Some of the basic targets set for 3GPP LTE (Third Generation partnership project Long term Evolution) are high data rate at cell edge, low delay, spectrum flexibility and maximum commonality between FDD(Frequency Division Duplex) and TDD(Time Division Duplex). Major upgrade that can help to achieve set targets is to increase bandwidth of operation. Thus give rise of wideband transmission. There is close relation between bandwidth and the rate of data transmission and can be understand by Shannon’s equation for channel capacity ‘C’
C = BW* log2(1+ S/N)
S = received signal power N = power of white Noise
Now assuming information rate be ‘R’ bits/sec
Therefore
S = Eb*R Eb = Energy per bit information
Also
N= No.BW No= Power spectral density of White Noise
But as information rate is always less than the channel capacity i.e. we have
R ≤ C = BW* log2(1+ S/N)
= BW* log2(1+ (Eb.R)/(No.BW))
Now a term define as radio link Bandwidth Utilization ‘γ’ as
γ = R/ BW ≤ log2(1+ (Eb/No) γ)
Hence using above equation‘s inequality we can find lower bound on the required received energy per bit of information normalised to the noise power density as
Eb/No ≥ min{ Eb/No } = (2 γ-1) / γ.
A plot for above equation is shown in Figure.1 (reproduce)
OFDM bandwidth utilization
Figure .1 Minimum required Eb/N0 at the receiver as a function of bandwidth utilization
As seen from figure 1 the operation performed for bandwidth utilization factor γ > 1 , the required signal power at the receiver must be high. Thus for energy efficient operation γ must be less than or equal to 1 i.e. for higher data rate , bandwidth must be increased in the same proportion and the main target for the evolution of 3G mobile communication is the provisioning of higher data rate with good coverage. However there are some critical issues related to wider bandwidth transmission like
· Spectrum is often a scarce and expensive resource, and it may be difficult to find spectrum allocations of sufficient size to allow for very wideband transmission, especially at lower-frequency bands.
· Wider transmission bandwidth has a direct impact on the transmitter and the receiver sampling rates, and thus on the complexity and power consumption of digital-to-analog and analog-to-digital converters as well as front-end digital signal processing. RF components are also, in general, more complicated to design and more expensive to produce, the wider the bandwidth they are to handle.
· Causes increased corruption of the transmitted signal due to time dispersion on the radio channel. Time dispersion occurs when the transmitted signal propagates to the receiver via multiple paths with different delays. In the frequency domain, a time-dispersive channel corresponds to a non-constant channel frequency response. This radio-channel frequency selectivity will corrupt the frequency-domain structure of the transmitted signal and lead to higher error rates for given signal-to-noise/interference ratios.
Multi-path propagation causing time dispersion and radio-channel frequency selectivity.
Receiver-side equalization has been used against radio channel frequency selectivity but is effective for narrowband transmission. However, if the transmission bandwidth is increased up to, for example 20 MHz, which is the target for the 3GPP Long-Term Evolution, the complexity of equalization is very high. In the following, two approaches to wider-band transmission will be discussed:
1. The use of different types of multi-carrier transmission, that is transmitting an overall wider-band signal as several more narrowband frequency-multiplexed signals. One special case of multi-carrier transmission is OFDM transmission.
2. The use of specific single-carrier transmission schemes, especially designed to allow for efficient but still reasonably low-complexity equalization, not discussed here anymore as out of the scope of topic.
Multi-carrier transmission
Multi-carrier transmission implies that, instead of transmitting a single more wideband signal, multiple more narrowband signals, often referred to as subcarriers, are frequency multiplexed and jointly transmitted over the same radio link to the same receiver. Block diagrams are shown for narrowband and wideband transmission.
Extension to wider transmission bandwidth by means of multi-carrier transmission
Figure 3 Extension to wider transmission bandwidth by means of multi-carrier transmission
A drawback of multi-carrier transmission is that, the parallel transmission of multiple carriers will lead to larger variations in the instantaneous transmit power. Thus, multi-carrier transmission will have a negative impact on the transmitter power-amplifier efficiency, implying increased transmitter power consumption and increased power-amplifier cost. Alternatively, the average transmit power must be reduced, implying a reduced range for a given data rate. For this reason, multi-carrier transmission is more suitable for the downlink (base-station transmission), compared to the uplink (mobile-terminal transmission), due to the higher importance of high power-amplifier efficiency at the mobile terminal.
The main advantage with the kind of multi-carrier extension outlined in Figure 3 is that it provides a very smooth evolution, in terms of both radio equipment and spectrum, of an already existing radio-access technology to wider transmission bandwidth and a corresponding possibility for higher data rates, especially for the downlink.
Now we will discuss, a different approach to multi-carrier transmission, based on so-called OFDM technique. OFDM has been adopted as the downlink transmission scheme for the 3GPP Long-Term Evolution (LTE) and is also used for several other radio technologies, e.g. WiMAX and the DVB broadcast technologies
Basic principles of OFDM
Transmission by means of OFDM can be seen as a kind of multi-carrier transmission. The basic characteristics of OFDM transmission, which distinguish it from a straightforward multi-carrier extension of a more narrowband transmission scheme are:
• The use of a relatively large number of narrowband subcarriers. In contrast, a straightforward multi-carrier extension as discussed previously, would typically consist of only a few subcarriers, each with a relatively wide bandwidth. As an example, a WCDMA multi-carrier evolution to a 20MHz overall transmission bandwidth could consist of four subcarriers, each with a bandwidth in the order of 5 MHz. In comparison, OFDM transmission may imply that several hundred subcarriers are transmitted over the same radio link to the same receiver.
• Simple rectangular pulse shaping as illustrated in Figure 4a. This corresponds to a sinc-square-shaped per-subcarrier spectrum, as illustrated in Figure 4b.
• Tight frequency-domain packing of the subcarriers with a subcarrier spacing ∆f =1/Tu, where Tu is the per-subcarrier modulation-symbol time (see Figure 5). The subcarrier spacing is thus equal to the per-subcarrier modulation rate 1/Tu.
Per-subcarrier pulse shape and spectrum for basic OFDM transmission
Figure 4. Per-subcarrier pulse shape and spectrum for basic OFDM transmission
OFDM subcarrier spacing.
Figure 5. OFDM subcarrier spacing.
In complex baseband notation, a basic OFDM signal x(t) during the time interval
mTu ≤ t < (m+1)Tu can thus be expressed as
OFDM notation
where xk(t) is the kth modulated subcarrier with frequency fk =k*∆f and ak(m) is the, modulation symbol applied to the kth subcarrier during the mth OFDM symbol interval, i.e. during the time interval mTu ≤ t <(m+1)Tu. OFDM transmission is thus block based, implying that, during each OFDM symbol interval, Nc modulation symbols are transmitted in parallel. The modulation symbols can be from any modulation alphabet, such as QPSK, 16QAM, or 64QAM.
OFDM modulation
Figure 6. OFDM modulation
The term Orthogonal Frequency Division Multiplex is due to the fact that two modulated OFDM subcarriers xk1 (t) and xk2 (t) are mutually orthogonal over the time interval mTu ≤ t < (m+1)Tu, i.e.
ofdm orthogoanl
The ‘physical resource’ in case of OFDM transmission is often illustrated as a time–frequency grid according to Figure 7 where each ‘column’ corresponds to one OFDM symbol and each ‘row’ corresponds to one OFDM subcarrier.
OFDM time–frequency grid
Figure 7. OFDM time–frequency grid
OFDM demodulation
Figure.8 illustrates the basic principle of OFDM demodulation consisting of a bank of correlators, one for each subcarrier. Taking into account the orthogonality between subcarriers, it is clear that, in the ideal case, two OFDM subcarriers do not cause any interference to each other after demodulation. Thus the avoidance of interference between OFDM subcarriers is not simply due to a subcarrier spectrum separation , rather due to the specific frequency-domain structure of each subcarrier in combination with the specific choice of a subcarrier spacing ∆f equal to the per-subcarrier symbol rate 1/Tu. To make an OFDM signal truly robust to radio-channel frequency selectivity, cyclic-prefix insertion is typically used, as will be further .
Basic principle of OFDM demodulation
Figure 8. Basic principle of OFDM demodulation
Cyclic-prefix insertion
As described in previous Section , an uncorrupted OFDM signal can be demodulated without any interference between subcarriers. However, in case of a time-dispersiv channel the orthogonality between the subcarriers will, at least partly, be lost. The reason for this loss of subcarrier orthogonality in case of a time-dispersive channel is that, in this case, the demodulator correlation interval for one path will overlap with the symbol boundary of a different path, as illustrated in Figure 9. Thus, the integration interval will not necessarily correspond to an integer number of periods of complex exponentials of that path as the modulation symbols ak may differ between consecutive symbol intervals. As a consequence, in case of a time-dispersive channel there will not only be inter-symbol interference within a subcarrier but also interference between subcarriers. Another way to explain the interference between subcarriers in case of a time dispersive channel is to have in mind that time dispersion on the radio channel is equivalent to a frequency-selective channel frequency response. As clarified, orthogonality between OFDM subcarriers is not simply due to frequency-domain separation but due to the specific frequency-domain structure of each subcarrier. Even if the frequency-domain channel is constant over a bandwidth corresponding to the main lobe of an OFDM subcarrier and only the subcarrier side lobes are corrupted due to the radio-channel frequency selectivity, the orthogonality between subcarriers will be lost with inter-subcarrier interference as a consequence. Due to the relatively large side lobes of each OFDM subcarrier, already a relatively limited amount of time dispersion or, equivalently, a relatively modest radio-channel frequency selectivity may cause non-negligible interference between subcarriers.
Time dispersion and corresponding received-signal timing
Figure 9 Time dispersion and corresponding received-signal timing.
To deal with this problem and to make an OFDM signal truly insensitive to time dispersion on the radio channel, so-called cyclic-prefix insertion is typically used in case of OFDM transmission. As illustrated in Figure 10, cyclic-prefix insertion implies that the last part of the OFDM symbol is copied and inserted at the beginning of the OFDM symbol. Cyclic-prefix insertion thus increases the length of the OFDM symbol from Tu to Tu +TCP, where TCP is the length of the cyclic prefix, with a corresponding reduction in the OFDM symbol rate as a consequence. As illustrated in the lower part of Figure 10, if the correlation at the receiver side is still only carried out over a time interval Tu =1/∆f , subcarrier orthogonality will then be preserved also in case of a time-dispersive channel, as long as the span of the time dispersion is shorter than the cyclic-prefix length.
Cyclic-prefix insertion
Figure 10. Cyclic-prefix insertion
Cyclic-prefix insertion is beneficial in the sense that it makes an OFDM signal insensitive to time dispersion as long as the span of the time dispersion does not exceed the length of the cyclic prefix. The drawback of cyclic-prefix insertion is that only a fraction Tu /( Tu +TCP) of the received signal power is actually utilized by the OFDM demodulator, implying a corresponding power loss in the demodulation. In addition to this power loss, cyclic-prefix insertion also implies a corresponding loss in terms of bandwidth as the OFDM symbol rate is reduced without a corresponding reduction in the overall signal bandwidth. One way to reduce the relative overhead due to cyclic-prefix insertion is to reduce the subcarrier spacing ∆f , with a corresponding increase in the symbol time Tu as a consequence.
Frequency diversity with OFDM: importance of channel coding
As discussed in previous Sections , a radio channel is always subject to some degree of frequency selectivity, implying that the channel quality will vary in the frequency domain. In case of a single wideband carrier, such as a WCDMA carrier, each modulation symbol is transmitted over the entire signal bandwidth. Thus, in case of the transmission of a single wideband carrier over a highly frequency-selective channel (see Figure 11a), each modulation symbol will be transmitted both over frequency bands with relatively good quality (relatively high signal strength) and frequency bands with low quality (low signal strength). Such transmission of information over multiple frequency bands with different instantaneous channel quality is also referred to as frequency diversity.
Transmission of single wideband carrier and OFDM transmission over a frequency- selective channel.
Figure 11. Transmission of single wideband carrier and OFDM transmission over a frequency- selective channel.
On the other hand, in case of OFDM transmission each modulation symbol is mainly confined to a relatively narrow bandwidth. Thus, in case of OFDM transmission over a frequency-selective channel, certain modulation symbols may be fully confined to a frequency band with very low instantaneous signal strength as illustrated in Figure 11b. Thus, the individual modulation symbols will typically not experience any substantial frequency diversity even if the channel is highly frequency selective over the overall OFDM transmission bandwidth. As a consequence, the basic error-rate performance of OFDM transmission over a frequency-selective channel is relatively poor and especially much worse than the basic error rate in case of a single wideband carrier.
However, in practice channel coding is used in most cases of digital communication and especially in case of mobile communication. Channel coding implies that each bit of information to be transmitted is spread over several, often very many, code bits. If these coded bits are then, via modulation symbols, mapped to a set of OFDM subcarriers that are well distributed over the overall transmission bandwidth of the OFDM signal, as illustrated in Figure 12, each information bit will experience frequency diversity in case of transmission over a radio channel that is frequency selective over the transmission bandwidth, despite the fact that the subcarriers, and thus also the code bits, will not experience any frequency diversity. Distributing the code bits in the frequency domain, as illustrated in Figure 12, is sometimes referred to as frequency interleaving. Thus, in contrast to the transmission of a single wideband carrier, channel coding (combined with frequency interleaving) is an essential component in order for OFDM transmission to be able to benefit from frequency diversity on a frequency selective channel. As channel coding is typically anyway used in most cases of mobile communication this is not a very serious drawback, especially taking into account that a significant part of the available frequency diversity can be captured already with a relatively high code rate.
Channel coding in combination with frequency-domain interleaving to provide frequency diversity in case of OFDM transmission
Figure 12. Channel coding in combination with frequency-domain interleaving to provide frequency diversity in case of OFDM transmission.
OFDM as a user-multiplexing and multiple-access scheme
The discussion has, until now, implicitly assumed that all OFDM subcarriers are transmitted from the same transmitter to a certain receiver, i.e.:
• downlink transmission of all subcarriers to a single mobile terminal.
• uplink transmission of all subcarriers from a single mobile terminal.
However, OFDM can also be used as a user-multiplexing or multiple-access scheme, allowing for simultaneous frequency-separated transmissions to/from multiple mobile terminals (see Figure 13.). In the downlink direction, OFDM as a user-multiplexing scheme implies that, in each OFDM symbol interval, different subsets of the overall set of available subcarriers are used for transmission to different mobile terminals (see Figure 13a). Similarly, in the uplink direction, OFDM as a user-multiplexing or multiple-access scheme implies that, in each OFDM symbol interval, different subsets of the overall set of subcarriers are used for data transmission from different mobile terminals(see Figure 13b).
OFDM as a user-multiplexing/multiple-access scheme: (a) downlink and (b) uplink.
Figure 13. OFDM as a user-multiplexing/multiple-access scheme: (a) downlink and (b) uplink.
Distributed user multiplexing
Figure 14. Distributed user multiplexing.
In this case, the term Orthogonal Frequency Division Multiple Access or OFDMA is also often used. Figure 13. assumes that consecutive subcarriers are used for transmission to/from the same mobile terminal. However, distributing the subcarriers to/from a mobile terminal in the frequency domain is also possible as illustrated in Figure 14.. The benefit of such distributed user multiplexing or distributed multiple access is a possibility for additional frequency diversity as each transmission is spread over a wider bandwidth.
Thus as far as downlink transmission scheme is concern OFDM fulfill all the desired target for 3G-LTE. Along with the efficient utilization of available spectrum, OFDM provide smooth transition from already established narrow band transmission system to high data rate wideband transmission. There many other services that a network operator suppose to provide to network user, OFDM supports many of them, like MBMS(Multimedia Broadcast/Multicast Service) . Multiple Input / Multiple output (MIMO) wireless system, also can enjoy easier implementation with OFDM. As far as future exploration is concern , Channel Estimation is one of such area that in case of OFDM.
