major 3G vendor have shown that over 30% of allocated bandwidth on the IuB
interface has to be reserved for re-transmissions in a TDMA networking environment to make sure all the
functions of the UTRAN operate properly and the end user performance is acceptable. This, of course,
has a major impact on the margins a Mobile Network Operator can achieve from providing 3G services as
a significant portion of the bandwidth used over a TDMA satellite link will have to be reserved for retransmissions, which does not generate any revenue.
Figure 3: TDMA Access Method
Bandwidth sharing mechanism in TDMA introduces significant jitter, latency and packet overhead
-> sensitive protocols like IuB lose performance
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Performance Challenge of 3G over Satellite
May 2016
Methods for Increasing Revenue & Quality of Experience
Comtech EF Data, as the market leader in satellite mobile backhaul, understands this challenge well. We
have thousands of Mbps of IuB backhaul in service that are low in both jitter and delay, and exhibit
significantly less of the issues described above and introduced by the use of TDMA-based backhaul. We
offer two primary solutions for IuB backhaul.
A SCPC point-to-point system offers dedicated bandwidth channels to each individual remote with
guaranteed bandwidth. Jitter, delay and IP overhead are kept at minimums, and this topology lends itself
especially well for 3G networks with a high share of symmetrical services like voice and Short Message
Service (SMS).
Figure 4: SCPC Access Method
Each remote requires its own fixed space segment -> Bandwidth is sometimes underutilized
because space segment is designed for peak period
Our Heights Networking Platform offers a shared outbound channel received by all remotes and dynamic
SCPC return carriers. This architecture brings out the best of SCPC while supporting dynamic bandwidth
in both directions. Jitter and latency is still kept at a minimum while supporting a wide range of modulation
and coding schemes plus Adaptive Coding and Modulation (ACM) in both directions. This topology lends
itself particularly well for Mobile Network Operators with a high degree of 3G data traffic and changing
bandwidth requirements.
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Performance Challenge of 3G over Satellite
May 2016
Figure 5: dSCPC Access Method
Each remote has all the benefits of SCPC (Advanced Modulations & FECs, Header and Payload
Compression and Bi-Directional ACM) while supporting changes in bandwidth demand -> the best
solution for 3G backhaul
Conclusion
Even without considering greater efficiency in terms of header and payload compression, as well as RF
performance and packet per second capabilities, both Comtech EF Data solutions for 3G backhaul over
satellite deliver more value per Mbps than TDMA. With SCPC’s low jitter and latency and with everything
else equal, 3G performance can be significantly improved compared to a TDMA system. Applications
function better with less retransmissions, which improves Quality of Experience for end users and
translates into higher usage. Additionally, more revenue-generating traffic can traverse the network in a
given Mbps, increasing margins and revenue for Mobile Network Operators. The result is a win-win for
everyone involved.
For additional information, please contact us.
Email: [email protected]
Voice: +1.480.333.2200
Fax: +1.480.333.2540
Web: www.comtechefdata.com
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