Orthogonal Frequency Division Multiplexing (OFDM) PDF

Chapter 4 Orthogonal Frequency Division Multiplexing (OFDM) Introduction OFDM created great expansion in wireless networks – Greater efficiency in bps/Hz Main air interface in the change from 3G to 4G – Also expanded 802.11 rates Critical technology for broadband wireless access...

Chapter 4 Orthogonal Frequency Division Multiplexing (OFDM) Introduction OFDM created great expansion in wireless networks – Greater efficiency in bps/Hz Main air interface in the change from 3G to 4G – Also expanded 802.11 rates Critical technology for broadband wireless access – WiMAX How OFDM Works Also called multicarrier modulation Start with a data stream of R bps – Could be sent with bandwidth Nfb 1 – With bit duration R OFDM splits into N parallel data streams – Called subcarriers – Each with bandwidth fb – And data rate R/N (bit time N/R) Figure 8.1 Conceptual Understanding of Orthogonal Frequency Division Multiplexing OrthOgonality (1 of 3) The spacing of the fb frequencies allows tight packing of signals – Actually with overlap between the signals – Signals at spacing of fb , 2fb , 3fb , etc. The choice of fb is related to the bit rate to make the Signals orthogonal – Average over bit time of s1(t )  s2 (t )  0 – Receiver is able to extract only the s1(t ) signal ▪ If there is no corruption in the frequency spacing OrthOgonality (2 of 3) Traditional FDM makes signals completely avoid frequency overlap – OFDM allows overlap which greatly increases capacity Figure 8.2 Illustration of Orthogonality of OFDM OrthOgonality (3 of 3) Given an OFDM subcarrier bit time of T – fb must be a multiple of 1 T Example: IEEE 802.11n wireless LAN – 20 MHz total bandwidth ▪ Only 15 MHz can be used – 48 subcarriers – fb = 0.3125 MHz – Signal is translated to 2.4 GHz or 5 GHz bands Benefits of OFDM Frequency selective fading only affects some subcarriers – Can easily be handled with a forward error-correcting code More importantly, OFDM overcomes intersymbol interference (ISI) – ISI is a caused by multipath signals arriving in later bits – OFDM bit times are much, much longer (by a factor of N) ▪ ISI is dramatically reduced – N is chosen so the root-mean-square delay spread is significantly smaller than the OFDM bit time – It may not be necessary to deploy equalizers to overcome ISI ▪ Eliminates the use of these complex and expensive devices. OFDM Implementation Inverse Fast Fourier Transform (IFFT) – The OFDM concept (Figure 8.1 see slide 4) would use N oscillators for N different subcarrier frequencies ▪ Expensive for transmitter and receiver – Discrete Fourier Transform (DFT) processes digital signals ▪ If N is a power of two, the computational speed dramatically improves by using the fast version of the DFT (FFT). – Transmitter takes a symbol from each subcarrier ▪ Makes an OFDM symbol ▪ Uses the Inverse FFT to compute the data stream to be transmitted ▪ OFDM symbol provides the weights for each subcarrier ▪ Then it is sent on the carrier using only one oscillator Figure 8.3 IFFT Implementation of OFDM Cyclic Prefix OFDM’s long bit times eliminate most of the ISI OFDM also uses a cyclic prefix (CP) to overcome the residual ISI – Adds additional time to the OFDM symbol before the real data is sent ▪ Called the guard interval ▪ ISI diminishes before the data starts – Data from the end of the OFDM symbol is used as the CP ▪ Simplifies the computations Figure 8.4 Cyclic Prefix Difficulties of OFDM (1 of 4) Peak-to-average power ratio (PAPR) – For OFDM signals, this ratio is much higher than for single-carrier signals – OFDM signal is a sum of many subcarrier signals ▪ Total can be very high or very low Power amplifiers need to amplify all amplitudes equally Vout  KVin Difficulties of OFDM (2 of 4) – Should have a linear characteristic with slope K on a Vout VS. Vin curve – Yet practical amplifiers have limited linear ranges ▪ Causing distortion if outside the linear range Figure 8.5 Ideal and Practical Amplifier Characteristics Figure 8.6 Examples of Linear and Nonlinear Amplifier Output Difficulties of OFDM (3 of 4) PAPR problem – Expensive amplifiers have wide linear range Solutions – Could reduce the peak amplitude ▪ Called input backoff ▪ But this would increase the signal to interference plus noise ratio (SINR) – Noise and interference would be relatively stronger because signal is weaker – Specific PAPR reduction techniques can be used ▪ Specialized coding, phase adjustments, clipping, etc. ▪ Single-carrier FDMA (SC-FDMA) Difficulties of OFDM (4 of 4) Intercarrier Interference (ICI) – OFDM frequencies are spaced very precisely – Channel impairments can corrupt this – Cyclic prefix helps reduce ICI ▪ But CP time should be limited so as to improve spectral efficiency ▪ A certain level of ICI may be tolerated to have smaller CPs – Doppler spread, mismatched oscillators, or even one subcarrier can cause ICI ▪ Spacing between subcarriers may need to be increased ▪ Could also use different pulse shapes, self-interference cancellation, or frequency domain equalizers. OFDMA Orthogonal Frequency Division Multiple Access (OFDMA) uses OFDM to share the wireless channel – Different users can have different slices of time and different groups of subcarriers – Subcarriers are allocated in groups ▪ Called subchannels or resource blocks ▪ Too much computation to allocate every subcarrier separately Subchannel allocation – Adjacent subcarriers – similar SINR ▪ Must measure to find the best subchannel – Regularly spaced subcarriers – diverse SINR – Randomly space subcarriers – diverse SINR and reduced adjacent-cell interference Figure 8.7 OFDM and OFDMA Opportunistic Scheduling (1 of 2) Schedule subchannels and power levels based on – Channel conditions – Data requirements Adjust in a dynamic fashion – Use channel variations as an opportunity to schedule the best choice in users ▪ Hence the term opportunistic scheduling Opportunistic Scheduling (2 of 2) – Criteria (maybe more than one used simultaneously) ▪ System efficiency – pick users with best throughput ▪ Fairness – proportional fairness considers the ratio of users’ current rates to the users’ average rates to know when a channel is best for them ▪ Requirements – audio, video ▪ Priority – public safety, emergency, or priority customers Single-Carrier FDMA SC-FDMA has similar structure and performance to OFDMA – But lower PAPR – Mobile user benefits – battery life, power efficiency, lower cost – Good for uplinks Uses extra DFT operation and frequency equalization compared to OFDM – DFT prior to IFFT – Spreads data symbols over all subcarriers – Every data symbol is carried by every subcarrier Multiple access is not possible – At one time, all subcarriers must be dedicated to one user – Multiple access is provided by using different time slots Figure 8.8 Simplified Block Diagram of OFDMA and SC-FDMA Figure 8.9 Example of OFDMA and SC-FDMA