Orthogonal Waveform Multiple Access Schemes

The multiple access using orthogonal channel for each user is very simple and effective technique and it has been the primary choice for traditional voice and low rate data traffics. Since the cen- tral receiver demodulates each user’s signal separately without any interference from other users’ signals, the error performance of such schemes resembles that of single user systems [5]. The orthogonal channels are obtained by dividing the total available bandwidth into finite number of non-overlapping frequency bands, time slots, or orthogonal codes. It can be noticed that the user capacity of OWMA is unity i.e., the maximum number of simultaneous users K supported is N . The most common OWMA multiple access schemes and their main characteristics are briefly reviewed next.

2.4.1.1 Frequency Division Multiple Access (FDMA)

FDMA scheme [5] employs N non overlapping frequency bands for accommodating data access for up to K ≤ N users as shown in Figure 2.3. To avoid possible interference because of carrier frequency offset due to imperfection in electronics components, the scheme also employs addi- tional guard bands between adjacent frequency bands for the users. Since each user is allocated a single frequency band, the central receiver requires N separate demodulators, which may be very inefficient in terms of hardware requirements. In this discourse we are interested in the user capacity of FDMA schemes, as it can be noted that the FDMA scheme has the user capacity of Cuser= 1.

2.4.1.2 Orthogonal Frequency Division Multiple Access (OFDMA)

The high speed data communications over realistic channels may incur severe inter-symbol inter- ference (ISI) due to longer delay spread of channels that spans over several symbol periods. The ISI leads to irreducible error floor and equalization of channel becomes inevitable in such case. Since most equalization techniques require rather complex algorithm, a novel approach to FDMA,

which is called Orthogonal FDMA [30], is now becoming more popular for broadband wireless communications. In OFDMA, each user is assigned one or more flat fading subcarriers (subbands) that preserve orthogonality even when the users’ signals are transmitted through frequency selec- tive fading channels. This considerably saves the computational complexity required for multi tap equalization of the wideband signals and user signal demodulation is performed by very effi- cient discrete Fourier transform at the receiver [30]. The use of OFDMA with channel coding is a promising technique for broadband wireless communications and has been adopted in many stan- dards such as digital video broadcasting, cable networks and fourth generation wireless networks such as WIMAX [2]. The user capacity of this scheme is often Cuser ≤ 1, due to bandwidth loss

due to insertion of cyclic prefix and guard interval etc. 2.4.1.3 Time Division Multiple Access (TDMA)

TDMA is popular multiple access scheme adopted in second generation cellular wireless network, such as GSM. The scheme accommodates multiple users by assigning non-overlapping time slots as shown in Figure 2.3. The data frame of multiple users are demultiplexed and demodulated separately at the central receiver. The scheme is more efficient than FDMA in the sense that it supports asymmetric user data traffic in uplink and downlink by varying the period of time slot assigned for the users. TDMA is also considered for third generation wireless cellular networks for providing full duplex capabilities for mobile users. Since TDMA uses orthogonal time slots for each user, its user capacity Cuser = 1.

Figure 2.3: Orthogonal multiple access schemes

2.4.1.4 Code Division Multiple Access (CDMA)

The multiple access method of CDMA can be achieved by assigning distinct code to each user, where the user modulates its data using a distinct spreading code spanned over the entire fre- quency bandwidth as shown in Figure 2.3. The scheme differs from previous ones in that all users share the whole frequency band rather than a fraction for each user. A CDMA scheme, where

the users employ orthogonal spreading sequences are termed as Orthogonal CDMA (OCDMA) and it shares the same properties of other schemes such as FDMA and TDMA in that the scheme allows interference free signal demodulation when the transmission channels are non-dispersive [2]. In OCDMA, the user specific sequences can be obtained using the rows of Hadamard Ma- trix [5]. Since the orthogonality can only be ensured when the signals of all users are perfectly synchronous, the OCDMA is usually employed in the downlink (forward link) of CDMA cellular wireless systems [25]. The user capacity of OCDMA is as noted is Cuser = 1. The interfer-

ence free performance in underloaded conditions of the OCDMA also serves as the basis for more spectrally efficient schemes which allows system to be overloaded i.e. Cuser > 1. The so-called

OCDMA/OCDMA schemes [25, 26], achieve the user capacity Cuser> 1 by employing two sets

of orthogonal sequences followed by iterative multistage detection. 2.4.1.5 Space Division Multiple Access (SDMA)

The aforementioned orthogonal multiple access techniques assume that signals from all users are received from the same spatial direction. Hence regardless of where the signals are transmitted from, the receiver can separate users’ data based on the unique structure of their transmitted wave- forms. It is now well recognized that the capacity of multiple access can be significantly increased if the additional spatial dimension (which is available for free) is also used see [31, 32, 33, 34, 35]. These different techniques have somewhat different purposes to exploit the spatial dimension like increasing diversity, spectral efficiency or user capacity. The schemes in [31, 33] are directly rele- vant to uplink multiple access channels and the method for achieving this is often termed as Space Division Multiple Access. The idea behind the SDMA is to exploit the space to increase the ca- pacity with the use of a receiver with uniform linear antenna arrays consisting of several antenna elements. The elements are placed in a given space such as to ensure good array response towards desired direction. The composite response of the whole array is controlled by weighting each el- ement by some weights. This approach also called beamforming is used to steer the higher gain beams towards desired users and low gain beams or nulls towards interfering users, see Figure 2.4. With this arrangement of K antenna arrays and signal processing tools, the SDMA may support up to K − 1 number of users without subdivision in time, frequency or orthogonal codes [31, 33]. As can be noted here, the user capacity of the SDMA is usually Cuser= K − 1 >> 1.