Agilent Measurement Journal - Issue 4 - 2008 - (Page 21)

The CP is chosen to be slightly longer than the longest expected delay spread in the radio channel. For the cellular LTE system, the standard CP length has been set at 4.69 µs, enabling the system to cope with path delay variations up to about 1.4 km. Note that this ﬁgure represents the difference in path length due to reﬂections, not the size of the cell.* Inserting a CP between every symbol reduces the data handling capacity of the system by the ratio of the CP to the symbol length. For LTE, the symbol length is 66.7 µs, which gives a small but signiﬁcant seven percent loss of capacity when using the standard CP. The ideal symbol length in OFDM systems is deﬁned by the reciprocal of the subcarrier spacing and is chosen to be long compared to the expected delay spread. LTE has chosen 15 kHz subcarrier spacing, giving 66.7 µs for the symbol length. In a single-carrier system, the symbol length is closely related to the occupied bandwidth. For example, GSM has 200 kHz channel spacing and a 270.833 ksps symbol rate, giving a 3.69 µs symbol length that is 18 times shorter than that of LTE. In contrast, W-CDMA has 5 MHz channel spacing and a 3.84 Msps symbol rate, producing a 0.26 µs symbol length — 256 times shorter than LTE. It would be impractical to insert a 4.69 µs CP between such short symbols because capacity would drop by more than half with GSM and by a factor of 20 with W-CDMA. Systems that use short symbol lengths compared to the delay spread must rely on receiver-side channel equalizers to recover the original signal. The link between channel bandwidth and symbol length puts single-carrier systems at a disadvantage versus OFDM when the channel bandwidths get wider. Consider a radio channel with 1 µs of delay spread: A 5 MHz single-carrier signal would experience approximately ﬁve symbols of ISI and a 20 MHz signal would experience approximately 20 symbols of ISI. The amount of ISI determines how hard the equalizer has to work and there exists a practical upper limit of about 5 MHz beyond which equalizer costs rise and performance drops off. Each 15 kHz subcarrier in LTE is capable of transmitting 15 ksps, giving LTE a raw symbol rate of 18 Msps at its 20 MHz system bandwidth (1200 subcarriers, 18 MHz). Using 64QAM — the most complex of the LTE modulation formats — in which one symbol represents six bits, the raw capacity is 108 Mbps. Note that actual peak rates as described in the LTE sidebar are derived by subtracting coding and control overheads and adding gains from features such as spatial multiplexing. OFDM’s other main advantage over single-carrier systems is the ease with which it can adapt to frequency and phase distortions in the received signal, whether caused by transmitter impairments or radio-channel imperfections. Transmitted and received signals are represented in the frequency domain by subcarrier phase and amplitude. By seeding the transmitted signal across the frequency domain with many reference signals (RS, known in other systems as pilots) of predetermined amplitude and phase, the receiver can easily correct for frequency-dependent signal distortions prior to demodulation. This correction is particularly necessary when using higher-order modulation formats (e.g., 16QAM, 64QAM) that are susceptible to erroneous symbol demodulation caused by even small errors in phase and amplitude. This ability to easily manipulate phase and frequency also lends itself to the processing required for multiple-input/multipleoutput (MIMO) antenna techniques such as spatial multiplexing and beamforming. The required manipulations of signal phase and amplitude are much easier to implement in OFDM systems than in single-carrier systems, which represent signals in the time domain. To summarize the advantages, OFDM systems transmit multiple low-rate subcarriers — resistant to multipath — that combine by the hundreds and thousands to provide a truly scalable system bandwidth and associated data rates. In addition, the frequency-domain representation of signals simpliﬁes the correction of signal errors in the receiver and reduces the complexity of MIMO implementation. By contrast, single-carrier systems do not scale well with bandwidth and are impractical at much above 5 MHz if path delay differences are long. *Longer CP lengths are available for use in larger cells and for specialist multi-cell broadcast applications. This provides protection for up to 10 km delay spread but with a proportional reduction in the achievable data rates. Agilent Measurement Journal 21

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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