LTE TDD as Capacity Expansion for Existing LTE FDD

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 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.

LTE TDD and LTE FDD Differences

From a technical standpoint, few significant differences exist between FDD and TDD on the physical layer and, in particular, with the frame structure. On the higher layers, the differences are limited to configurability of the physical layer and negligible timing relations due to the discontinuous nature of uplink and downlink. Some of the major advantages of TDD follow:

Frame structure. TD-LTE supports a special subframe for switching from DL to UL and to ensure coexistence with other TDD systems.

Random access. Additional short random access format and multiple random access channels are available in special subframes.

Scheduling. Multi-subframe scheduling is supported for the uplink flow. ACK/NACK. Bundling of acknowledgements or multiple acknowledgements are allowed on the

uplink control channel. H-ARQ process number. Variable number of H-ARQ processes is possible, depending on UL/DL

allocation.

Overall, the biggest benefit of TD-LTE is the flexibility it offers to operators in adjusting the DL/UL ratio. This feature allows operators to configure the DL/UL ratio to suit the traffic ratio on their network.

Figure 1. LTE TDD Performance Flexibility

A common perception in the industry is that LTE TDD does not match LTE FDD for coverage. It is true that FDD systems transmit data over half the bandwidth, for twice as long as TDD Systems. For example a 2x5MHz FDD system transmits data over 5MHz bandwidth for the entire frame; by comparison, a 10 MHz TDD system will transmit data over a 10 MHz bandwidth for about half of the frame. If both systems transmit at 200 mW, power is more concentrated for FDD (40mW/MHz for FDD, 20mW/MHz for TDD), so the range of an FDD system will be larger than TDD’s. To achieve identical range, the power at the antenna of a TDD system needs to be 2x (3dB) higher than the power at the antenna of an FDD system. However, an FDD system transmits 2x longer than a TDD system, so the net energy (Power x Time) transmitted from the antenna is identical for both cases. Neglecting component losses, FDD and TDD systems are identical in terms of energy consumption. Additionally, in an FDD system, as the UL and DL operate in separate frequencies, there is a need for a Duplexer Filter to protect the sensitive receiver from Tx Noises. Nonetheless, the duplexer module, based on the duplex spacing, will add to the power consumption of the power amplifier (PA). As there is no need for a duplexer in TDD Systems, typically the TDD systems will have >50% better battery life than FDD systems.

Table 1. Power Amplifier and Battery Consumptions Comparison

Peak Spectral Efficiency (PSE)

The PSE is defined in ITU-R (2008). It is basically the highest theoretical data rate normalized by bandwidth assignable to a single mobile station assuming error-free conditions. The WINNER+ IEG evaluated PSE for LTE-A FDD mode and TDD mode in UL and DL. In addition to evaluation configuration parameters provided in (ITU-R 2009) with up to 4 Rx and 4 Tx antennas at the base station and up to 4 Rx and 2 Tx antennas at the mobile station, configurations with up to eight antennas were investigated for information purposes. From a mathematical point of view the PSE calculation is not demanding. It is simply the number of data bits that can be transmitted divided by the bandwidth and the time needed for that transmission. But LTE-A, like any other mobile radio system, needs overheads that do not contribute to the data rate. Reference and synchronization signals as well as broadcast channels and control signaling with channels carrying different indicators and control information from such overheads. Depending on the mode and the direction of transmission, different overhead types have to be taken into account. In TDD mode the guard period (GP), which separates DL and UL transmission in time domain adds additional overheads. The TD-LTE spectral efficiency is slightly degraded because of the additional overhead due to GAP period.

Figure 2. LTE Peak Spectral Efficiency Comparison

LTE TDD as Capacity Expansion

TDD spectrum can provide substantial additional bandwidth per operator in almost all countries. Even so, it will be critical for operators to use the available spectrum and other network resources as efficiently as possible if they hope to keep pace with demand and provide an excellent high-speed mobile broadband service for customers. 3GPP has defined 10 frequency bands for TD-LTE and 23 for FDD-LTE. The TD-LTE frequency bands currently attracting the most interest are 2.6 GHz and 2.3 GHz, while the recently ratified 3.4-3.8 GHz bands are also likely to play an important role by making 400 MHz of fresh spectrum bandwidth available in many countries. In fact, TDD spectrum can provide substantial additional bandwidth per operator in almost all countries.

Figure 3. TDD can be used to provide more DL capacity with the flexible DL/UL ratio

There are possible co-deployment scenarios for FDD and TDD using different frequency bands:

Co-located cells may use different frequency bands over the same footprint. Both networks offer full coverage of the same area. This can be an economical option, since both technologies occupy the same real estate, use the same backhaul and potentially the same core network.

Alternatively, co-located sites may not have the same cell footprint. This might be the natural result of using two very different frequencies, with different path loss characteristics resulting in concentric rings around each base station that are best served by the different frequency bands.

Some operators may opt to deploy TDD to cover hotspots, enabling traffic to be offloaded from the wider FDD network.

Figure 4. Example of LTE TDD deployment on top of LTE FDD

Existing FDD networks can leverage LTE TDD for targeted capacity expansions, ensuring a larger economy of scale by utilizing common EPC network architecture wherever possible. TDD is excellent for hot-spot expansions. LTE TDD is an excellent indoor complement for small nodes because it does not interfere with the FDD network. It is the ideal technology to leverage unpaired spectrum, which is typically available at higher frequency bands optimal for capacity expansion, making it less fragmented. A mix of LTE TDD hot-spots with LTE FDD macro cells will boost capacity and expand coverage. A mix of LTE TDD hot-spots with LTE FDD macro cells will boost capacity and expand coverage.

Normally there are two phases in deploying LTE TDD as complement for existing LTE FDD Network:

Phase I, interoperability between LTE FDD and LTE TDD is implemented using coverage and load balancing based handover

Phase II, upon the availability of supporting terminal, joint operation of LTE FDD and LTE TDD is implemented using Carrier Aggregation

Previously, CA (Carrier Aggregation) has been possible only between FDD and FDD spectrum or between TDD and TDD spectrum. 3GPP has finalized the work on TDD-FDD CA, which offers the possibility to aggregate FDD and TDD carriers jointly. The main target with introducing the support for TDD-FDD CA is to allow the network to boost the user throughput by aggregating both TDD and FDD toward the same UE. This will allow the network to boost the UE throughput independently from where the UE is in the cell (at least for DL CA).

Figure 5. FDD-TDD LTE Carrier Aggregation

TDD and FDD CA would also allow dividing the load more quickly between the TDD and FDD frequencies. In short, TDD-FDD CA extends CA to be applicable also in cases where an operator has spectrum allocation in both TDD and FDD bands. The typical benefits of CA – more flexible and efficient utilization of spectrum resources – are also made available for a combination of TDD and FDD spectrum resources. The Rel-12 TDD-FDD CA design supports either a TDD or FDD cell as the primary cell.

CA is an effective tool to combine together fragmented operator spectrum and deliver higher throughputs with a bigger spectrum pipe. However, CA has several additional benefits:

Higher speeds. Aggregation of carriers increases spectrum resources, which provides higher speeds across the cell coverage

Capacity gain. Aggregating multiple carriers increases spectrum but also includes trunking gains from dynamically scheduling traffic across the entire spectrum. This in turn increases cell capacity and network efficiency and improves the experiences of all users. A user previously facing congestion on one carrier can be seamlessly scheduled on a carrier with more capacity and maintain a consistent experience.

Optimum utilization of an operator’s spectrum resources. The majority of operators have fragmented spectrum covering different bands and bandwidths. Carrier Aggregation helps combine these into more valuable spectrum resource.

ZTE LTE FDD + LTE TDD Solution for Telkomsel

With the multi-mode era coming, ZTE, who is one of the top players in providing comprehensive network solutions and delivering the future-oriented quality network for operators, has developed the cutting-edge SDR Unified platform with the essential feature to support multi-mode and multi-band radio access.

The SDR Unified platform aim is to design a unified network which can bring seamless experience to operators. For any network in GSM, UMTS, LTE FDD or LTE TDD, ZTE proposed solution enables Telkomsel to save CAPEX and OPEX significantly because they only need to deploy a Uni-Radio Access Network. The following figure and table show typical upgrade that is required to upgrade eNodeB from LTE FDD to LTE FDD + LTE TDD in Telkomsel network:

Figure 6. FDD LTE Upgrade to LTE FDD + LTE TDD

Table 2. FDD LTE Upgrade to LTE FDD + LTE TDD Module Requirements

The B8200 BBU is shared between LTE FDD and LTE TDD, including its PM, SA, FAM, and CC boards. Because LTE TDD is operating in different frequency, new RRU is required to deploy LTE TDD in the designated band. For LTE TDD in 2300MHz ZTE propose R8984E as RRU. R8984E features 4T4R, and is capable to support beam forming and 4x4 MIMO.

Unified solution from ZTE also enables the implementation of common transmission between LTE FDD and LTE TDD as follow:

Backhaul is shared by FDD and TDD and different IP/VLAN is allocated to FDD/TDD Transmission priority can be configured according to the service type of TDD/FDD Congestion control is carried by transmission equipment according to TDD/FDD’s service

message

Figure 7. FDD LTE and LTE TDD Co-Transport

ZTE NetNumen U31 system can provide a centralized solution to manage the whole radio network, including GSM, UMTS, and LTE FDD and LTE TDD infrastructure. U31 is located in the Element Management Layer of TMN (TelecommUnication Management Network) model, and interconnects with

the upper level NMS (Network Management System) through northbound interface. Below figure shows the role of U31 in network in managing both LTE FDD and LTE TDD:

Figure 8. Unified O&M Solution

The main functions of NetNumen U31 includes: fault management, configuration management, performance management, security management, topology management, system management, software management, signaling tracing and northbound interfaces, etc. The dimension of OMC is based on number of cells in the whole network. Based on the cell number in each region, ZTE propose EMS solution with corresponding capacity.

Glossary

3GPP 3rd Generation Partnership Project

AAA Authentication, Authorization, Accounting

BBU Base band Unit

BS Base Station

CAPEX Capital Expenditures

CC Control Central

CN Core Network

DL Downlink

eNodeB Evolved Node B

EPC Evolved Packet Core

EPS Evolved Packet System

E-UTRA Evolved Universal Terrestrial Radio Access

E-UTRAN Evolved Universal Terrestrial Radio Access Network

FDD Frequency Division Duplex

LTE Long Term Evolution

MAC Medium Access Control

MIMO Multiple In Multiple Out

OMC Operation and Maintenance Centre

OAM Operation Administration Maintenance

OPEX Operating Expense

QoS Quality of Service

RRU Remote Radio Unit

SAE System Architecture Evolution

SISO Single In Single Out

UE User Equipment

UL Uplink