LTE TDD versus FDD Debate:

Dr. Hossein Eslambolchi
Date: January 2012

LTE stands for Long Term Evolution. It’s the technology behind some of the most advanced 4G networks in the world today. LTE-Advanced is a new standard currently in development, and many of our competitors have announced future deployment plans that include it. We refer to our future network as “LTE-Advanced ready”, because we plan to deploy our network in such a way that once LTE-Advanced mobile devices are available, we will simply flip the switch to take advantage of this technology.

There’s been discussion in the industry about two variants of LTE: FDD-LTE (Frequency Division Duplexing LTE) and TDD-LTE (Time Division Duplexing LTE). Historically, global spectrum has been allocated in both paired (FDD) and unpaired (TDD) configurations. Older 1G, 2G and 3G wireless technologies were all rooted in voice services, which required paired spectrum. Newer generations of wireless technologies, which focus exclusively on packet data services, tend to used unpaired spectrum. LTE was designed from inception to combine FDD and TDD into a single technology solution for 4G and beyond: The LTE network architecture, protocol stack, radio management, and MAC layers are identical, and there are only minor differences in about 15 percent of the physical layer 1. All of the key features of LTE and LTE­Advanced are identical for both FDD and TDD.

For most operators, deciding which variant to use is simply a function of the regulatory rules associated with their spectrum and the legacy technologies they already support in their network. FDD-LTE is the natural choice — really the only choice — for most operators around the world, since they are adopting 4G by transforming their existing 2G and 3G FDD networks.

In contrast, newer operators who deploy Greenfield 4G networks — or whose networks have evolved from earlier 3G/4G technologies which used TDD spectrum — will naturally gravitate to TDD-LTE. I think the industry as a whole should do so as well. This recommendation is driven by some real cost advantages in technology and network migration. And, over time, TDD-LTE mobile devices in higher frequency spectrum such 2.5 GHz band or higher will be used by the world’s largest operators, serving the most dense population centers where the majority of traffic data is generated. For example — by 2014, 91 percent of IP traffic will be video; 55 percent of that will be wireless-driven, using LTE technology over multiple smart phones such as iPad.

TDD-LTE is also very scalable and future-proof. Incumbent operators are deploying 4G using basic LTE radios which only have two transceivers. They have no prior experience base with MIMO deployments. In the future, new 5G Wireless Technology will use advanced antenna processing techniques to deliver unsurpassed data rates and capacity beyond what even wire-line can offer today. When LTE Advanced (Rel-10) standards and devices become available, these same radios will be able to aggregate large swaths of spectrum to generate even higher data rates and capacity. In addition, cognitive radios produce wireless speeds in the 1-10 Gbps range. That type of speed may drive worldwide service providers to use wireless LTE, even for their back-haul traffic at 10 Gbps.

TDD has a key advantage: TDD offers the flexibility to configure channel capacity with respect to asymmetric downlink or uplink traffic. This is where configurability is most needed, and it’s the reason why it has always been the preferred approach for nearly all Internet-centered wireless technologies including WiFi, Expedience, 802.20, PHS, IP-Wireless.

Additionally, unlike FDD systems, TDD systems allow these configurations to change. FDD systems use a fixed, symmetric ratio of 50/50 which is often sub-optimal and cannot be altered. TDD-LTE, however, allows carriers to dynamically dedicate more of their spectrum to downlink traffic.

Two key points:

1. Mobile Internet, unlike other LTE networks, will be able to alter this downlink/uplink ratio in the future depending on how usage evolves.

Other U.S. carriers are suddenly making rather unsubstantiated claims of “LTE-Advanced” on aggressive schedules. The reality is that no carrier in the United States has anything larger than a 10 MHz channel in any frequency band, and will not in the foreseeable future. Several operators don’t have anything larger than a 5 MHz channel in any band. In order for any of these carriers to reach any truly wide bandwidth, they must put their hopes entirely into an LTE-Advanced feature known as carrier aggregation.

Carrier aggregation allows an operator to stitch together disparate pieces of narrow-band spectrum in different bands to create a 10 or 20 MHz channel. This requires a great deal of radio complexity in both base stations and devices, and will not be properly supported until Rel-11 specifications, which are still in development. These specifications are also mired in operator-specific band nuances, which have become a real problem for those developing the standard. For example, the base stations and devices of major North American carriers who wish to support LTE-Advanced will have to aggregate various combinations of 700 MHZ, 800 MHZ, and 1900 MHZ channels. This creates a lot of multi-band radio RF and system-design challenges.

Additionally, LTE devices have to support an incredible amount of baseline LTE bands to begin with — there is no harmonized global band for LTE, unlike 2G/GSM or 3G/UMTS. Finally, inter-band radio systems have never been deployed before, and their behavior in practical applications is not well understood.

LTE­Advanced carrier aggregation for carrier is simply the pairing up of the simple. For example, 20 MHz channels today are aggregated within the same 2.5 GHz band, which is emerging to be one of the most viable candidates for global LTE adoption.

Standards bodies are still working out technical specifications for LTE-Advanced — in particular, the key carrier aggregation feature. In addition to future standards, service providers need to start thinking about LTE ecosystems to support their various business plans. Low-cost, multi-mode, multi-band devices which can support LTE are central to the adoption of mobile broadband. 4G/LTE market economics will be forced to support the RF spectrum which has the highest populations of paying consumers who need 4G service.

The world population reached 7 billion in 2011, with the highest urban population densities in China, Japan, India, other parts of Asia and the United States. These regions are widely understood to be the key drivers of the 4G/LTE era. These same areas of the globe are primarily serviced with spectrum within the 2.3-2.7 GHz range, which can be accommodated by a simple, low-cost device architecture.

2. In February of this year, Global TDD-LTE Initiative (GTI) was established at the Mobile World Congress by China Mobile, Vodafone, and Softbank.

GTI currently includes more than 31 members. These operators serve over 1 billion subscribers. For the past year, the International Wireless Industry Consortium (IWPC) has been conducting industry surveys and workshops to study operator requirements and the availability and performance of electronic components for various RF bands for LTE. Particular attention was given to carrier aggregation and other LTE-Advanced features and their implications for RF front-end designs. These surveys concluded that:

– Sub-2GHz bands for 4G are fragmented

– Some of the largest LTE deployments occur in 2.3-2.7 GHz bands

– 25 percent of the world’s 4G LTE deployments use 2.3-2.7 GHz bands

The combination of Band 7 (FDD) & Band 38/40/41 (TDD) constitutes a single frequency range for 4G worldwide roaming and mass-market opportunity. RF front-end-modules (FEMs) of multi-mode-multi-band (MMMB) devices are facing large technical challenges related to supporting so many fragmented RF bands. The problem is significantly exacerbated by the LTE-Advanced carrier aggregation feature for inter-band FDD bands in the sub-2 GHz spectrum. FEM designs for unified common 2.3-2.7 GHz are supported by a large majority of RF component vendors; 2.3 to 2.7GHz is the only global band for 4G wireless technologies.

I predicted that IP would eat everything back in 2001. Given the trends I’ve outlined above, I predict today that wireless IP and LTE will eat IP itself, and the world will become seamless through virtual wireless IP.

Dr. Hossein Eslambolchi