Agilent Measurement Journal - Issue 4 - 2008 - (Page 22)

Examining the disadvantages of OFDM OFDM has two big disadvantages when compared to singlecarrier systems. First, as the number of subcarriers increases, the composite time-domain signal starts to look like Gaussian noise, which has a high peak-to-average ratio (PAR) that can cause problems for ampliﬁers. Allowing the peaks to distort is unacceptable because this causes spectral regrowth in the adjacent channels. Modifying an ampliﬁer to avoid distortion often requires increases in cost, size and power consumption. There exist techniques to limit the peaks (e.g., clipping and tone reservation*) but all have limits and can consume signiﬁcant processing power while degrading in-channel signal quality. The other main disadvantage of OFDM systems is caused by tight spacing of subcarriers. To minimize the lost efﬁciency caused by inserting the CP, it is desirable to have very long symbols, which means closely spaced subcarriers; however, apart from increasing the required processing, close subcarriers start to lose their orthogonality (independence from each other) due to frequency errors. Three key problems associated with close subcarriers cause lost performance. First, any frequency error in the receiver will cause energy from one subcarrier’s symbol to interfere with the next. Second, phase noise in the received signal causes similar ISI in the subcarriers but on both sides. Third, Doppler shift can cause havoc. It is easy to remove a ﬁxed Doppler shift but consider the case when multipath is involved and signals are arriving at the receiver from both front and back: The received signals are shifted both higher and lower in frequency and it takes considerable processing power to recover the original signal. To balance the desire for long symbols with the problems caused by close subcarrier spacing, LTE has adopted 15 kHz spacing, with a narrower 7.5 kHz chosen for use with LTE’s solution for mobile TV, the evolved Multimedia Broadcast Multicast Service (eMBMS).1 Introducing SC-FDMA The undesirable high PAR of OFDM led 3GPP to choose a different modulation format for the LTE uplink. This difference contributed to the inability of TTA, the Korean standards body, to persuade 3GPP (in 2006) to merge LTE and WiMAX. Pure OFDM is used in the WiMAX uplink but LTE continued to use SC-FDMA, a new hybrid modulation scheme that cleverly combines the low PAR of single-carrier systems with the multipath resistance and ﬂexible subcarrier frequency allocation offered by OFDM. When a new concept in modulation comes along (e.g., OFDM or CDMA), it can take a long time before the literature starts to make sense. Yet, after everyone “gets it,” we often look back at what previously seemed to be impenetrable explanations and wonder what the fuss was about! So it may be with SC-FDMA. The Release 8 3GPP speciﬁcations do little to explain the concept. For a formal deﬁnition of SC-FDMA, a student of signal processing need look no further than TS 36.211, which gives the mathematical description of the time-domain representation of an SC-FDMA symbol.2 For the majority of us who ﬁnd the formal mathematical approach hard to follow, we will present here a graphical comparison of the differences between OFDM and SC-FDMA. Comparing OFDM and SC-FDMA Figure 2 shows how a series of QPSK symbols are mapped into time and frequency by the two different modulation schemes. Rather than using OFDM, we will now shift to the term OFDMA, which stands for orthogonal frequency-division multiple access. OFDMA is simply an elaboration of OFDM used by LTE and other systems that increases system ﬂexibility by multiplexing multiple users onto the same subcarriers. This can beneﬁt the efﬁcient trunking of many low-rate users onto a shared channel as well as enable per-user frequency hopping to mitigate the effects of narrowband fading. For clarity, the example here uses only four (N) subcarriers over two symbol periods with the payload data represented by QPSK modulation. Real LTE signals are allocated in units of 12 adjacent subcarriers (180 kHz) called resource blocks that last for 0.5 ms and usually contain seven symbols whose modulation can be QPSK, 16QAM or 64QAM. *Tone reservation is an advanced form of clipping in which the time-domain signal is shaped such that the error energy falls on speciﬁc, reserved in-channel frequencies, ensuring less distortion in the wanted part of the signal. 22 Agilent Measurement Journal

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 4 - 2008

Agilent Measurement Journal - Issue 4 - 2008 Delivering Confidence through Compliance with Standards Contents Emerging Innovations Trying Early Device Implementations at the IEEE 1588 PlugFest Achieving Greater Confidence in Measurement Accuracy through Consistency in Calibration Services Overcoming the Challenges of RFID Component Testing 3GPP LTE: Introducing Single- Carrier FDMA Ensuring Reliable Operation and Performance in Converged IP Networks Testing Storage Area Networks and Devices at 8.5-Gb/s Fibre Channel Rates Choosing an Appropriate Calibration Method for Vector Network Analysis Making Traceable EVM Measurements with Digital Oscilloscopes Exploring Terahertz Measurement, Imaging and Spectroscopy: The Electromagnetic Spectrum’s Final Frontier Interpreting Quoted Specifications when Selecting Digitizers

Agilent Measurement Journal - Issue 4 - 2008

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