Use of FDD in LTE mobile networks is under the spotlight as the availability of paired spectrum wanes and operators consider TDD, which will need adequate conformance testing, writes Lindsay Harris.
With the arrival of smartphones, mobile broadband use is exploding, putting a strain on already well-used networks and spectrum.
To meet this demand, significant additional spectrum and more efficient mobile broadband technologies are needed.
Fortunately, these new demand patterns were not unforeseen.
In 2004, when 3GPP first set requirements on what LTE needed to achieve, spectrum flexibility was acknowledged as a prerequisite, i.e. the ability to operate in different spectrum allocations and to exploit paired and unpaired spectrum.
To this end, it was envisaged that paired, frequency division duplex (FDD) and unpaired time division duplex (TDD) spectrum would need to be part of the LTE equation.
While both FDD and TDD versions of LTE were defined, the industry first implemented FDD, but with the availability of paired spectrum waning, operators have started applying TDD, where there is greater availability of spectrum.
In the past, TDD has been realised using different 3G radio-interface specifications: WCDMA for FDD; TD-SCDMA and TD-CDMA for TDD). With LTE, 3GPP has specified dual-mode terminals that can be used for both FDD and TDD.
The main advantage of TDD is, like Wimax, its single channel nature – a single continuous band serves both the downlink and the uplink. For data-driven networks this makes sense, because far more bandwidth tends to be required for the downlink compared to the uplink.
Broadband services in paired spectrum can waste spectrum assigned to the uplink, making unpaired spectrum a more efficient use of the available spectrum.
TDD shares the same technical foundations as LTE FDD and so makes use of existing investment in LTE development.
For example, LTE TDD uses OFDMA for downlink and SC-FDMA for uplink, just like LTE FDD, and the same modulation schemes up to 64QAM are applied.
There are few differences between the FDD and TDD specification, the main one being the frame structure. The transmitted signal for both LTE FDD and LTE TDD is organised into subframes of a millisecond, with 10 subframes making up each radio frame.
With a single carrier frequency used for uplink and downlink TDD transmissions, the base station and the user equipment (UE) need to switch from transmission to reception, and vice versa.
Thus, a guard frame (a time delay) is used to separate the transmit and receive periods of the data stream, allowing the base station and UE time to switch between different modes, preventing interference due to channel delay spread.
As a part of the 3GPP family of standards, LTE needs to coexist with other 3GPP air interfaces, such as WCDMA, HSPA, GSM and CDMA2000. Particular attention is required during handover.
This is referred to as inter radio access technology (IRAT) handover, whereby LTE equipment needs to be able to seamlessly pass transmissions back and forth with older 2G, 3G and CDMA2000 network technologies and between different LTE modes i.e. TDD and FDD.
A series of handover test cases have been specified to ensure conformity, designed to ensure in-call service continuity for users.
Verification of all throughput requirements is necessary to ensure that the terminal protocol stack and its applications are capable of handling high data rates.
Just as important is RF conformity testing for UEs. With LTE’s high data throughput requirements and a new air interface to be tested, new modulation schemes, MIMO and handover between multiple technologies, it presents a challenging environment for RF engineers.
Typical transmitter measurements required for LTE include subcarrier error vector magnitude (EVM), magnitude and phase error, resource block allocation, spectrum flatness and inband emissions.
For the receiver, block error rate (BLER) and ACK/NACK counting are necessary. These UE RF conformance tests are defined in 3GPP Technical Specification 36.521-1.
To perform RF conformance testing, customers need a flexible combination of LTE radio access network simulation, with fading, interference signals, power measurements, spectrum measurements and modulation analysis.
Lindsay Harris is an account manager at Rohde and Schwarz UK