Copyright ©2019 Thomas Schwengler. A significantly updated and completed 2019 Edition is available.
We present here the fundamentals of Orthogonal Frequency Division Multiplex (OFDM), which is used in standards like Wi-Fi, WiMAX, and LTE.
OFDM techniques consist in splitting a user data stream into several sub-streams, which are sent in parallel on several subcarriers. These sub-streams and subcarriers benefit from a number of properties that we now review in details.
Recall the classic example of continuous wave to encode information: the carrier frequency in itself in not capable of encoding information. The quantity of information s(t) is encoded by changes or modulation of the wave, and affects the amount of spectrum required Δfc as shown on figure 8.1.
One can of course use several carriers fi,i ∈{1,2,…,Nc}, and filter them separately. That is a common approach and is used extensively in FDMA systems: in particular multiple network operators who own licenses over a same area must take care not to exceed allowed levels of adjacent channel interference into one-another’s bands.
OFDM improves on the idea by using orthogonal properties of functions to increase spectral efficiency by choosing a specific interval Δf = fi+1 -fi between subcarriers. Multiple parallel signal streams are used: si(t) = exp(jωit) (where ωi = 2πfi), and in frequency domain: Si(f) = δ(f - fi).
In fact time signals are limited in a time window, and a user information symbol has a time interval for transmission: [0, Ts], so
| (8.1) |
(where ui is a user information symbol) and the frequency domain representation of the signal is modified from a perfect Dirac function δ(f - fi) to a sinc function1:
| (8.2) |
This last expression is derived from Fourier transform, using definitions from the next section.
Now recall the duality between time domain and frequency domain, with Fourier transform (and inverse Fourier transform) to switch from one domain to the other. Several definitions exist; let us the following definition of Fourier transform
| (8.3) |
and inverse Fourier transform
| (8.4) |
(Other definitions exist, with different signs under the exponent and different 2π factors, so it is important to always specify what definitions are used.) With this definition, the reader can readily derive formula (8.2):
| (8.5) |
OFDM is a multicarrier modulation in which a user bit stream (of rate Ru) is transmitted over Nc subcarriers, each having a symbol rate Rs = , or a symbol duration Ts = = . The advantage of that parallel transmission is that the symbol time may be increased, which mitigates inter-symbol interference.
Each symbol stream is multiplied by a function ϕk from a family of orthogonal functions {ϕk},k ∈{0,…,Nc - 1}. In CDMA, these functions were Walsh codes, in OFDM, they are windowed complex exponentials (or possibly cosine functions):
| (8.6) |
So in a similar manner to the CDMA forward link presented in section 7.1, multiple channels are multiplexed and combined, using exponential functions instead of Walsh code sequences:
| (8.7) |
(where ui = sigi; si represents the information symbol (+1 or -1), gi is the individual channel gain) or for many successive bits m = 0,1,2,..., etc.
| (8.8) |
That sequence is manipulated further and sent over the air; on the receiver side, that sequence may decoded by using orthogonal properties of {ϕk}.
| (8.9) |
where δk-l is the Kronecker symbol (i.e. 0 if k = l, 1 otherwise), and the asterisk (*) denotes the complex conjugate of the expression. Equations (8.6) and (8.9) gives us the orthogonal condition for subcarriers’ spacing: the right hand side equals zero if and only if (ωk -ωl)Ts = 2πn, for n non-zero integer. This leads to the condition Δf = n∕Ts, and 1∕Ts is the smallest separation between two subcarriers.
On the receiving side, the sequence may be decoded by simply integrating for each channel; for channel k, the information bit is retrieved from the sign of the integral:
| (8.10) |
Although in this case an additional trick is used, and direct and inverse Fourier transforms are used for decoding.
If we then examine the Fourier transform of our functions given in equation (8.6), we obtain a sinc function of pseudo-period Ts, which means that in the frequency domain subcarriers are spaced exactly such that the peak of the next one corresponds to the previous one’s first zero – see figure 8.3.
The overall envelope looks a bit like a spread spectrum signal, and may be tapered further to reduce out of band spectral power density.
The above choice of orthogonal basis functions has another useful property, relating to Fourier transform. Indeed sampling Stot turns the above expressions into the usual discrete Fourier Transform (DFT), and therefore, instead of multiplying, summing, and then integrating for decoding, OFDM allows to simply carry out a DFT and its inverse (IFT), which are very efficient operations.
Looking back at the Fourier transform (8.3), and sampling the time function and its Fourier transform (with N samples), one may define the following notations: uk = u(tk),tk = kτ,k ∈ {0,1,…,N - 1}, and Un = U(fn),fn = ,n ∈ {-N∕2,…,N∕2}. And one obtains the discrete Fourier transform
| (8.11) |
and the inverse discrete Fourier transform
| (8.12) |
Now comparing these discrete transforms to above S′tot with the particular OFDM orthogonal functions, one sees that the sk,m ⋅gk,m coefficients are Fourier transforms 2 of the complex amplitude of the subcarriers.
Consequently, encoding-decoding of an OFDM signal is practically not done with integration like (8.10), but by simple FFT. The transmitter builds: {S(ωn)} = DFT{sk,m ⋅ gk,m}. And the receiver decodes the received spectral signal: Stot = IDFT{S(ωn)}.
If sampling is made as a power of 2 (N = 2p), DFT (and IFT) algorithms are in O(Nlog2N), referred to as fast Fourier transform (FFT) and very efficient to implement. OFDM schemes are therefore based on the nearest power of two, and when fewer subcarriers are users, the same order N = 2p is used for the FFT algorithms, but with zero entries for a subset Nz = N - Nc.
Note that increasing the number of subcarriers in a given band of spectrum does not increase capacity but provides a useful parameter to optimize: there is an interesting tradeoff between number of subcarriers Nc and subcarriers symbol time Tc. The more subcarriers are used, the longer their symbol rate is, which means that the overall rate of information remains the same, but a longer symbol rate is useful for multipath mitigation (recall conditions when equalizers are required). Consequently subcarrier spacing is a fundamental parameter to chose for an OFDM standard like Wi-Fi vs. WiMAX.3
A few more standard techniques are used in combination with the above OFDM definition in practical radio systems. [121]
Copyright ©2018 Thomas Schwengler.
The combination of guard interval and cyclic prefix is interesting: an empty guard interval is typical in non-OFDM systems, and is better to limit ICI, but here it would cause such ICI, that instead the guard interval is filled with some redundant data; the interesting problem is that the data causes ISI, but the redundancy allows some of that to be lost.
Wi-Fi is a standard for interoperable equipment, certified by the Wi-Fi alliance, and based on various iterations of IEEE 802.11, which uses OFDM for its highest throughput profiles. Wi-Fi has been the most successful local area network standard, and it is worth spending some time examining some of its OFDM parameters.
Details of the 802.11 air interface can be found in a number of references, and recent books have good overview of the latest efforts [123], [124]. We only examine here some aspects of 802.11 as they relate to OFDM in order to provide some insight on performance goals and limitations.
General Parameters: 802.11a/g uses an N = 64 point FFT in a 20MHz channel, δf = 1∕Ts = 312.5kHz, Ts = 3.2μs, 4 μs time block is used, with cyclic prefix, 52 of the 64 subcarriers are populated, 4 are pilots (for phase and frequency training and tracking), 48 actually carry data.
The 802.11g packet structure includes the following:
802.11n is a high throughput amendment to 802.11 containing improvements over 802.11a/g. Major throughput improvements in the physical (PHY) and MAC layers are explained in this section.
A number of improvements in the physical (PHY) layer were designed to increase throughput in some situations, although these improvements may come at a cost, which will be pointed out. Actual data rates are lower than these maximum rates, and are detailed further in section 10.1.1.
Modul. | bits/ Hz | bits/ subcx | 48cx | 11g PHY | 11n PHY 40MHz 800ns GI | 11n PHY 40MHz 400ns GI |
BPSK1/2 | 1 | 1/2 | 24 | 6 | 13.5 | 15 |
BPSK3/4 | 1 | 3/4 | 36 | 9 |
|
|
QPSK1/2 | 2 | 1 | 48 | 12 | 27 | 30 |
QPSK3/4 | 2 | 1.5 | 72 | 18 | 40.5 | 45 |
16QAM1/2 | 4 | 2 | 96 | 24 | 54 | 60 |
16QAM3/4 | 4 | 3 | 144 | 36 | 81 | 90 |
64QAM2/3 | 6 | 4 | 192 | 48 | 108 | 120 |
64QAM3/4 | 6 | 4.5 | 216 | 54 | 121.5 | 135 |
64QAM5/6 | 6 | 5 |
|
| 135 | 150 |
The media access control (MAC) layer deals with multiple element addressing, channel access prioritization, and control. It transmits among other things beacons with regulatory and management information (such as country code, allowed channels, max power), and scans channels for beacons. Scanning is usually done passively but when regulations allow it, active probe requests can be sent for specific SSIDs or BSSIDs. MAC improvements for 802.11n include:
802.11ac is another high throughput amendment to 802.11 containing improvements over 802.11a/g/n. It leaves the 2.4 GHz band unchanged but provides very high throughput at 5 GHz. Its improvements are similar those already standardized in 802.11n, but taken even further.
Preamble and cyclic prefix is left as in 802.11n, but the following parameters are increased further:
The standard reaches for each spatial stream: 200 Mbps at 256 QAM (5/6) with 400 ns cyclic prefix in 40MHz, 866.7 Mbps in 160MHz. The maximum throughput in 8 spatial stream reaches 6.92 Gbps throughput.
Modulation | bits/ subcx | 20MHz 52cx | 40MHz 108cx | 80MHz 234cx | 160MHz 468cx |
BPSK1/2 | 1/2 | 7.22 | 15 | 32.5 | 65 |
QPSK1/2 | 1 | 14.22 | 30 | 65 | 130 |
QPSK3/4 | 1.5 | 21.67 | 45 | 97.5 | 195 |
16QAM1/2 | 2 | 28.89 | 60 | 130 | 260 |
16QAM3/4 | 3 | 43.33 | 90 | 195 | 390 |
64QAM2/3 | 4 | 57.78 | 120 | 260 | 520 |
64QAM3/4 | 4.5 | 65 | 135 | 292.5 | 585 |
64QAM5/6 | 5 | 72.22 | 150 | 325 | 650 |
256QAM3/4 | 6 | 86.67 | 180 | 390 | 780 |
256QAM5/6 | 6.67 | 96.3 | 200 | 433.33 | 866.67 |
The improvements follow the general 802.11n ideas. RTS/CTS is improved to deal with secondary channels efficiently. A-MPDU aggregation size is further increased to 1MBytes. Other concepts, like reduced inter-frame space (RIFS) is deemed less efficient than aggregation and becomes obsolete (except for backward compatibility).
Copyright ©2018 Thomas Schwengler.
A number of further development are in the work for 802.11. They produce new amendments to the specification with the following goals:
Conveniently, these channel widths are exactly 10 times narrower than 802.11ac’s (20, 40, 80, 160MHz); so the standard judiciously chose to use the same number of FFT subcarriers, to increase subcarrier spacing 10 times (31.25kHz, compared to 312.5kHz of Wi-Fi), hence gaining an order of magnitude for symbol time, and therefore acceptable delay-spreads without risk of inter-symbol interference. For example the 2MHz channel still uses 64 FFT (and 52 of these subcarriers carry data), and has the two guard band option (now 4 or 8μs). Recalling 802.11ac rates from table 8.2, 802.11ah rates are one tenth these rates in the 2-16MHz channels.
The 1MHz profile is slightly different: it maintains the same subcarrier spacing of 31.25 kHz, resulting in 32 FFT, 24 of which carry data (similar to half the number of 802.11g subcarriers). Finally an additional profile is created with a simple 2x symbol repetition thus gaining more link budget, and lowering the data rate to 150kbps.
The MAC layer also has a number of improvements to: associate many more devices, save power, improve channel access. Other techniques such as MIMO are similar (though MIMO is restricted to 4 streams).
A different 802 standard, 802.22 addresses Wireless Regional Area Networks (WRAN); it defines PHY, MAC, policies and procedures for operations in TV white spaces (TVWS); the standard was published in 2011, and was widely reported on in the press, nicknamed super Wi-Fi. Given the TVWS spectrum landscape, 802.22 defines 6, 7, or 8MHz channels, it uses OFDMA, 2048FFT, QPSK, 16QAM to 64QAM, code rates 1/2, 3/4, 5/6, and 200-300μs symbol time (with cyclic prefix of 1/4 to 1/32 of OFDMA symbol), which adapts well to wider area delay spreads. 5 The standard is therefore analogous to 802.16e in some respects.
The increasingly inaccurately named third generation partnership project (3GPP) is standardizing 4G around a long-term evolution (LTE) standard and architecture, aiming to:
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 to 256QAM). LTE offers a flexible range of channel bandwidth (1.4 to 20MHz, and aggregation of multiple channels), both in FDD and TDD bands.
LTE uses OFDMA for the downlink, with a fairly simple frame structure, and SC-FDMA for the uplink.
LTE FDD uses 10ms frames, divided into 20 sub-frames or slots (of 0.5ms each). Each sub-frame uses 7 OFDM symbols, each with a cyclic prefix. Subchannels separation is Δf =15kHz. (For multimedia broadcast multicast service MBMS dedicated cell, reduced carrier spacing can be used in the downlink Δf=7.5kHz). A cyclic prefix (CP) is used to duplicate part of the symbol: total symbol duration Ts = Tu + Tcp. For normal 15kHz subcarrier spacing, the normal CP is 7 OFDM symbols per slot, which works well in typical urban multipath (Tu = 66.7μs, and Tcp = 5.21μs for first symbol, 4.7μs for the following symbols). An extended CP for larger cells or heavy multipath is available: Tcp = 16.67μs.
This splits radio resources into time and frequency elements, called resource blocks. On the frequency scale a resource block is 12 subcarriers wide (180kHz), on the time scale it is one slot (0.5ms).
CP | OFDMA symbols | Subcx | CP symbols | CP (μs) |
Δf=15kHz, normal | 7 | 12 | 160 first | 5.2 first |
|
|
| 144 after | 4.7 after |
Δf=15kHz, extended | 6 | 12 | 512 | 16.7 |
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.
The uplink standard is departing from the usual OFDMA approach: it uses single carrier FDMA (SC-FDMA). SC-FDMA is a type of frequency domain equalization (FDE). In SC-FDMA, a bit stream is converted into single carrier symbols, then a Discrete Fourier transform (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 cyclic prefix to limit ICI, and pulse shaping is used to limit ISI.
Similar parameters are used as for downlink: subcarrier spacing 15kHz, 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. Resource blocks are defined in the same manner, with NSCRB = 12 subcarriers and NRB depends on bandwidth: 6, 15, 25, 50, 75, or 100.
The reasons for preferring SC-FDMA over OFDMA are mainly that transmitting mobile units have strict limitations on the transmit power, and that peak-to-average power ratios (PAPR) are high for OFDMA. [139]
LTE physical layer throughput calculations are easily derived from the 3GPP specifications: 1 Radio Frame has 10 sub-frames, each sub-frame 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).
A Resource Block (RB) uses 12 Sub-carriers. Assume 20MHz channel bandwidth (100 RBs), normal CP. The number of bits in a 1ms sub-frame is 100RBs x 12 sub-carriers x 2 slots x 7 modulation symbols x 6bits=100800 bits. So the data rate is 100.8 Mbps. For 4x4 MIMO the peak data rate is simply four time that, or 403Mbps. (Of course, a more robust FEC coding, lowers the bitrate to 336Mbps at 64QAM 5/6, or 302Mbps at 64QAM 3/4.)
Note that the above accounts for every resource block, which has to carry overhead signaling, reference signals, etc. Practically, looking at resource elements in a resource block for one (1ms) subframe, some resource elements are reserved (for instance with control frame indicator CFI=2).
Out of the 12x14 RE, 36 are used for control (PDCCH) and reference signals, so only 132 can carry data. (See figure 8.9). So 20% of the physical layer data rate is reserved. So the maximum physical layer data rate is 80.64Mbps (or 322.56Mbps in 4x4 MIMO).
Commonly cited numbers are 75Mbps uplink, and 300Mbps downlink for LTE, this because layer 2 has additional transport block size (TBS) restrictions and frame overhead – typically around 9-10%, leading to 75Mbps and 300Mbps rates (for 4x4 MIMO in 20MHz).
A difficult aspect of wireless systems is quality of service (QoS). LTE systems use different service definitions to provide different quality of services, some have guaranteed rates (GBR), others not (non-GBR). LTE uses several QoS parameters, and sorts them in different QoS Class Identifiers (QCI), which are treated with different requirements and priorities throughout the network. The intent is that the UE requests a QCI and sees similar user experience on every eNodeB. QCI include:
LTE network operators define a different service class for each CQI, that define target bitrates, packet error rates, and delay budgets, according to the type of service they intend as well as a target user of experience. Key performance indicators (KPI’s) are defined for each of these types of service flows, and are monitored and optimized by network providers. The RAN scheduler will handle different QCI with different priorities and requirements. And the core routed network may create different queues and priorities for each QCI.
LTE QoS can be used for instance to differentiate types of services (voice, video, or best effort data). QoS can also vary depending on time of day, emergency services, or service levels defined in the HSS customer profiles and priorities. 6
Bearer services can be defined statically in MME and HSS with the following parameters:
QoS is important for services such as voice; and wireless carriers are looking at adding voice over the LTE physical layer (VoLTE), and potentially migrate to an LTE only network in the long run. Several initiatives (IMS, VoLGA) are in the works, as well as fallback solutions to circuit switched standards (CSFB).
Codecs Several voice coders-decoders codecs are available from very efficient bitrate to high rate high quality voice transmission.
Voice Capacity Capacity aims at exceeding 3G voice (see §2.2 and equation (2.5)). These estimates are now well verified from commercial systems; although they still vary with terrain, cell sized, mobility, soft handoff, and various key voice quality metrics, capacity estimates tend to revolve around 100 simultaneous calls in a 5MHz spectrum block (FDD 5+5MHz). Various LTE estimates have been devised, tested, and published for data, but voice capacity estimates are still in their modeling phase, and few reports exist with good field data. [145]
Typical mobile wireless VoLTE can assume the G722.2 12.65kbps (plus some overhead); Further assuming 5MHz FDD LTE, 2x2 MIMO, leads to estimates ranging in the 70 to 375 simultaneous VoLTE calls depending on modulation. Empirical verification is still needed; loaded systems will be more readily available soon for a more accurate estimation.
Important parameters for VoLTE capacity include frequency (700MHz to 2.5GHz), propagation environment, link budget, shadowing statistics, channel bandwidth (e.g. 5MHz FDD), modulation (QPSK–64QAM), FEC (rate 1/2 to 1), interference mitigation features (eICIC, CoMP), MIMO rank, protocol (RTP), header compression (ROHC for RTP/UDP/IP), and of course vocoder rate (e.g. 12.2kbps G.722.2–AMR).
VoIP Calls over LTE were tested in different environments and showed the following key parameter indicators (KPI), and how they compare to typically accepted values for acceptable quality (in parentheses)
In summary: LTE capacity for carrying voice has been demonstrated, and is likely to work well in future LTE-only systems. Major wireless carriers will continue to rely on well established 3G voice until their data plans need to reclaim that spectrum, and subsequently rollout voice over LTE systems.
The LTE physical layer is described above, and remains similar for all releases so far. LTE-Advances provides further features that aim at significantly improving throughput in different ways.
The term enhanced local area access refers to techniques dealing with the increasing base station density and presence of small base stations in larger umbrella coverage. Rel-12 will continue Rel-11’s effort, allowing for instance the network to send stationary traffic to small cells, but high speed traffic to larger cells. It will also focus on different frequency modes such as: dual connectivity to macro cell and small cell (even at different frequencies such as 3.5GHz in small cells); uplink and downlink separation on different frequencies; refinement of mobility and handover commands (between wide and local cells).
LTE started with a flexible channel assignment between 1.4MHz and 20MHz. As few carriers own 20MHz contiguous, yet all want to increase throughput, carrier aggregation within a band, and then across different bands was added in LTE-Advanced.
Carrier aggregation can combine channels up to 5 carriers, therefore up to 100MHz (though that limit might be removed in later releases). When channels are non continuous (and optionally in different bands), multiple transmitters (and receivers) are powered simultaneously.
The primary carrier operates as usual, and the other component carriers are secondary. The scheduler allocates data for transmission in each available carrier (according to their DCI).
Inter-cell interference cancellation (ICIC) is important for efficiency at the edge. Rel-8-9 implemented ICIC, which allows for exchange of load balancing information between sites over the X2 interface. It then lowers power of some subchannels to limit interference (in the frequency domain) in these channels at the edge.
Rel-10 enhances the scheme to eICIC, which adds the ability to produce almost blank subframes (ABS), which is a time domain interference cancellation scheme at the UE (with resulting backhaul timing implications). Rel-11 further enhances ICIC (feICIC) by providing Cell specific Reference Symbols (CRS) information to the UE to mitigate its interference. And Rel-12 further enhances interference cancellation schemes with network assisted interference cancellation and suppression NAICS.
CoMP is an important feature of LTE for efficiency at the edge, where it coordinates resource between sectors to minimize interference. Both uplink and downlink CoMP are being standardized.
In the downlink: multiple sector transmission is coordinated to the mobile unit (data is present at multiple CoMP cooperating sets at different locations). Signal is either transmitted coherently from multiple points (coherent joint transmission), or only transmitted from the one point at a given time (dynamic point selection).
In the uplink, the idea is similar: mobile unit data transmitted is either received jointly by multiple sites, or coordinated scheduling / beamforming can be used for a specific site. (Uplink CoMP is especially efficient, much like soft handoff was).
A 3GPP CoMP study reports that data rates could be improved between 25 to 50% for mobiles at cell edges with neighboring interference. So CoMP enhancements continue, especially on downlink multipoint CSI feedback (which were introduced in Rel-11). CoMP will also focus on relaxing backhaul requirements (currently requiring dark fiber for CPRI often limits its practical applicability too much).
LTE MIMO was extended in release 11 to 8 dowlink spatial streams, and 4 uplink. Release 12 will continue these multiantenna enhancements:
Device to device (D2D) communication provides direct communication between devices without tying base station resources. It has several advantages in some close proximity of device scenarios, and saves resources, power, and improves throughput and latency. Applications related to proximity detection and social media, as well as Internet of things make the feature very interesting. There are of course drawbacks in the complexity of spectrum/RB management (as well as authentication and security and privacy concerns). Also battery drain of discovery techniques is a major issue.
Initial techniques investigated the use of Bluetooth and wi-fi, but hey are limited in range. Within an operator, all devices can listen to a common announcement. To achieve operator interoperability, devices would have to be allowed to listen to another operator band (though still only transmit in its own band). Alternatively, operators in a country could agree on one unique announcement spectrum (or a couple for fairness of who shares that capacity).
LTE can handle many MTC scenarios, but more work is underway to enhance support for low-cost, simple, low power devices, extended coverage, large number of devices per cell, and infrequent transmission needs and high sleeping time, e.g. allow high cycles of discontinuous reception (DRX) and minimize signaling.
LTE currently focuses on license bands (both FDD and TDD), but an important new area for LTE expansion is in unlicensed bands, especially 5GHz and 3.5GHz. Of course operations in 5GHz will have to follow part 15 rules in the US, which restricts power levels. It will also have to coexist with existing Wi-Fi systems, which is the cause of some concerns in the industry. Also, some portions of the 5GHz band have pre-existing protected systems, such as radar – to that effect, the FCC mandates dynamic frequency selection (DFS) in the 5.470-5.725GHz range.
Some countries (especially in the EU) require that any unlicensed operations scan for existing signal prior to transmitting (a mode of operations called listen-before-talk or LBT). In the US however, unlicensed (part 15) bands require no such scheme. A spectrum etiquette exists that encourages operations to limit interference with existing service, but no strict rules are in place, nor does the FCC get involved in any potential dispute.
The initiative caused some concerns in the industry worried about coexistence with existing Wi-Fi services. The LTE physical layer normally uses pilots that never back off (like Wi-Fi) and would significantly impact colocated Wi-Fi services. MuLTEfire therefore relies on a duty-cycle approach, and only transmits a portion of the time to allow for coexistence with Wi-Fi.
Subsequent releases of 3GPP LTE have been published:
Many other wireless standards exist and are continually in development for a wide range of applications. Figure 8.10 shows a summary of the most popular ones with their typical throughput, range, and domain of applications.
Such table is difficult to keep up-to-date as standards work focuses on new needs and new opportunities; and incorporate the latest technology innovations into them as needed. Some of them become extremely successful and successful, while others miss their window of opportunity and disappear. They range over a wide industry interest such as wireless cellular, LAN’s, smart grids, RFID tagging, entertainment, consumer electronics, and much more.
Other non standard solutions are becoming popular, such as one by Flarion (later owned by Qualcomm). The proposal was the basis of work to another IEEE group to be created: 802.20. The proposal initially used 113 subcarriers, 17 of which are used for pilots. (The next four questions refer to this solution)
(For simplicity, ignore guard bands, cyclic prefix, etc, and assume that the entire symbol duration is for user data).
Then create a second table if you now assume a long guard interval of 0.8μs instead of short GI (hint: the useful symbol length remains 3.2μs, the total symbol length is increased to 4μs instead of 3.6μs used in table 8.2.)
Copyright ©2019 Thomas Schwengler. A significantly updated and completed 2019 Edition is available.