In LTE the multiple access transmission scheme is based on the Frequency Domain Multiplexing (FDM). Two different versions are used: Orthogonal Frequency Do- main Multiple Access (OFDMA) for the downlink, and Single Carrier Frequency Domain Multiple Access (SC-FDMA) for the uplink. OFDMA is a very efficient transmission scheme which is widely employed in many digital communication systems, e.g., Digital Video Broadcasting (DVB), WiMax, Wireless Local Area Network (WLAN). The reason behind the popularity of OFDMA comes from the fact that it has very robust characteristics against frequency selective channels. Frequency selectivity is one of the transmission problems that can be overcome through equalization, but the complexity of the equalization technique is very high. Another reason for choosing OFDMA as the downlink transmission scheme is the bandwidth flexibility it offers, since changing the number of sub-carriers used can increase or decrease the used frequency bandwidth.
SC-FDMA is the transmission scheme in the LTE uplink. It provides a low peak- to-average ratio between the transmitted signal; it is a very desirable characteristic for the uplink to have an efficient usage of the power amplifier. This provides a high battery life time for mobile devices.
3.2.1 OFDM
The basic principle of multi-carrier systems is the splitting of the total bandwidth into a large number of smaller and narrower bandwidth units, which are known as sub-channels. Due to the narrow bandwidth sub-channels frequency selectivity does not exist. As a result, only the gains of the sub-channels has to be compen- sated and no complex equalization techniques is required.
In OFDM the sub-channels are orthogonal to each other. This nice property does not require the addition of guard intervals between the sub-channels and hence
it increases the system spectral efficiency. Figure 3.2 shows the orthogonality principle of OFDM; the frequency representation of one OFDM sub-channel is a Sinc1 function, where if the sampling is done at the exact spacing the result will only be at the sub-carrier of that sub-channel and zeros at every other sub-carrier frequency. This means that the sub-channels are orthogonal to each other.
Frequency
Time
Sub-Carriers Spectrum (Bandwidth)
Figure 3.2: OFDM signal in frequency and time domain [Hoa05]
3.2.2 OFDMA
Orthogonal Frequency Division Multiple Access (OFDMA) is an access scheme that uses the OFDM principle to orchestrate the distribution of the scarce radio resources among several users enabling multi user communications. This is done by using the Time Domain Multiple Access (TDMA), where users dynamically get some resources at the different time instances of the scheduling.
The LTE MAC Scheduler (explained in chapter 6) makes use of the differ- ent user channel conditions to distribute the frequency resources (sub-carriers) to where it best fits. This can mean giving them to the users, for example with the best instantaneous channel conditions (Max-CI scheduling). This distribution process is determined by the used scheduler discipline.
1The sinc function, sometimes also known as the sampling function, is a function that is widely used
Figure 3.3 shows an example of channel dependent scheduling between two users, where the sub-carriers of the system are distributed between the two users based on who has the best channel. A system with such channel dependent schedul- ing is often very robust with a better system capacity and higher spectral efficiency than a single user OFDM system.
User #2 scheduled Frequency User #1 channel User #1 scheduled Time User #2 channel
Figure 3.3: An example of channel dependent scheduling between two users [ADF+09]
3.2.3 SC-FDMA
As mentioned earlier, the Single Carrier Frequency Division Multiple Access is chosen as the transmission scheme for the LTE uplink. The motivation behind choosing SC-FDMA was the attractive characteristics it possesses, that is having
a low peak to average ratio which is considered to be a very desired property for having efficient power amplifier that can save battery power of the mobile device for the uplink transmission.
SC-FDMA is a special type of OFDM that combines the low peak to average power ratio with multi path resistance and flexible and efficient frequency alloca- tion. It still uses orthogonal sub-carriers similar to OFDMA, but with one differ- ence, that is the sub-carriers used for transmission are chosen to be sequential and not in parallel. A small comparison between SC-FDMA and OFDMA can be seen in Figure 3.4.
15 kHz
Frequency Data symbols occupy
15 kHz for one OFDMA symbol period
Time
Frequency
Time
SC-FDMA
Data symbols occupy M*15kHz For 1/M SC-FDMA symbol periods
60 kHz CP CP Cons tant subc arrier powe r dur ing ea ch SC -FDM A sym bol per iod 1,1 -1,1 -1,-1 1,-1 Q I
QPSK modulated data symbols
1,1 -1,-1 -1,1 1,-1 -1,-1 1,1 1,-1 -1,1
Sequence of QPSK data symbols to be transmitted
OFDMA symbol OFD MA symbol SC-FDMA symbo l SC-FDMA symbol
Figure 3.4: An example comparing SC-FDMA to OFDMA [Agi09]