Orthogonal Multiple Access (OMA) Schemes

In OMA, multiple users transmit on orthogonal channels such that there is no interfer- ence in the users signal waveform. Thus, the receiver detects the signal for each user without interference from other users with the error performances similar to that of a single user. The total system resource/bandwidth W in time and frequency is divided into M frequency channels between the M users to ensure orthogonality. Examples of OMA techniques include time division multiple access (TDMA), frequency division multiple access (FDMA) (Figure 2.6), orthogonal division multiple access (OFDMA) and other MAC scheme which assign orthogonal signal waveforms.

2.2.1.1 Time Division Multiple Access (TDMA)

Time Division Multiple Access (TDMA) allows each user to transmit using the entire available bandwidth for a portion of the time [24]. Each frequency channel is divided

Figure 2.6: Orthogonal TDMA and FDMA resource allocation

into a number of periodically recurring time slots and multiple users are allocated a number of time slots which can vary according to their rate requirements. The number of slots in a frame, how many time slots are assigned to each user, and how this assign- ment is performed, depends on a number of factors such as permissible delay, available bandwidth, modulation technique etc.

2.2.1.2 Frequency Division Multiple Access (FDMA)

Frequency Division Multiple Access (FDMA) is the most classic MAC scheme used in mobile communications systems. The available system bandwidth W is divided into M equal narrow band frequency channels serving M users simultaneously where each user is allocated its exclusive channel. Frequency spacing between the user channels is re- quired to minimize inter-channel interference caused by the non-linear effects of power amplifiers, operating near saturation which spread the signal bandwidth and generate inter-modulation frequencies [24, 4].

The basic challenges of classic FDMA are the requirement of M modulators and de- modulators at the base station to serve M users simultaneously, which leads to excess- ive cost and complexity where the BS must handle large number of users (hundreds to

rate requirements due to the fixed allocation of narrowband channels. Thirdly, it suffers bandwidth wastage where no sub-channel is reallocated to other users if it is not in use by the assigned user [3, 25].

2.2.1.3 Orthogonal Frequency Division Multiplexing (OFDMA)

OFDM [26, 27] is a narrowband OMAS scheme used in 4G networks. In OFDM, the signals to be transmitted are mapped onto several parallel orthogonal sub-carriers (Fig- ure 2.7). The orthogonality between subcarriers is ensured by spacing the subcarriers by n/Ts, where Ts is the symbol time and n is a non-zero integer usually chosen as one. Guard intervals called CP are added to each OFDM symbol to mitigate ICI. Fur- thermore, the bandwidth of each sub-carrier is narrower than the coherence bandwidth of the channel, which ensures flat fading in an otherwise frequency-selective channel. Practical implementations of OFDM is performed with a more efficient and faster Inverse Fast Fourier Transform (IFFT) for modulation, while fast fourier transform (FFT) is used for demodulation.

OFDMA [26, 28, 29] is a hybrid combination of FDMA and OFDM. It is currently used in wireless LAN, WiMAX (IEEE 802.16), and LTE downlink systems. The system bandwidth is divided into many sub-channels and each user is allocated multiple ded- icated sub-channels, allowing M users transmit simultaneously. The number of sub- carriers allocated to each user is flexible dependent on its rate and QoS requirement. Additionally, sub-carrier allocation to different users can be either adaptive tor fixed [30, 31, 32, 33, 34, 35]. Fixed sub-carrier allocation does not adapt to users’ channel conditions and remain unchanged throughout the communication session leading to a simpler implementation without incurring high overheard. Furthermore, the users can be allocated adjacent subcarriers, which simplifies frequency and time synchronization

Figure 2.7: OFDMA resource allocation and modulation

on the expense of vulnerability to deep fading, or separated by more than the coherent bandwidth of the channel to exploit maximum frequency diversity, at the expense of a minimum separation between sub-carriers from different users requiring strict cross- user synchronization to avoid ICI.

Adaptive sub-carrier allocation dynamically allocates sub-carriers to users based on their channel condition so as to optimize some performance criteria.

The main challenges with OFDMA is that it suffers from a high Peak-to-Average Power Ratio (PAPR) which leads to inefficient operation of power amplifiers [36, 11, 31]. This is especially critical in uplink where user transmit powers are limited Secondly, OFDM is very sensitive to errors in time and frequency synchronization which leads to frequency and phase offset causing ICI and ISI.

2.2.1.4 Single Carrier-Frequency Division Multiple Access (SC-FDMA)

SC-FDMA can be described as normal OFDM with a FFT-based precoding to reduce the PAPR of the transmitted signal envelope [37, 38, 39, 40, 41, 42, 43, 44]. This is also known as DFTS-OFDM. A block of O modulation symbols from some modulation alphabet are first applied to a size-N DFT NDFT. The output of the DFT is then mapped to consecut- ive subcarriers of an OFDM modulator where a size-K IFFT KIFFT is implemented with KIFFT > NFFT. The unused IFFT subcarriers are set to zero. This is illustrated in Fig- ure 2.8. A more computationally efficient radix-2 IFFT processing, KIFFT = 2k, for some integer k is used in place of IFFT. Similar to normal OFDM, a cyclic prefix is inserted for each transmitted block. When the FFT size M equals the IFFT size K, the FFT-IFFT

Figure 2.8: SC-FDMA resource allocation and modulation

processing would imply cancel each other out. However, when M is smaller than K with the remaining inputs to the IFFT set to zero, the output of the IFFT will be a signal with ‘single-carrier’properties such as low power variations which increase power amplifier efficiency, and a bandwidth that depends on M.