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Overview of OFDM

Orthogonal frequency division multiplexing OFDM is being targeted for the Broadband wireless access solutions. The scheme works on the principle of multiplexing the data transmission across orthogonal frequency carriers. Access to multiple users is provided by one of the following schemes:

1. OFDM-TDMA: at a point of time all the subcarriers are allocated to one user. Multiple users are scheduled on time axis.
2. OFDM-FDMA (OFDMA): A subset of subcarriers is allocated per user in each time slot.
3. OFDM-CDMA: Division of subcarriers amongst multiple users is done by code division.

Out of these schemes OFDMA is gaining popularity and is proposed scheme for Wimax systems. OFDMA has multiple advantages like

• complete flexibility in subcarrier allocation to multiple users along frequency and time axes
• combats the effect of frequency selective fading
• user specific power control
• adaptive modulation and coding scheme per user

Prime problem in OFDMA scheme is high PAPR. OFDMA involves simultaneous transmission on multiple carriers, these carriers are independently modulated and thus cause high peak to average ratio. High PAPR renders the scheme power inefficient especially for use in terminals since they are power limited (battery life).

Understanding SC-FDMA

Single carrier FDMA is modified OFDMA scheme to overcome the high PAPR problem. LTE is deploying SC-FDMA scheme for uplink access due to its low PAPR advantage.

For more details, see equalization? The reason for the good performance of OFDMA is the ability to implement frequency domain equalization rather than time-domain equalization. In a high mobility environment, the spread of a signal (the timing difference in arrival due to movement, reflections, etc.) can be as high as 20 microseconds. For a very high bandwidth signal, this represents a very large number of symbols; consequently, the corresponding time-domain equalizer has to use a large number of taps and a complex adaptive equalization algorithm. On the other hand, if the equalization is done in the frequency domain, it can be non-adaptive and computationally less expensive.

SC-FDMA seeks to utilize the frequency domain equalization related benefits of OFDMA with a low PAPR of a standard single carrier signal [Falconer1]. In the original form, in fact, the transceiver-receiver chain was expected to be as below:

Înter%

. The transmitter would be a standard single carrier transmtter. The receiver would first do an FFT of the received signal, do equalization in the spectral domain and then do an inverse FFT to get back the signal in a time-domain. This would have the additional advantage that the transmitter operations would be very simple; ideal for a low power, low complexity mobile unit. This model is still valid for localized mode operation in modern day SC-FDMA systems, though off course it won't work for distributed mode and other types of subcarrier allocations.

Transmitter and Receiver Structure

A data stream to be transmitted using SC-FDMA follows these steps:

• Modulation

The input data stream is modulated as per the scheme supported by the wireless protocol. e.g. LTE supports /2 offset BPSK, QPSK, 8-PSK and 16-QAM schemes. The modulation scheme exercised on signal to be transmitted adapts with the channel conditions.

• FFT spread

The modulated data stream is input to FFT engine, which spreads the stream across K outputs frequencies.

• Subcarrier Mapping

These K outputs are then mapped to the subcarriers scheduled for this terminal. The subcarriers are orthogonal and cause no interference with each other. Let the total number of subcarriers be L, L>K. The K subcarriers are mapped on assigned subset of L subcarriers.

• Mapping to time domain

The L subcarriers are then input to IFFT engine. Out of these L inputs, K inputs have non-zero values. The IFFT engine converts the frequency domain signal to time domain signal.

• Overcoming IBI

Cyclic Prefix is added to the signal. The CP should be equal to the channel spread in order to come over the IBI (inter block interference).

Receiver side operation is the inverse of process defined above.

Subcarrier allocation schemes

Let the total number of subcarriers be L and the DFT spread in SC-FDMA be done on K subcarriers.

L=K.U

U defines the number of users accessing the spectrum simultaneously. Allocation of subcarriers to the users is done in either localized or distributed manner.

Localized mode

In this scheme consecutive subcarriers are allocated to the terminal.

This scheme is advantageous for Channel dependent scheduling (CDS). In this scheme, base station allocates the channels to the terminals as per the channel quality experienced by the terminals.

Distributed mode

Distributed mode involves assignment on non continuous sub carriers to the terminals.

The interspersing of carriers has advantage in frequency selective fading scenarios. When the subcarriers are spread out in the spectrum range and fading happens in a frequency range, it causes lesser bit errors and the chances of correcting that error are better. A special case of distributed mode is Interleaving mode. In interleaving mode subcarriers assigned to one user are equally distributed across the L subcarriers.

The diagram below shows four SC-FDMA signals multiplexed over the air. Each signal consists of a sinusoid starting at a random phase angle Înter% . The output signal using distributed mode in time-domain is Înter% . The same signal in localized mode permutation is shown in Înter%

A basic derivation of the effect of the two permutation modes is provided in here and a more detailed derivation is provided by Hyung in [ScFdma]

Comparison with OFDMA

Signal processing

Operation of SC-FDMA scheme is very similar to OFDMA scheme. The only additional module in signal processing is FFT engine on transmitter (and IFFT engine on receiver), which spreads the modulated data stream on multiple subcarriers. Due to this SC-FDMA scheme is also referred as DFT spread OFDMA/ DFT precoded OFDMA.

Scheduling multiple users

Both SC-FDMA and OFDMA schemes provide flexibility of allocating resources on both frequency and time axes.

PAPR

SC-FDMA transmits a single modulated stream on multiple sub carriers and thus experiences lower PAPR. When U terminals simultaneously transmit independently modulated streams to the base station, base station faces PAPR caused by these U modulated streams.However, this PAPR is substantially lower than PAPR experienced in OFDMA. Interleaved subcarrier allocation scheme has lower PAPR as compared to distributed/localized allocations.

Inter symbol interference

The terminals transmit on orthogonal subcarriers, which experience different multi path propagation. Due to this base station receives signals with significant inter symbol interference. CQI information at the base station indicates the corrections required for signals received on various frequencies. Base station thus incorporates frequency selective equalization to support SC-FDMA uplink.

References

[Sohl] Anja Sohl, Tobias Frank and Anja Klien, Channel Estimation for DFT precoded OFDMA with blockwise and interleaved subcarrier allocation, available online

[Lim] Junsung Lim, Hyung G. Myung, Kyungjin Oh, and David J. Goodman, Channel-Dependent Scheduling of Uplink Single Carrier FDMA Systems, available online

[Dahlman] Erik Dahlman, Hannes Ekstrm, Anders Furuskr, Ylva Jading, Jonas Karlsson, Magnus Lundevall, Stefan Parkvall , The 3G Long-Term Evolution Radio Interface Concepts and Performance Evaluation, Vehicular Technology Conference, 2006. VTC 2006-Spring. IEEE 63rd, May 2006

[ScFdma] Hyung G. Myung, Introduction to Single carrier FDMA, available online

[Falconer1]Falconer, Ariyavisitakul, Benjamin-Seeyar, Eidson, "Frequency Domain Equalization for Single Carrier Broadband Wireless Systems", IEEE Communications Magazine, vol.40, April, 2002

[Myung] Hyung G. Myung, Junsung Lim and David J. Goodman, Peak-to-average power ratio of single carrier FDMA signals with pulse shaping, 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'06)

[Zhang] Jianhua ZHANG*, Chen HUANG, Guangyi LIU, Ping ZHANG, Comparison of the Link Level Performance between OFDMA and SC-FDMA, Communications and Networking in China, 2006. Chinacom'06. First International Conference ,25-27 Oct. 2006

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Categories: Wireless

 LTE 

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