OFDM/OFDMA in Wireless Communication Systems

With the advancement of mobile telecommunication and the success of internet more importantly services like video chat/sharing, email and other multimedia services has led users demanding for higher data rates. This has posed a great challenge on the existing technologies based on CDMA like CDMA 2000 and UMTS which uses WCDMA. UMTS with HSPA+ has been used as an earlier version of 4G but does not meet the data rate specifications set by ITU for the next generation 4G networks. Both Wimax and 3GPP led 4G LTE-Advanced have since adopted OFDM/OFDMA and have been approved by the ITU as true 4G.

The 4G systems are expected to integrate all the existing technologies to give the freedom of roaming to the user. The key feature which led to the consideration of OFDMA are its high transmission rate, multicarrier modulation, efficiency at combating ISI and multipath fading, high spectral efficiency. Basic OFDMA can be considered as a multiuser version of OFDM which assign one block to single user where as OFDMA which allows simultaneous frequency transmission (subcarriers) to different users.

OFDM has been around since 1950s. In 1966 and 1970 Robert W. Chang first published paper on transmission of simultaneous data stream without ISI and patented OFDM. One of the main drawbacks of OFDMA is its high Peak to Average Power Ratio. Imran Baig and Varun Jeoti (June 21-22, 2011) published their finding and proposed the use of zadoff-chu matrix transform (ZCMT) precoding based OFDMA system. (Baug and Jeoti, 2011)Tahir and priantoro published their finding for the advantages of VSB-OFDMA in terms of BER.

OFDM is a multi-carrier transmission scheme. In this scheme a block of data is divided and transmitted over several such carriers of low data rate. The main difference between FDM and OFDM is that here the carriers are orthogonal to each other so like in FDM there is no need of separate guard bands of each carrier. In OFDM the sub-carriers can even overlap still no interference is observed at receiver due to orthogonality. Hence the frequency is efficiently utilizes in OFDM. Fig 2.1 shows comparison between OFDM and FDM.

A basic transmitter block diagram of OFDM is shown below. The modulator works as an inverse discrete Fourier transform block for N number of information bits. After modulation IDFT samples are converted from analogue to digital. The IDFT sample can be obtained by using Inverse fast Fourier transform. Let Sk be a stream of N bits which are modulated using QAM or phase shift keying. The IDFT of data block is given as

Sn = ∑ _k=0^N-1s_k exp{j2πnk/N}, n=0,1,2…….n-1

The channel delay spread may cause ISI, so before each N IFFT block cyclic prefix (CP) or guard band is added such that CP length is equal to channel length. The guard interval helps in synchronisation of time and frequency in receiver on the cost of power and bandwidth. For transmit the data parallel to serial conversion is done and after filtering transmission done.

The receiver side of the OFDM is also shown below. The received OFDM data is converted to parallel from serial and adding error handling code to it. After filtering FFT is performed and signal is transferred to the equalizer. In OFDM equalizer liner distortions are compensated. To recover the original data stream error detection and parallel to serial data conversion is performed.

1. OFDM-TDMA
2. OFDMA
3. OFDM-CDMA
4. MIMO Aided OFDM

1. OFDM-TDMA
In this type of access scheme a particular user has access to all the subcarriers of the system foe specific time/symbol duration. Thus the users are divided o the basis of time slots. The OFDM-TDMA frame consists of a combination of all the symbols allocated to all the users. The advantage of this scheme is that it enables the mobile station to reduce its power consumption as it only has to process OFDM symbols dedicated to it.
2. OFDMA
In this type of access scheme the subcarriers are divided among the users for the entire duration of its use. OFDMA can support differ data rate simultaneously catering to different users. For security and resilience frequency hopping spread spectrum is implemented. The advantage of OFDA is that the system can be multi-rate without increasing system complexity.
3. OFDM-CDMA
Spreading codes are used for spreading the user data over subcarriers. OFDM-CDMA combines the advantages of both OFDM and CDMA. While CDMA provides frequency diversity and multiuser access OFDM on the other hand provides protection against ISI.

MIMO can be basically described as employment of multiple antennas on both receiver and transmitter side. They are the fundamental building block for the next generation of wireless broadband networks. There are two main advantages of

• The capacity and spectral efficiency of the network increases linearly with increase in MIMO antennas
• Reduced fading as all the channel fade independent of each other.

There are many advantages of using OFDM like

• Due to the use of guard intervals Inter Symbol Interference (ISI) is reduced
• The need for equalization is avoided or is minimum in comparison to single carrier modulation.
• Sub carriers suffering from deep fade are corrected using Forward Error Correction. This makes it resistant to fading.
• OFDM can tolerate delay spread.
• Robustness in multipath propagation environment.

Some of the design challenges are

• The oscillator used should be stable and noise free.
• Frequency offset correction is require at the receiver side
• Power efficiency is reduced due to large peak to average ratio.

Peak to average power ratio is a calculated by dividing the peak power amplitude of the waveform by the RMS value of it, which is expressed in decibels (dB). PAPR is defined as

PAPR = max │S(n)│²/ E│S(n)│² 0 ≤ n ≤ N – 1

Where max│S(n)│²) denotes the maximum instantaneous power and

E{│S(n)│²} is the average power of the signal.

In OFDM, time domain signal envelope change with different data symbols. The distortions are caused by the non-linearity effect. This causes noise in the receive side. The subcarriers become out of order and results in cross talk as orthogonality of signals ias affected. Higher the value of PAPR more is the amplitude fluctuation in the signal. This disrupts the operating point of the amplifier hence reducing efficiency.

Some of the existing solutions for the problem are:

• PAPR Reduction Capability
• BER Increase at the Receiver
• Loss in Data Rate
• Power Increase in Transmit Signal

OFDMA is used as the primary air interface technology in WiMAX (up-link and down-link) and used in down-link in LTE due to the peak to average power ratio SC-FDMA is used as up-link air interface.

Air interface defined in IEEE 802.16-2009 (WiMax only OFDMA portion)

• WirelessMAN-OFDM: Frequency of operation is below 11 GHz having TDD or FDD duplexing. It can work in both near LOS (line of sight) or Non LOS. Requires multiple antennas, power management and interference mitigation
• Wireless MAN-OFDMA: Frequency of operation is below 11 GHz having TDD or FDD duplexing. It can support both Point To Multipoint (PMP) and near LOS or NLOS.

Air interface LTE

LTE and LTE-Advance employ OFDMA in downlink an OFDM as a physical modulation scheme. The use of OFDMA in the downlink to allocate physical resources. OFDM helps in frequency selectivity and one cell reuse. It provides low complexity with flexible operation. It also helps in increasing signal power with the use of suitable cyclic prefix design. Thus aiding the broadcasted signal.

The future of mobile communication will be governed by the data rates which could be provided. One of the factors affecting the data rates is the multiple access scheme considered. 3 GPP is pinning its hopes on OFDM for the future of the wireless technology and is also being supported by the vendors on the infrastructure side. The use OFDM/MIMO has further helped in increasing the data rates. OFDM is a cost-effective solution to achieve high data rates at higher bandwidths.

Besides the many advantages the biggest obstacle is the PAPR problem. Due to this the uplink in LTE is done using SC-FDMA instead of OFDMA. SC-FDMA also can be considered as an altered form of OFDM to avoid the PARP problem. In SC-FDMA before the standard OFDM modulation DFT is applied to the signal first, making it suitable for uplink due to its reduced PAPR. Though there are some existing solutions for it more work is require to solve the PAPR problem and use the OFDM/OFDMA effectively