1 | Page OFDM DEMODULATOR My background knowledge with OFDM is mostly related to my association with the field of WIMAX. During my MSC thesis I used it in the development of UWB IEEE 802.15.3a and WI HUMAN IEEE 802.16 m. 802.16m Standard for an advanced air interface expects data rates of 100 Mbps for mobiles and 1Gbps for fixed users. OFDM was used as one of the modulation techniques for multiband transmissions based upon the concept multicarrier OFDM. OFDM is also called Discrete Multi-tone modulation (DMT) was used in the system to combat both frequencies selective as well as flat fading. Purely, I used it as a multi-carrier technique where a transmission of a serial bit stream is converted into parallel bit stream to be transmitted simultaneously, using two or more coordinated pass band (BPSK, QAM...Etc) signals each being transmitted over a distinct carrier over a parallel sub-channel. The technique enabled me to squeeze all the multiple, parallel, modulated sub-carriers together, thereby reducing the required bandwidth by inducing orthogonal aspect amongst the different sub-carriers. All my simulations were carried out in MATLAB. MY BACKGROUND KNOWLEDGE My TX And RX 1. OFDM to COMBAT FLAT FADING 2. OFDM to COMBAT MULTI-PATH FADING 3. OFDM for RANDOMIZATION and INTERLEAVING The following parameters were used by me. Parameter Value NSD: Number of data subcarriers 100 NSDP: Number of defined pilot carriers 12 NSG: Number of guard carriers 10 NST: Number of total subcarriers used 122 (= NSD + NSDP + NSG) ÄF: Subcarrier frequency spacing 4.125 MHz (= 528 MHz/128) TFFT: IFFT/FFT period 242.42 ns (1/ÄF) TCP: Cyclic prefix duration 60.61 ns (= 32/528 MHz) TGI: Guard interval duration 9.47 ns (= 5/528 MHz) TSYM: Symbol interval 312.5 ns (TCP + TFFT + TGI)
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OFDM DEMODULATOR
My background knowledge with OFDM is mostly related to my association with the field of WIMAX. During my MSC thesis I used it in the development of UWB IEEE 802.15.3a and WI HUMAN IEEE 802.16 m. 802.16m Standard for an advanced air interface expects data rates of 100 Mbps for mobiles and 1Gbps for fixed users. OFDM was used as one of the modulation techniques for multiband transmissions based upon the concept multicarrier OFDM. OFDM is also called Discrete Multi-tone modulation (DMT) was used in the system to combat both frequencies selective as well as flat fading. Purely, I used it as a multi-carrier technique where a transmission of a serial bit stream is converted into parallel bit stream to be transmitted simultaneously, using two or more coordinated pass band (BPSK, QAM...Etc) signals each being transmitted over a distinct carrier over a parallel sub-channel. The technique enabled me to squeeze all the multiple, parallel, modulated sub-carriers together, thereby reducing the required bandwidth by inducing orthogonal aspect amongst the different sub-carriers. All my simulations were carried out in MATLAB.
MY BACKGROUND KNOWLEDGE
My TX And RX
1. OFDM to COMBAT FLAT FADING 2. OFDM to COMBAT MULTI-PATH FADING 3. OFDM for RANDOMIZATION and INTERLEAVING
The following parameters were used by me.
Parameter Value NSD: Number of data subcarriers 100
NSDP: Number of defined pilot carriers 12 NSG: Number of guard carriers 10
NST: Number of total subcarriers used 122 (= NSD + NSDP + NSG)
I joined Wateen in 2007 after MSc from UK. My thesis helped me in getting job in Wateen where I was involved in initially testing of Wimax equipment of Motorola. I also got training by Motorola related to OFDM.
Wateen Experience
Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA) are digital encoding techniques used by WiMAX to transmit large amounts of data while minimizing the effects of multipath interference. These techniques divide the data into multiple sub-channels and transmit the sub-channels all at the same time. Using frequency division multiplexing allows the symbol duration to be greater. But it also requires guard bands between each sub-carrier.
OFDM is a more efficient method of multiplexing data onto multiple carriers. The main difference between OFDM and OFDM is that OFDM uses orthogonal frequencies for it's sub-carriers. Each carrier is spaced at set intervals and is a multiple integer of the lowest carrier frequency.
Scale able OFDMA (S-OFDMA)
OFDMA may be scaled based on the available channel bandwidth. The fast Fourier Transform (FFT) size can be varied between 128 and 2048. This changes the number of subcarriers while maintaining the subcarrier spacing.
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The Channel Assignment, called the Downlink Centre Frequency, is associated with a sector. This parameter is located in the OFDMA Downlink Channel tab. The EMS automatically sets the Uplink Centre Frequency value equal to the configured Downlink Centre Frequency value.
Although IEEE 802.16 describes single carrier, OFDM and OFDMA operation, the WiMAX Mobile System Profile and Motorola WiMAX equipment only support OFDM/OFDMA Physical Layer operation. OFDMA adds multiple access sub-channels to OFDM: sub-channels group sub-carriers together for more efficient operation. Note that 802.16d described OFDMA operation in the Up Link direction for OFDM. 802.16e and the WiMAX Mobile System Profile add OFDMA sub-channels in the Down Link direction.
IEEE 802.16d describes OFDM with 256 sub-carriers (256 FFT). 802.16e enhances OFDM to support more (or less) sub-carriers based on the channel bandwidth. The parameter wmanIfBsFFTSize specifies the number of sub-carriers (FFT) for the selected channel bandwidth. Essentially the FFT number varies based on the configured channel size. If channel bandwidths are integer multiples of each other, the sub-carrier spacing and symbol characteristics are identical; only the FFT size and channel bandwidth vary. The channel sizes are multiples of 1.25 MHz, their sub-carrier characteristics are identical. However, a channel that is not an integer multiple of 1.25 MHz would have different sub-carrier characteristics. The multiples of 1.75 MHz would be different. The table below shows the number of FFT (NFFT) used for each supported channel size. Although 10MHz and 7MHz use the same NFFT, their sub-carrier characteristics are quite different.
OFDM groups many symbol times into fixed-length, time-dependent Physical Layer frames. The symbol time interval used to carry modulated data is called the Useful Symbol Time, or TB. TB is the reciprocal of the Sub-carrier Spacing. Using the following formula, calculate TB for each channel size:
TB = 1 / Δf
The Cyclic Prefix guarantees a whole number of Hz per symbol time (a requirement of OFDM). It also allows the receiver to sample and demodulate the signal anywhere within the Total Symbol Time (TS). Using the following formula, calculate the Guard Interval: NOTE This formula assumes a cyclic Prefix of 1/8.
TS = TB + Tg
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Different commands were used for configuring OFDM and OFDMA parameters in the WiMax equipment. These commands enabled different parameters embedded in the hardware. These were entered in the command line interface.
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Proposed Solution Methodology:
The proposed solution consists of 3 main steps
1. The signal will be down converted to firstly to baseband. 2. Timing synchronization will be performed. 3. Frequency synchronization will be performed.
The figure below shows the basic diagram of OFDM transceiver
The above figure depicts a typical transmitter and receiver chain of an OFDM modem. Unlike signal-carrier modulation, the OFDM modem is performed on a block by-block basis. At the transmitter, a block of information-bearing symbols are first serial-to-parallel converted onto K subcarriers. The orthogonal waveform modulation is carried out using an inverse FFT and a parallel-serial converter. Following the converter, the last L points are appended to the beginning of the sequence as the cyclic prefix. The resulting samples are then shaped and transmitted. Each transmitted block is referred to as an OFDM symbol. The receiver reverses the process using an FFT operation. In particular, the sampled signals are first processed to determine the starting point of a block and the proper demodulation window. By removing the CP (which now contains ISI), an N (N = K) point sequence is serial-to-parallel converted and fed to the FFT. The outputs of the FFT are the symbols modulated on the K subcarriers, each multiplied by a complex channel gain. Depending on the availability of the channel information, different demodulation/decoding schemes are then used to recover the information bits. Considering an OFDM system with N subcarriers and a time-domain sampling rate l/Ts, a time and frequency representation of an OFDM symbol is depicted below
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In practice, several additional operations are often needed at the transmitter and the receivers:
Cyclic prefix and postfix: the cyclic prefix provides a guard interval for all multipath following the first arrival signal. As a result the timing requirement of the observation window is quite relaxed (up to Tmax ambiguity). On the other hand, timing estimation often hinges on the multipath signal with the highest strength, which in some cases may not be the first arrival signal. To increase the robustness of the receiver, the guard interval is often split into cyclic pre-fix and post-fix as in figure below to guard against early and late (relative to the strongest path) multipath signals.
Pulse shaping: since the time-limited signal waveforms have strong raised cosine side lobes in the frequency-domain, OFDM has been shown to be sensitive to frequency offset which leads to inter-carrier interference. The rectangular time window also leads to high out-of-band emission which is undesirable in radio communications. An effective way to reduce the ICI sensitivity is to pulse shape (time domain multiplication) the OFDM symbol with a pulse-shaping window. The tradeoff is a reduced guard-inteval and increased complexity. Alternatively, one can apply filters to limit the spectrum of the OFDM signals. The filtering introduces the same convolutional effects as the multipath channel, therefore reduces system tolerance to delay spread given a CP.
Virtual carriers: In order to guard against neighboring band interference and the out-of-band emission, a portion of the subcarriers at the two edges of the band may not be modulated. These unused subcarriers are termed as the virtual carriers. As a result, the number of subchannels that carry the information is generally smaller than the size of the DFT block, i.e., P < N = K. The virtual carriers provide a guard band for neighboring channels. In the absence of adjacent-channel-interference (ACI), the outputs from these virtual carriers are zero.
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Demodulation has to be done using FFT and we need to calculate frequency and phase offsets. These must be performed on every symbol.
Another Important suggestion is related to the discovery of a software which is used for Narrow band OFDM demodulation which can greatly facilitate in getting our desired results. This software is called OC-6040 and its brochure has been attached with the email. This software is by MEDAV a German based company. Its estimated cost is I guess either 100 0r 100000 Euros not clear from the email received by the company. Email also attached for reference.
