- Understand the need for OFDM signalling in telecommunications networks.
- Comprehend and understand some of the theory behind multi-carrier modulation and the design of OFDM.
- Observe and examine OFDM performance and behaviour under different conditions using real radio experimentation equipment.

- Exposure to advanced Future Internet Research and Experimentation (FIRE) testbed infrastructure.
- Experience using GNU Radio, an open-source software toolkit that provides signal-processing blocks to implement software radios.

OFDM was quickly recognized as an efficient data transmission method and research and standards continued to evolve in the 1980’s, 1990’s and 2000’s with the additional of (see Figure 1):

- Power-Line-Communication
- Broadcast: DVB-C2
- ADSL/-2/-2+
- Digital Subscriber Line (DSL) technologies,
- Broadcast: DAB, DVB-T/-T2, DVB-H, ISDB-T
- Wireless Personal Area Network (WPAN): WiMEdia
- Wireless Local Area Network (WLAN): IEEE802.11a/g/n/ac/ad, IEEE 802.15.4g, HiperLAN/2
- Wireless Metropolitan Area Network (WMAN): IEEE 802.16a WiMAX
- Mobile telephony: LTE (3.9G), LTE Advanced (4G)

Figure 1: OFDM Standards

“OFDM will certainly remain as the root framework for the new 5G waveform design in both the downlink and the uplink, with some optimization to support the new 5G use cases” - Alan Carlton in NetworkWorld, March 2016.

Figure 2: Signal Transmitter and Receiver

Figure 3: The Doppler Shift

Different propagation directions result in different Doppler shifts per multipath component. Received envelope power depends on constructive or destructive addition of signals. The following short video gives a good explanation of the Doppler Effect:

Transmitting shorter symbols => limited in case of multi-path propagation (Inter-symbol interference (ISI))

Transmitting more bits per symbol => limited by noise and other distortions

Figure 4: Single carrier/mono-carrier system

Symbol width: 1/W and data is transmitted over only one carrier. Disadvantages include:

- Event frequency selective fading
- Equalization is complex
- Very short pulses
- Inter-symbol interference (ISI) is long
- Poor spectral efficiency because of guard bands

Multicarrier modulation is a technique where multiple low data rate carriers are combined by a transmitter to form a composite high data rate transmission, see Figure 5. To improve the spectral efficiency, guard bands between carriers need to be eliminated. In a classic multi-carrier system, the available spectrum is split into several non-overlapping frequency sub channels. The individual data elements are modulated into these sub channels and are thus frequency multiplexed.

Figure 5: Multicarrier system

Symbol width= N_c/W and data stream is split up into multiple lower-data rate sub streams, see Figure 5. They are modulated and transmitted in parallel on different sub carrier frequencies i.e. Frequency Division Multiplexing (FDM). By parallel data transmission on NC sub-carriers, symbol duration TS can be increased by factor N_c to achieve the same data rate. Longer symbols are less susceptible against inter-symbol interference (ISI). Other advantages include:

- Flat fading per subcarrier
- N_c short equalizers
- N_c long pulses
- ISI is relatively short
- Poor spectral efficiency because of guard bands
- It is easy to exploit Frequency diversity
- 2D coding techniques are allowed
- Dynamic signalling is possible

Figure 6: OFDM subcarrier tones are separated by the inverse of the signalling symbol duration

Problem: If individual subcarriers are overlapping isn’t there interference between carriers?

Answer: No! If subcarrier tones are separated by the inverse of the signalling symbol duration, independent separation of frequency-multiplexed tones is possible. Additionally, sub-spectra may overlap in frequency domain, which supports more efficient use of available spectrum and greater data rates are achievable.

In the remainder of this section we give you a brief overview of some basic OFDM concepts that we will explore further in the experimentation section using TCDs IRIS FIRE testbed equipment. These include, symbol mapping/de-mapping, Inverse Discrete Fourier Transform (IDFT), Discrete Fourier Transform (DFT), equalization, cyclic prefix, and frequency sensitivity. Figure 7 illustrates the OFDM Systems Model, and how these concepts are interconnected.

Figure 7: The OFDM System Model

If number of sub-carriers NC is chosen as a power of 2 (2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048), the IDFT can be replaced by an IFFT, yielding a very efficient implementation of a OFDM modulator (FFT for demodulator at receiver). For example, 8-PSK, which has 8 Phase Shift Keying, has three bits per sub-carrier per symbol, see Figure 8.

Figure 8: 8-psk Constellation Diagram

- Insertion of known symbols (pilots) in the OFDM frame
- Evaluating their distortions at the receiver
- Assuming a relatively static channel, data symbols can be equalized

In OFDM, each carrier becomes a infinite sinusoid (i.e. eigenfunction). As a result, the out of channel is a scaled version of the same function. The eigenvalues of the (circular) channel are the complex scalar terms that multiply each carrier. Thus symbols only experience magnitude and phase change, which makes equalization simple. Convolution in time domain corresponds to multiplication in frequency domain. However, this fact does not hold in discrete time. Circular convolution in (discrete) time domain corresponds to multiplication in (discrete) frequency domain. OFDM wants simple multiplication in the frequency domain. So, circular convolution is needed and not the regular convolution i.e., real channel does regular convolution. The solution to this problem is to add a cyclic prefix, so regular convolution can be used to create circular convolution.

Figure 9: Cyclic prefix

T_cp ≥ σ↓τ

Data Rate | Bandwidth | N | Code Rate | Modulation |

6 Mbps | 15 | 48 | 1/2 | BPSK |

9 Mbps | 15 | 48 | 3/4 | BPSK |

12 Mbps | 15 | 48 | 1/2 | QPSK |

18 Mbps | 15 | 48 | 3/4 | QPSK |

24 Mbps | 15 | 48 | 1/2 | 16-QAM |

36 Mbps | 15 | 48 | 3/4 | 16-QAM |

48 Mbps | 15 | 48 | 2/3 | 64-QAM |

54 Mbps | 15 | 48 | 3/4 | 64-QAM |