Microwave Engineering Europe - November 2007 - (Page 16)

16 TESTING OFDMA RF testing for OFDMA in LTE base-stations By Jonathan Borrill, Director of Market Strategy, Anritsu s mobile communications systems move to faster data rates, higher bandwidths, and innovative new modulation and coding schemes, designers and testers are being challenged to implement and test these new technologies. One of the leading new systems for these next generation mobile systems is the Long Term Evolution (LTE) that is being designed by the Third Generation Partnership Programme (3GPP) as an evolution upgrade of today’s GSM/GPRS and 3G networks. From the RF point of view, LTE introduces several new technologies to the mobile design world such as MIMO and OFDMA/SC-FDMA. MIMO is introduced as a technique to use multiple antennas (spatially separated) at both the transmitter and receiver to enable the synthesis of separate data paths between pairs of antenna. So an antenna array with 2 transmit antennas and 2 receive antennas can synthesise 2 separate data paths and hence convey twice as much data from transmitter to receiver (the data rate is doubled). An array of 4 antennas would achieve a theoretical 4 times increase in data rate. LTE and OFDM modulation We will now look at the OFDM aspects of LTE in more detail, to see the exact implementation in LTE and how we can test this. LTE uses OFDMA (Orthogonal Frequency Domain Multiple Access) as the coding scheme in the downlink to transmit data to individual users. The key features of OFDMA are that it uses many ‘sub-carriers’ that are individual low bandwidth frequency channels. Each of these sub-carriers is modulated using QPSK, 16QAM or 64QAM. The sub-carrier is an RF channel, and the sub-carriers are spaced at exactly 15 kHz intervals. Each set of 12 adjacent frequency channels are grouped together, and for a period of 7 time slots they form a ‘Resource Block’ (1 slot = 0.5 ms, and is composed of normally 7 symbols). This Resource Block is the fundamental element in LTE, as each user is dynamically scheduled for different numbers of resource blocks according to data requirements, system capacity, and A Figure 1: Basic construction of the LTE downlink signal as deﬁned by 3GPP in Technical Speciﬁcation number TS36.211 version 1.2.0. propagation conditions. So this Resource Block becomes our basic element for measurement purposes. The key feature of OFDM based systems is that these sub-carriers are exactly spaced together so they minimise interference to each other. This is done by modulating at exactly the precise data rate so the ‘nulls’ and ‘side lobes’ of adjacent channels exactly align, that is the highest level of interference from one channel is positioned onto the point in the adjacent channel where no data is transmitted, and hence interference effects are minimised. This allows for a very close spacing of sub-carrier channels and hence very efﬁcient use of the spectrum. Figure 1 shows this basic construction of the LTE downlink signal as deﬁned by 3GPP in Technical Speciﬁcation number TS36.211 version 1.2.0. When a data transmission is made between the network and user, the user may be allocated many simultaneous resource blocks at the same time. As LTE is able to use up to 20 MHz of RF bandwidth, there are up to 100 simultaneous Resource Blocks available. This is because each Resource Block requires 12 x 15 kHz, which is 180 kHz, but there are a number of frequency channels that are reserved and un-used, due to frequency planning requirement functions (LTE can support operation in 1.4, 3, 3.2, 5, 10, 15 and 20 MHz versions), and so 100 of these 180 kHz blocks means that 18 MHz of the spectrum is available for direct user transmissions. This is still a large increase on today’s systems that have a maximum 3.84 MHz available with WCDMA. For measurement purposes we must be able to look across all of these 1200 simultaneous sub-carriers (100 resource blocks, each with 12 sub-carriers) to characterise the instantaneous transmissions. Measurement of the RF aspects of OFDM in LTE As LTE is using high order modulation such as 64QAM we must carefully evaluate the EVM levels to ensure that there will be sufﬁcient separation of the symbols at the receiver. Figure 2 shows an example of measuring EVM versus sub-carrier for all 1200 subcarriers in LTE in the bottom trace. In addition to the EVM measurement above (related to using higher order modulation e.g. 64QAM), we must also evaluate some new parameters. LTE requires that the transmitter has equal power level of all sub-carriers to ensure Microwave Engineering Europe ● November 2007 ● www.mwee.com 016_017_MWEE.indd 16 25/10/07 13:53:38 http://www.mwee.com

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Microwave Engineering Europe - November 2007 Contents News Comment Metamaterials: Metamaterials Tackle Communications Wavelengths Microwave Components — EM tools: Microwave Component Design Easier With New EM and EDA Tools Cover Feature: RF Testing for OFDMA in LTE Base-Stations Startup Eyes Battery-Free Wireless Sensor Nets High-speed ADC Technology Paves the Way for Software Defined Radios Planning a WiMAX network: Maximising the ROI by Using Advanced Optimisation Tools Transporting Video Over Wireless Networks Ultrawideband Under the Gun Specifying the Proper SAW Filter Products Product Feature: RF Test Solution Supports Emerging 4x4 MIMO as Well as Multiple Commercial Standards Calendar

Microwave Engineering Europe - November 2007

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