2

OFDMA

2.1. What is OFDM/OFDMA?

One of the key elements of Long Term Evolution (LTE) is the use of Orthogonal Frequency Division Multiplex (OFDM) as the signal bearer and the associated access schemes, OFDMA (Orthogonal Frequency Division Multiplex Access) and Single Carrier Frequency Division Multiple Access (SC-FDMA).

OFDM is a modulation format that is very suitable for carrying high data rates – one of the key requirements for LTE.

OFDM can be used in both frequency division duplex (FDD) and time division duplex (TDD) formats, and LTE operates in both FDD and TDD modes.

Therefore, LTE is an OFDMA-based technology standardized in 3rd Generation Partnership Project (3GPP) release 8 and the following releases 9, 10, 11 and 12 to date.

OFDMA stands for Orthogonal Frequency Division Multiplex Access. It is based on OFDM, a coding scheme invented in Centre Commun d’etudes en telecommunications et television (CCETT, Rennes, France) in 1982. The purpose of OFDM was, at that time, focused on digital television transmission. OFDM and OFDMA are two different variants of the same broadband wireless air interface that are often mistaken for one another. OFDMA is a form of OFDM, which is the underlying technology.

OFDM is a superior air access, which has been chosen for LTE and now adopted by most radio communication systems, such as WiMax and Wi-Fi. OFDM is the coding scheme of all digital television systems, especially terrestrial (DVB-T and DVB-T2) and satellite (DVB-S2) broadcasting – and also of DAB. It is also the basis of the last avatars of Communications on Power Lines (CPL). Also, OFDM is one of the key technologies that enable non-line of sight wireless services making it possible to extend wireless access system over wide areas.

The interfaces of both OFDM and OFDMA work by separating a single signal into subcarriers, called subchannels, or, in other words, by dividing one extremely fast signal into numerous slow signals that optimize mobile access, as the subchannels can then transmit data without being subject to the same intensity of multipath distortion faced by single carrier transmission. The numerous subcarriers are then collected at the receiver and recombined to form one high-speed transmission.

Normally, the signals carried by the numerous subcarriers would be expected to interfere with each other, but by making the signals orthogonal to each other there is no mutual interference. The data to be transmitted is split across all the carriers to give resilience against selective fading from multipath effects.

Figure 2.1. OFDM frequency and time domain

The difference between OFDM and OFDMA is that OFDMA has the ability to dynamically assign a subset of those subcarriers to individual users, making this the multiuser version of OFDM, using either Time Division Multiple Access (TDMA) (separate time frames) or Frequency Division Multiple Access (FDMA) (separate channels) for multiple users. OFDMA simultaneously supports multiple users by assigning them specific subchannels for intervals of time. Point-to-point systems are OFDM, and do not support OFDMA. Point-to-multipoint fixed and mobile systems use OFDMA.

2.1.1. Claimed OFDMA advantages

2.1.2. Recognized disadvantages of OFDMA

2.1.3. Characteristics and principles of operation

According to the channel performances, adaptive user-to-subcarrier assignment can be achieved. If the assignment is done sufficiently fast, this improves the OFDM robustness with fast fading and narrowband cochannel interference correction, and it makes it possible to achieve even better system spectral efficiency.

Different numbers of subcarriers can be assigned to different users, with a view to support differentiated Quality of Service (QoS), i.e. to control the data rate and error probability individually for each user.

OFDMA can be seen as an alternative to combining OFDM with TDMA or time-domain statistical multiplexing, i.e. packet mode communication. Low-data-rate users can send continuously with low transmission power instead of using a “pulsed” high-power carrier. Constant delay, and shorter delay, can be achieved.

OFDMA can also be described as a combination of frequency domain and time domain multiple access, where the resources are partitioned in the time-frequency space, and slots are assigned along the OFDM symbol index as well as OFDM subcarrier index.

OFDMA is considered as highly suitable for broadband wireless networks, due to advantages including scalability and use of multiple antennas (MIMO)-friendliness, and ability to take advantage of channel frequency selectivity.

In spectrum sensing cognitive radio, OFDMA is a possible approach to fill free radio frequency bands adaptively.

OFDMA is used in:

OFDMA is also a candidate access method for the IEEE 802.22 Wireless Regional Area Networks (WRAN). The project goal is to set the first radio-based standard that operates in the very high frequency (VHF) and spectrum ultra high frequency (UHF) TV spectrum.

Figure 2.2. OFDMA subcarriers

2.2. General principles

OFDM may be considered as a variant of the Frequency Division Multiplexing (FDM) scheme in which the frequency channel is divided into multiple smaller subchannels. In FDM, subchannelization requires provisioning of guard bands between two subchannels to avoid interference between them.

OFDM divides the frequency bandwidth in narrow orthogonal subparts called subcarriers. A subchannel is an aggregation of a number of these subcarriers. OFDM splits one fast carrier into many slow subcarriers. By spreading data across N carriers, one bit has N times the length of what would be using only a fast carrier, and this is achieved with roughly the same bandwidth. Being longer, each bit is more immune to noise and jamming.

Since the subcarriers that carry data are transmitted at a low rate, with higher symbol time, OFDM is more resilient to multipath effects. Therefore, it is more suitable for wide-area non-line of sight wireless access technology.

Also, using overlapping orthogonal subcarriers without guard bands makes it more efficient than the FDM scheme in terms of bits per Hertz. OFDM is a spread-spectrum technology in which energy generated at a particular bandwidth is spread across a wider bandwidth making it more resilient to interference and “jamming”.

Figure 2.3. OFDM frequency

OFDM is a technique for transmitting large amounts of digital data over a noisy channel, such as the power grid or Rayleigh/Rice radio channels. The technology works by splitting the signal into multiple smaller subsignals that are then transmitted simultaneously at different (orthogonal) frequencies. Each smaller data stream is then mapped to individual data subcarriers and modulated using some sort of Phase Shift Keying (PSK, e.g. BPSK, QPSK and 8 PSK) or Quadrature Amplitude Modulation (QAM, e.g. 16 QAM, 32 QAM, 64 QAM and event 256 QAM).

Besides its high spectral efficiency, an OFDM system reduces the amount of cross talk in signal transmissions and can efficiently overcome interference and frequency-selective fading caused by multipath.

Figure 2.4. Channels

While OFDM addresses communications in noisy environments, it is still insufficient to achieve reliable communications in very harsh conditions. To further improve reliability, the OFDM method can be combined with a multiple access scheme. This approach is called OFDMA.

OFDMA exploits frequency selectivity of the multipath channel with low complexity receivers. It allows frequency selectivity on top of frequency diverse scheduling and one cell reuse of available bandwidth.

Furthermore, due to its frequency domain nature, OFDM enables flexible bandwidth operation with low complexity devices. Smart antenna technologies are also easier to support with OFDM, since each subcarrier becomes flat faded and the antenna weights can be optimized on a per subcarrier (or block of subcarriers) basis.

OFDMA is a multiuser version of the OFDM scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual data streams. This allows simultaneous transmission of several individual data streams. OFDMA further improves OFDM robustness to fading and interference, but more importantly the individual data streams can be used either to communicate with multiple nodes (power meters) simultaneously or for redundancy, thus, greatly improving the reliability of the system.

The subcarriers include data carriers, pilot carriers and a data carrier (DC). The DCs are used to carry the traffic data; the pilot carriers are used for channel sensing purposes; and the DC marks the center of the channel. Each subcarrier is modulated with a conventional modulation scheme such as QAM or PSK at a low symbol rate. Each user is provided with an integer number of subchannels which is composed of a number of subcarriers. User data is carried in parallel on each subcarrier at a low rate. The combination of the parallel subcarriers at the destination provide for the high data rates.

OFDM allows adaptive assignment of subcarriers to subchannels based on channel conditions making it more robust and achieving higher spectral efficiency than all other schemes.

In order to share a piece of spectrum between many users, it is necessary that anyone does not suffer from the communications issued by the others. In 0 G or 1 G system, subscriber were given a precise frequency with the phase of frequency modulation, the different assigned frequencies, being sufficiently distant to avoid interferences.

With GSM and DAMS, voice has been digitized with vocoder. So the mobile systems are carrying only digital baseband signals for voice communication as well as data communication.

The allocated channels are set orthogonal, since they do not hamper one another. Orthogonal channel are provided by FDM (frequency division multiplex), TDM (time division multiplex) and CDM (code division multiplex). The last avatar of the orthogonal channel is provided by OFDM (offset frequency division multiplex). In OFDMA (offset frequency division multiplex access), the call is assigned a set of subcarrier frequencies, on which the different bits of the data stream are transmitted.

The advantage of DMA is that it mitigates or eventually eliminates the Rayleigh fading. This is due to the fact that the allocated subcarriers use different frequencies.

In the OFDMA process, the data stream has to be buffered in order that a long stream of bits can be properly sent on the chosen subcarriers.

Figure 2.5. OFDM techniques

The transmitted signal is obtained by a Fourier transform. The distance between subcarriers is n / Ts (the smallest separation being 1/Ts).

In the real system, each subcarrier does not carry only one bit in a Ts interval since it is modulated (in LTE) in QPSK up to 64 QAM. This is achieved by just processing at the same time a longer stream of data bits at the entrance of encoder. Symmetrically the decoder at the receiving side is made aware of the phase (/Amplitude) modulation that has been applied.

Since the process applies in a binary environment, instead of a Fourier transform, a discrete Fourier transform DFT is used on the encoding side and the inverse Fourier transform (IFT) at the receiving side. More, the sampling is made with a power of two DFT and IFT which is just a regular FFT (Fast Fourier Transform). This calculation can be easily implemented in regular DSP.

2.2.1. Cyclic prefixes

Signal propagation phenomena are modelled as the Rayleigh fading or multipath interference. For LTE and OFDMA this is creating an overlab of 2 symbols which is called intersymbol interference or ISI.

The modulation is based on the amplitude and the phase, so in case of overlapping there are two different amplitudes and phases. The receiver is not able to decode the state of the symbol.

Therefore, a few more standard techniques are used in combination with the above OFDM definition in practical radio systems:

Figure 2.6. Cyclic prefix

The CP transforms the classical channel convolution into a cyclic convolution, which permits easy demodulation after FFT.

Figure 2.7. Transformation

Therefore, OFDM systems are well suited to resolving rich multipath situations and slow time varying channels, which explains their popularity for standards like LTE. However, they are not ideal for Doppler shift and phase noise.

2.3. LTE channel: bandwidths and characteristics

One of the key parameters associated with the use of OFDM within LTE is the choice of bandwidth. The available bandwidth influences a variety of decisions including the number of carriers that can be accommodated in the OFDM signal and this in turn influences elements including the symbol length and so forth.

LTE defines a number of channel bandwidths. Obviously the greater the bandwidth, the greater the channel capacity.

The channel bandwidths that have been standardized for LTE are:

The subcarriers are spaced 15 kHz apart from each other. To maintain orthogonality, this gives a symbol rate of 1/15 kHz = 66.7 μs.

Each subcarrier is able to carry data at a maximum rate of 15 ksps (kilosymbols per second). This gives a 20 MHz bandwidth system a raw symbol rate of 18 Msps. In turn this is able to provide a raw data rate of 108 Mbps as each symbol using 64 QAM is able to represent six bits.

These rates do not align with the figures given in the LTE specifications. The actual peak data rates are derived by first subtracting the coding and control overheads.

2.3.1. LTE OFDM cyclic prefix, CP

OFDM shows an excellent resilience to multipath delays and spread. However it is still necessary to implement methods of adding resilience to the system. This helps in overcoming the ISI.

In areas where ISI is expected, it can be avoided by inserting a guard period into the timing at the beginning of each data symbol. It is then possible to copy a section from the end of the symbol to the beginning. This is known as the CP. The receiver can then sample the waveform at the optimum time and avoid any ISI caused by reflections that are delayed by times up to the length of the CP.

Figure 2.8. Effect of multipath propagation

The length of the CP is important. If it is not long enough then it will not counteract the multipath reflection delay spread. If it is too long, then it will reduce the data throughput capacity. For LTE, the standard length of the CP has been chosen to be 4.69 μs. This enables the system to accommodate path variations of up to 1.4 km. With the symbol length in LTE set to 66.7 μs.

The symbol length is defined by the fact that for OFDM systems the symbol length is equal to the reciprocal of the carrier spacing so that orthogonality is achieved. With a carrier spacing of 15 kHz, this gives the symbol length of 66.7 μs.

Figure 2.9. LTE OFDMA in the downlink

2.3.2. LTE OFDMA in the downlink

The OFDM signal used in LTE comprises a maximum of 2,048 different subcarriers having a spacing of 15 kHz. Although it is mandatory for the mobiles to have the capability to be able to receive all 2,048 subcarriers, not all need to be transmitted by the base station, which only needs to be able to support the transmission of 72 subcarriers. In this way all mobiles will be able to talk to any base station.

The LTE OFDM signal can be carried on three types of modulation:

Figure 2.10. 16 QAM modulation: 4 bits per symbol

The exact format is chosen depending upon the prevailing conditions. The slower form of modulation (QPSK) does not require such a large signal-to-noise ratio but it is not able to send the data as fast. When there is a sufficient signal-to-noise ratio the higher order modulation format should be used.

2.3.3. Downlink carriers and resource blocks

In the downlink, the subcarriers are split into resource blocks (RBs). This enables the system to be able to compartmentalize the data across standard numbers of subcarriers.

Figure 2.11. LTE RB allocation

RBs comprise 12 subcarriers, regardless of the overall LTE signal bandwidth. They also cover one slot in the time frame. This means that different LTE signal bandwidths will have different numbers of RBs.

Table 2.1. Number of resource block by channel bandwidth

Figure 2.12. Uplink

2.3.4. LTE SC-FDMA in the uplink

For the LTE uplink, a different concept is used for the access technique. Although still using a form of OFDMA technology, the implementation is called SC-FDMA, a modified form of OFDMA, also called DFT-Spread OFDM (DFT_SOFDM). SC-FDMA improves the peak-to-average power ratio (PAPR) compared with OFDM. The PAPR will be equivalent to a single carrier’s one. It reduces the cost of the power amplifier of the mobile. It reduces power amplifier backoff and thus improves coverage.

One of the key parameters that affects all mobiles is that of battery life. Even though battery performance is improving all the time, it is still necessary to ensure that the mobiles use as little battery power as possible. With the RF power amplifier that transmits the radio frequency signal via the antenna to the base station being the highest power item within the mobile, it is necessary that it operates in as efficient mode as possible. This can be significantly affected by the form of radio frequency modulation and signal format. Signals that have a high PAR and require linear amplification do not lend themselves to the use of efficient RF power amplifiers.

As a result it is necessary to employ a mode of transmission that has as near a constant power level as possible when operating.

OFDM has a high PAR. While this is not a problem for the base station where power is not a particular problem, it is unacceptable for the mobile. As a result, LTE uses a modulation scheme known as SC-FDMA – Single Carrier Frequency Division Multiplex – which is a hybrid format. This combines the low PAR offered by single carrier systems with the multipath interference resilience and flexible subcarrier frequency allocation that OFDM provides.

SC-FDMA groups together the RBs in such a way that reduces the need for linearity, and so power consumption, in the power amplifier. A low PAPR also improves coverage and the cell-edge performance. SC-FDMA can be interpreted as a linearly precoded OFDMA scheme, in the sense that it has an additional DFT processing step preceding the conventional OFDMA processing.

In OFDM, each modulation symbol “sees” a single 15 kHz subcarrier (flat channel). In SC-FDMA, each modulation symbol “sees” all the bandwidth (i.e. N blocks of 180 kHz). Therefore equalization is compulsory in the SC-FDMA receiver.

Figure 2.13. SC-FDMA spreads the data symbols all over the system bandwidth

DFT-Spread OFDM (DFTS-OFDM) combines the desired properties for the mobile set:

2.3.5. Transmitter and receiver structure of LP-OFDMA/SC-FDMA

The transmission processing of SC-FDMA is very similar to that of OFDMA. For each user, the sequence of bits transmitted is mapped to a complex constellation of symbols (BPSK, QPSK or M-QAM). Then different transmitters (users) are assigned different Fourier coefficients. This assignment is carried out in the mapping and demapping blocks. The receiver side includes one demapping block, one IDFT block, and one detection block for each user signal to be received. Just like in OFDM, guard intervals (called CPs) with cyclic repetition are introduced between blocks of symbols with a view to efficiently eliminate ISI from time spreading (caused by multipath propagation) among the blocks.

In SC-FDMA, multiple access among users is made possible by assigning different users different sets of non-overlapping Fourier coefficients (subcarriers). This is achieved at the transmitter by inserting (prior to IFFT) silent Fourier coefficients (at positions assigned to other users), and removing them on the receiver side after the FFT.

Figure 2.14. Localized mapping and distributed mapping

The distinguishing feature of SC-FDMA is that it leads to a single carrier transmit signal, in contrast to OFDMA which is a multicarrier transmission scheme. Subcarrier mapping can be classified into two types: localized mapping and distributed mapping.

In localized mapping, the DFT outputs are mapped to a subset of consecutive subcarriers, thereby confining them to only a fraction of the system bandwidth. In distributed mapping, the DFT outputs of the input data are assigned to subcarriers over the entire bandwidth non-continuously, resulting in zero amplitude for the remaining subcarriers. A special case of distributed SC-FDMA is called interleaved SC-FDMA (IFDMA), where the occupied subcarriers are equally spaced over the entire bandwidth.

Owing to its inherent single carrier structure, a prominent advantage of SC-FDMA over OFDM and OFDMA is that its transmit signal has a lower PAPR, resulting in relaxed design parameters in the transmit path of a subscriber unit. Intuitively, the reason lies in the fact that where OFDM transmit symbols directly modulate multiple subcarriers, SC-FDMA transmit symbols are first processed by an N-point DFT block.

In OFDM, as well as SC-FDMA, equalization is achieved on the receiver side, after the FFT calculation, by multiplying each Fourier coefficient by a complex number. Thus, frequency-selective fading and phase distortion can easily be combatted. The advantage is that frequency domain equalization (FDE) using FFTs requires less computation than conventional time-domain equalization.

Figure 2.15. SC-FDMA and OFDMA. DFT: discrete Fourier transform

IDFT: inverse discrete Fourier Transform

CP: Cyclic Prefix

PS: Pulse Shaping

DAC: Digital to analog Conversion

RF: Radio Frequency SIGNAL

ADC: Analog to Digital Conversion

LP-OFDMA: Linearly Precoded OFDMA

2.4. OFDM applied to LTE

2.4.1. General facts

Subsequent releases of 3GPP LTE:

The goal of LTE is to provide 3GPP with further evolutions, improving its architecture, throughput and spectrum efficiency. LTE can:

LTE’s air interface, like other 4G standards, revolves around OFDMA.

MIMO is used to either enhance data rates or increase data integrity (diversity and MRC). And the other usual tools are used as well: convolutional and turbo codes, and adaptive modulation (QPSK, 16 QAM, 64 QAM).

LTE offers a flexible range of channel bandwidth (1.4, 3, 5, 10 or 20 MHz), which is well adapted to the current cellular bands and to the future newly opened bands.

2.4.2. LTE downlink

LTE FDD uses 10 ms frames, divided into 20 subframes or slots (of 0.5 ms each). Each subframe uses 7 OFDM symbols, each with a CP. Subchannels separation is Δf = 1/T =15 kHz, where T is the OFDM symbol period. (For multimedia broadcast multicast service MBMS dedicated ell, reduced carrier spacing can be used in the downlink Δf = 7.5 kHz). A CP is used to duplicate part of the symbol: total symbol duration Ts = Tu + Tcp. For normal 15 kHz subcarrier spacing, the normal CP is 7 OFDM symbols per slot, which works well in typical urban multipath (Tu = 66.7 μs, and Tcp = 5.21 μs for first symbol, 4.7 μs for the following symbols). An extended CP for larger cells or heavy multipath is available: Tcp = 16.67 μs.

This splits radio resources into time and frequency elements, called RBs. On the frequency scale a RB is 12 subcarriers wide (180 kHz), on the time scale it is one slot (0.5 ms)

Table 2.2. LTE cyclic prefix lengths in number of symbols, subcarriers and time

There are three downlink channels in the physical layer, shared, control and common control. And there are two uplink channels, the shared and the control channel. Modulation techniques used for uplink and downlink are QPSK, 16 QAM, 64 QAM while the broadcast channel uses only QPSK.

Figure 2.16. LTE OFDMA physical layer structure LTE physical layer uses multiple OFDMA subcarriers and symbols separated by guard intervals

Figure 2.17. LTE resource blocks and resource elements (from the 3GPP standard)

2.4.3. Uplink

The uplink standard is departing from the usual OFDMA approach: it uses SC-FDMA. SC-FDMA is a type of FDE. In SC-FDMA, a bit stream is converted into single carrier symbols, then a DFT is applied to it, subcarriers are mapped to these DFT tones and an inverse DFT (IDFT) is performed to convert back for transmission. Much like in OFDMA, the signal has a CP to limit ICI, and pulse shaping is used to limit ISI.

Similar parameters are used as for downlink: subcarrier spacing 15 kHz, CP normal or extended (note that CP is the same for all UE in cell, and the same as downlink). The uplink uses the same symbol period and resource elements as in the downlink. RBs are defined in the same manner, with NSCRB = 12 subcarriers and NRB depends on bandwidth: 6, 15, 25, 50, 75 or 100.

Figure 2.18. CDF PAPR comparison for OFDMA used in the LTE downlink, and SC-FDMA localized mode (LFDMA) used in the LTE uplink – 256 total subcarriers, 64 subcarrier per user, 0.5 roll-off factor, a) QPSK, b) 16 QAM

LTE physical layer throughput calculations are easily derived from the 3GPP specifications: 1 Radio Frame has 10 subframes, each subframe has 2 time-slots, each time-slot is 0.5 ms long, 1 time-slot has 7 modulation symbols or OFDMA symbols (when normal CP length is used). Each modulation symbol = 6 bits at 64 QAM (note that these are physical layer bits, not actual user information).

An RB uses 12 subcarriers. Assume 20 MHz channel bandwidth (100 RBs), normal CP. The number of bits in a 1 ms subframe is 100 RBs × 12 subcarriers × 2 slots × 7 modulation symbols × 6 bits = 100,800 bits. So the data rate is 100.8 Mbps. For 4 × 4 MIMO the peak data rate is simply four time that, or 403 Mbps. (Of course, a more robust FEC coding lowers the bitrate to 336 Mbps at 64 QAM 5/6, or 302 Mbps at 64 QAM 3/4).

Note that the above accounts for every RB, which has to carry overhead signaling, reference signals, etc. Practically, looking at resource elements in a RB for one (1 ms) subframe, some resource elements are reserved (for instance with control frame indicator CFI = 2).

Figure 2.19. Some LTE resource elements are reserved for control channel and reference signals only a subset are used for user data, thus lowering actual throughput

Out of the 12 × 14 RB, 36 are used for control (PDCCH) and reference signals, so only 132 can carry data. So 20% of the physical layer data rate is reserved. So the commonly cited numbers are 75 Mbps uplink, and 300 Mbps downlink for LTE, this because layer 2 has additional transport block size (TBS) restrictions and frame overhead – typically around 9–10%, leading to 75 Mbps and 300 Mbps rates (for 4 × 4 MIMO in 20 MHz).

Figure 2.20. Conventional OFDMA with cyclic prefix

2.5. OFDMA in the LTE radio subsystem: OFDMA and SCFDMA in LTE

2.5.1. The downlink physical-layer processing of transport channels

The downlink physical-layer processing of transport channels consists of the following steps:

Figure 2.21. Downlink: OFDMA transmission scheme: downlink physical layer processing chain

2.5.2. Downlink multi-antenna transmission

Multi-antenna transmission with 2 and 4 transmit antennas is supported.

The maximum number of codewords is two, irrespective of the number of antennas with fixed mapping between code words to layers.

Spatial division multiplexing (SDM) of multiple modulation symbol streams to a single user using the same time-frequency (-code) resource, also referred to as single user MIMO (SU-MIMO), is supported.

2.5.3. Uplink basic transmission scheme

For both FDD and TDD, the uplink transmission scheme is based on single carrier FDMA, more specifically DFTS-OFDM, SC-FDMA, low PAR and good qualities of OFDM like multipath resistance and flexible subcarrier allocation.

The uplink subcarrier spacing f = 15 kHz. The subcarriers are grouped into sets of 12 consecutive subcarriers, corresponding to the uplink RB. 12 consecutive subcarriers during one slot correspond to one uplink RB.

Figure 2.22. Transmitter scheme of SC-FDMA

Figure 2.23. OFDMA and SC-FDMA

OFDMA transmits the four QPSK data symbols in parallel, one per subcarrier.

SC-FDMA transmits the four QPSK data symbols in series at four times the rate, with each data symbol occupying N × 15 kHz bandwidth.

Visually, the OFDMA signal is clearly multi-carrier and the SC-FDMA signal looks more like single carrier, which explains the “SC” in its name.

One SC-FDMA symbol in the time domain by computing the trajectory traced by moving from one QPSK data symbol to the next. This is done at N times the rate of the SC-FDMA symbol such that one SC-FDMA symbol contains N consecutive QPSK data symbols.

2.5.4. Physical-layer processing

The uplink physical layer processing of transport channels consists of the following steps:

2.5.4.1 Uplink multi-antenna transmission

The baseline antenna configuration for uplink MIMO is MU-MIMO. To allow for MU-MIMO reception at Node B, allocation of the same time and frequency resource to several UEs, each of which is transmitting on a single antenna, is supported.

Closed loop type adaptive antenna selection transmit diversity shall be supported for FDD (optional in UE).

2.5.4.2. Comparison of LTE with Wi-Fi and WiMAX, the latest versions of which also use OFDM techniques

Table 2.3. Comparison of LTE with Wi-Fi and WiMAX

2.5.4.3. Carrier aggregation in LTE Advanced

LTE Advanced has been specified to fulfill ITU requirements for a “4G” standard (“IMT-advanced”):

Spectrum aggregation scenarios:

Asymmetric bandwidth for FDD:

Figure 2.24. Number of DL/UL component carriers

2.6. Appendix 1: the constraints of mobile radio

Any mobile communication system has to be designed and deployed taking into account the constraints that are a consequence of the laws of physics. Electromagnetic wave propagation follows complicated rules, depending from the frequency. The description of these phenomena is the subject of many studies and books. Cellular systems do not operate in low frequencies, so they avoid plenty of singularities, such as long distance propagation, which affects low frequencies, up to the lower part of VHF. Cellular systems are deployed mainly in UHF, where the propagation is less dependent of fancy effects.

Cellular systems use high speed data communication over radio waves. The transmission of data is suffering of fading and also of frequency shift. Fading and frequency shift are related with well-known processes, inherent to the fact that the mobile is moving in its environment.

2.6.1. Doppler effect

The Doppler effect impacts all kinds of electromagnetic wave received or transmitted by a mobile terminal on the move. The received frequency is shifted from the transmitted value F by:

where v is the speed of the mobile, c the speed of the light, in the air c = 3.10⁸ m/s, and αi the angle between the direction of the speed vector of the mobile and the direction of the transmitted wave.

For terrestrial mobiles, the waves are coming from a set of diffractors and reflectors, the position of which is at random around the mobile. Therefore, the angle of arrival may be considered to be equally distributed around the mobile, producing a distribution of energy on a frequency bandwidth (F – fd, F + fd). A monochromatic wave is transformed by the Doppler effect into a distribution of power on a frequency interval.

The Doppler effect has little impact on mobile communications when frequency (or phase) modulation is used. The Doppler shift is negligible compared to the imprecision of the carrier frequency.

However, on the contrary, when OFDMA is the access scheme, the Doppler effect has its importance and has to be managed, e.g. by introducing the guard interval.

2.6.2. Rayleigh/Rice fading

The transmission between the mobile and the base station is subject to multipath travel due to the many diffractors and reflectors in the environment. The resulting reception shows a variation of amplitude, which is very characteristic of mobile communications. It is called the Rice channel when the mobile and the base station are in line of sight without having the direct beam completely obstructed by buildings or natural obstacles. When there is no direct line of sight beam, the phenomenon is called Rayleigh fading. The received amplitude shows deep changes of level, often 40 dB.

Rayleigh fading results from the resonance pattern of the electromagnetic wave being radiated either by the base station or by the mobile. The wave incoming to the mobile is reflected or diffracted by a lot of obstacles. To have a representation of the situation, just look to the Kundt tube experience where sound waves are reflected inside a glass tube: the “bellies” (maxima) and “knots” (minima) of the sound wave are made visible with some powder. The distance between two consecutive “knots” is half the wavelength of the signal. Of course, the frequency of mobile communications is far higher and the electromagnetic wave does not propagate as slowly as the sound wave, but the resonance pattern phenomenon is the same.

The mobile is moving in an array of stationary waves. Therefore, it comes through peaks and nulls of the signal power. The result is a random distribution of the received amplitudes.

With a recording receiver, it is easy to keep track of the signal variation.

At the transmitter, the electromagnetic wave is produced for digital symbols with a constant amplitude. The received amplitude is a random variable, which is characterized by short and deep fadings. The mobile integrates the different contributions:

The received field is Ez = Ea + Σ Ei = Ea + Er

with Ea = A cos(2πFt + 2πtfda + Øa)

and EoCi cos(2πFt + 2πtfdi + ϕa).

Ea and Er are independent random variables, when measured at different times on an interval which is sufficiently short to avoid the shadow effect on top of the Rayleigh fading, a few wavelengths covers at the speed v.

The characteristic function of Ez is the product of the characteristic functions of Ea and Er. The one for Ea is a Bessel function Jo (Av).

For Er, the calculation is less simple. The received field from Ei has a cosine component and a sine component:

Er = Ec. cos2πFt + Es. sin2πFt

Ec and Es are centered processes.

The total received field R has an amplitude of R² = (A + Ec)² + Es².

The probability of receiving an amplitude in the interval (R, R + dR) is:

where Eo is the variance of Er, supposed to be Gaussian .

The calculations were made by Rice in 1944 and give the following curves with σ = Eo :

In a town, the direct wave is most often absent. In that case, Ec and Es can be approximated by a Gaussian process with σ as variance and a probability density:

and a cumulative function:

with the error function being:

The density of probability of the envelope is given by the curve:

More detailed calculations are available in [REM 92].

Some practical results:

P is the probability of receiving a signal with an amplitude lower than average. (The real average value is Eo – 1 dB). This means that to receive the signal with 99% probability having tuned the receiver on, the average strength needs a margin of 19 dB.

To transmit data packets, we are more interested by the duration of the fading. When the signal disappears in a fading hole, the data are lost. So, focus has to be put on the duration of the Rayleigh fading holes.

The calculation made by Rice introduces NR, number of crossings of a value R by the envelope, ascending. Considering Ti to be the duration of the ith hole:

T being the length of the observation.

The average length of the hole will be

This result shows that the network engineering, including the performance of the error correcting code, should provide a protection against holes of 2t, which appear 10% of the time. The issue is that t depends of the speed of the mobile. For a car travelling at 50 km/h in a network using 900 MHz frequencies, T can be approximated to 1 ms. With lower frequencies, t will obviously be lower.

NOTE.– Multipath propagation, which is the doomed fate of mobile services, also introduces another fading, which is selective in frequencies. This is due to the discrepancies in the length of each individual path, in the range of microseconds. The consequence is the existence of a coherence band of a few tens of thousand kHz.

How can we combat Rayleigh fading and frequency selective fading?

In LTE, the different subcarriers have different distributions of peaks and nulls of the received signal. The distance between one null and the consecutive peak is function of the wavelength. It is λ/4.

Having , this gives for 900 MHz λ = 0.3 m = 30 c and = 7 cm.

The different subcarriers in the OFDM of LTE are 15 kHz apart, approximately the distance between two subsequent peaks of subcarriers is less than 10^-7m, under one micron. For this reason, the data flow transmitted by LTE (OFDM) is never subject to heavy losses needing an error correcting code able to correct error bursts.

On top of that, LTE makes use of correction of two powerful means:

2.6.3. Area of service

Electromagnetic waves show a propagation, which becomes more and more quasi-optical as the frequency rises. On top of this, some terrains absorb the energy. An example in the north of France, a high tower radiates very far to the north in Belgium and Netherland having just a flat country to cover, but the signal disappears completely in the south east direction behind a small hill covered with forest.

The usual process for evaluating the effective service provided by a base station tower is to realize vertical cuts of the surrounding land in a set of azimuths. For radio waves, the Earth shows an “increased” radius of 8,500 km. Travelling further and further from the tower, the signal dims depending of the different terrain it travels through (swamps, lakes, forestry, etc.). If the line of sight ray is obstructed by some obstacle, behind that obstacle the signal is severely dimmed, depending of the shape and nature of this obstacle. Typically for 900 MHz, one diffraction costs 20 dB, maybe more. For higher frequencies, the damage is worse; over 3 GHz, the propagation may be considered optical. The modeling of the propagation in general adopts a logarithmic formula which has the advantage of providing easy computer calculation. There are quite a few of these formulas, the most well-known of which are Okumura-Hata or COSR231. Many others are available, e.g. Epstein & Peterson, Millington and Deygout. Generally, mobile operators implement their own variant which is based on the measurements actually made in the field.

For the coverage of urban areas, such formulas are not accurate enough. The area of service is calculated from ray tracing computer calculation. For this purpose, the exact location of the base stations has to be very accurate (a few centimetres) as well as the digitized terrain including all existing buildings. Moreover, since the request from the customers includes the servicing of underground parking spaces or the service in cellars, inside shops and shopping malls, the prediction of the service area has to take many sophisticated processes.

2.6.4. Shadow effect

When the coverage is calculated with a simple algorithm, the results have to be mitigated to take into account the variation of the situation on a given area. Digitized maps are sold with a certain level of quantization. The usual available granulometry is scarcely under 5 × 5 m; prediction systems make use of 10 × 10 m, 50 × 50 m, 100 × 100 m, 200 × 200 m or 500 × 500 m. On each square, an average ground height is provided. Of course, the mobile may stand anywhere in this square, therefore its service is modeled as a Gaussian distribution in dB (log-normal law). The calculation gives the mean value of the electromagnetic field, and the Gaussian law provides the standard deviation σ. σ ranges from 4 dB to 6 dB in a rural environment, but up to 10 dB in difficult urban environments.

The shadow effect must be taken into account when measurements are made in order to calibrate the prediction tool. To obtain a valid value for the mean of the field strength, some 100 individual pieces of data have to be collected along a distance coherent with the size of the quantization square.

2.7. Appendix 2: Example of OFDM/OFDMA technological implementation Innovative DSP

This implementation uses FPGA from Xilink (Virtex-5 and Virtex-6).

A maximum likehood algorithm is implemented which provides superior performance in nosy and impaired channel conditions. For more details, see www.innovative-dsp.com.

2.8. Appendix 3: LTE error correction on the radio path [WIK 14d]

Hybrid automatic repeat request (hybrid ARQ or HARQ) is a combination of high-rate forward error-correcting coding and ARQ error-control. In standard ARQ, redundant bits are added to data to be transmitted using an error-detecting (ED) code such as a cyclic redundancy check (CRC). Receivers detecting a corrupted message will request a new message from the sender. In Hybrid ARQ, the original data is encoded with a forward error correction (FEC) code, and the parity bits are either immediately sent along with the message or only transmitted upon request when a receiver detects an erroneous message. The ED code may be omitted when a code is used that can perform both FEC in addition to error detection, such as a Reed-Solomon code. The FEC code is chosen to correct an expected subset of all errors that may occur, while the ARQ method is used as a fall-back to correct errors that are uncorrectable using only the redundancy sent in the initial transmission. As a result, hybrid ARQ performs better than ordinary ARQ in poor signal conditions, but in its simplest form this comes at the expense of significantly lower throughput in good signal conditions. There is typically a signal quality cross-over point below which simple hybrid ARQ is better, and above which basic ARQ is better.

The simplest version of HARQ, Type IHARQ, adds both ED and FEC information to each message prior to transmission. When the coded data block is received, the receiver first decodes the error-correction code. If the channel quality is good enough, all transmission errors should be correctable, and the receiver can obtain the correct data block. If the channel quality is bad, and not all transmission errors can be corrected, the receiver will detect this situation using the error-detection code, then the received coded data block is rejected and a retransmission is requested by the receiver, similar to ARQ.

In a more sophisticated form, Type II HARQ, the message originator alternates between message bits along with error detecting parity bits and only FEC parity bits. When the first transmission is received error free, the FEC parity bits are never sent. Also, two consecutive transmissions can be combined for error correction if neither is error free.

To understand the difference between Type I and Type II Hybrid ARQ, consider the size of ED and FEC added information: error detection typically only adds a couple of bytes to a message, which is only an incremental increase in length. FEC, however, can often double or triple the message length with error correction parities. In terms of throughput, standard ARQ typically expends a few percent of channel capacity for reliable protection against error, while FEC ordinarily expends half or more than half of all channel capacity for channel improvement.

In standard ARQ a transmission must be received error free on any given transmission for the error detection to pass. In Type II Hybrid ARQ, the first transmission contains only data and error detection (no different from standard ARQ). If received error free, it is done. If data is received in error, the second transmission will contain FEC parities and error detection. If received error free, it is done. If received in error, error correction can be attempted by combining the information received from both transmissions.

Only Type I Hybrid ARQ suffers capacity loss in strong signal conditions. Type II Hybrid ARQ does not, because FEC bits are only transmitted on subsequent retransmissions as needed. In strong signal conditions, Type II Hybrid ARQ performs with as good capacity as standard ARQ. In poor signal conditions, Type II Hybrid ARQ performs with as good sensitivity as standard FEC.

2.8.1. Hybrid ARQ with soft combining

In practice, incorrectly received coded data blocks are often stored at the receiver rather than discarded, and when the retransmitted block is received, the two blocks are combined. This is called Hybrid ARQ with soft combining. While it is possible that two given transmissions cannot be independently decoded without error, it may happen that the combination of the previously erroneously received transmissions gives us enough information to correctly decode. There are two main soft combining methods in HARQ:

Several variants of the two main methods exist. For example, in partial chase combining only a subset of the bits in the original transmission are retransmitted. In partial incremental redundancy, the systematic bits are always included so that each retransmission is self-decodable.

HARQ can be used in stop-and-wait mode or in selective repeat mode. Stop-and-wait is simpler, but waiting for the receiver’s acknowledgment reduces efficiency. Thus multiple stop-and-wait HARQ processes are often done in parallel in practice: when one HARQ process is waiting for an acknowledgment, another process can use the channel to send some more data.

There are other forward error correction codes that can be used in an HARQ scheme besides Turbo codes, e.g. extended irregular repeat-accumulate (eIRA) code and efficiently-encodable rate-compatible (E2RC) code, both of which are low density parity check (LDPC) code. LTE has standardized LDPC, processing the received data. The original bitstream is previously encoded with Bose Chauduri Hockenghem (BCH).

2.9. Appendix 4: The 700 MHz frequencies in the USA for LTE

The 700 MHz frequency band is depicted in the following figure.

2.9.1. Upper and lower 700 MHz

The 700 MHz band is divided into 2 bands by the FCC upper 700 MHz and lower 700 MHz.