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LTE
Did you know that WiFi could disappear?
posted 27 May 2016, 03:43 by Behind The Sciences [ updated 13 Jun 2016, 13:50 ]
Mobile network operators are constantly developing new techniques to provide better quality of service to their clients, but resources and specially the spectrum, are limited. One option that has been studied since last year is the named LTE-U (LTE Unlicensed). However, as we will see later, companies frequently discuss if this will be a real benefit for the user or it will be a problem for users with WiFi at home. Why? Would it be a real problem? LTE-U is a new multi-band wireless communication system initially promoted by Ericsson (known as the Licensed Assisted Access-LAA) that aims to re-use the 3.5 and 5 GHz bands to provide high speed LTE connectivity. Figure 1. LTE-U and WiFi coexistence In theory, the basic LTE-U protocol shouldn't be a problem because is based on the "listening before transmitting" protocol; first, it looks for free channels in the 5 GHz band avoiding the ones used by WiFi connections. However, a study carried by the Signal Research Group (SRG) in collaboration with Verizon Wireless and Qualcomm tested an LTE-U version by making different VoIP calls and Voice over WiFi at the same time that LTE-U and WiFi traffic were active. Figure 2. WiFi and LTE-U study In addition, Google, Microsoft and Comcast published the possible risks of using LTE-U showing that the standard will be promoted by the mobile network operators only due to commercial interest. However, almost at the same time, Huawei and NTT DoCoMo published the results of tests carried in dense user spots, using LAA and WiFi, and they proved that this combination is positive; the reason is that, unlike LTE-U, LAA is supported by the 3GPP, which make it more compatible with WiFi. Therefore, the dangers come from the competition between WiFi and LTE operators. CableLabs is a a consortium funded by various cable and WiFi companies and they also have get involved in this issue: they have tested LTE-U and WiFi coexistence in their offices and they published that "WiFi performance suffered disproportionately in the presence of LTE-U". But they not only found a throughput degradation, the tests showed that there is a significant impact in latency: In addition to WiFi, LTE-U and LAA, MulteFire is another technology recently developed taking advantage of the un-licensed spectrum in order to provide more capacity. There are different opinions about the coexistence of these technologies. The most positive one comes from the Qualcomm infographic that shows some "guide lines" to make these technologies coexist together: Figure 5. Screenshot of Qualcomm's infographic More recently, in the last Mobile World Congress, the Senza Fili Consulting President, Monica Paolini also talked about the relevance of this topic: MWC2016: Monica Paolini talks LTE-U, LAATo conclude, let's just mention that the amount of wireless clients in the United States is three times bigger than the broadband cable clients, and wireless growth as well as the mobile devices sells are increasing more and more. What do you think? Is LTE-U a new technology developed for the users' benefit or is it just a new business strategy from mobile network operators? Share |
4G Speeds
posted 26 Mar 2016, 01:44 by Behind The Sciences [ updated 13 Jun 2016, 13:50 by Behind The Sciences ]
In this post we are going to teach you how to calculate the data rate in LTE. In order to follow the process, you need to download the 3GPP document, TS 36.213. Now, let's see an example for the calculation procedure for downlink(PDSCH):
3. refer to TS 36.213 Table 7.1.7.2.1 4. go to column header indicating
the number of RB (Let's assume that RB is 50) 6. we would get 1800 (if the
number of RB is 50 and I_TBS is 1) 7. (This is Transport Block Size
per 1 ms
for one Antenna)
If we use 1 antenna, the throughput is 1800 bits * 1000 subframes/sec = about 1.8 Mbps
Now, how these 1800 bits are obtained? What's the formula behind this number? Have a look to the following set of Resource Blocks: Source: http://www.ece.drexel.edu/walsh/Gwanmo-Nov11-2.pdf For data, there are: 10 OFDM symbols per subframe x 12 subcarriers = 120 (we are counting the 6 Reference Signal symbols and excluding the PDCCH symbols) Therefore, 120 symbols x 50 RBs x 2 bits/symbol (QPSK) = 12000 bits For QPSK we know that the efficiency is 0.15 (approx.), so:
12000 x 0.15 = 1800 bits, as in table Table7.1.7.2.1 3GPP TS 36.312 for 50 RBs and I_TBS=1 Note that a more accurate way to do this calculation, which you can find in other sources as well, uses this approach: Source: http://www.ece.drexel.edu/walsh/Gwanmo-Nov11-2.pdf Here, we are considering: Number of non-sync symbols per RB= 12×7 – 4 (RS symbols)=80 OFDM symbols per slot 80 non-sync symbols x 50 RBs x 2 bits/symbol x 0.0762 x 2 slots =1219 bits
1219 bits -24 bits for the CRC = 1195 bits The 0.0762 is the code rate, which is calculated from table 7.2.3-1: 4-bit CQI Table (3GPP document TS. 36.213): The calculation procedure for uplink(PUSCH) is the same as the downlink: same steps as above except that you have to refer to 36.213 Table 8.6.1-1 at step 1. Hope this was helpful! Any feedback or comment on this post will be very welcomed! :)
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OFDM in LTE
posted 14 Mar 2016, 00:21 by Behind The Sciences [ updated 13 Jun 2016, 13:51 by Behind The Sciences ]
On January 2014, more than 200 million of users demanded mobile communications services based on the 4G technology LTE (Long Term Evolution). This standard improves the performance of predecessor wireless systems by optimizing spectral efficiency and managing the challenges of hostile wireless channels. The principal requirements for this high demand are a faster network and robustness. These requirements are covered by using OFDM (Orthogonal Frequency Division Multiplexing), MIMO (Multiple-Input, Multiple-Output) and multi-level modulation schemes. OFDM is suitable for the enhancement of data-rates and capacity which compiles the increasing demand of mobile services around the world. In addition, it is well known that OFDM offers a high performance in multipath environments that characterize the mobile wireless systems. In fact, OFDM was the key technology of other standards like DBV-T; this system transmits audio, video and information data through a MPEG-2 stream by using COFDM (Coded OFDM). LTE or 4G provides a series of improvements over UMTS (Universal Mobile Telecommunications System) and it is introduced from 3GPP (3rd Generation Partnership Project). Final users will find data-rates up to 150 Mbps in downlink and 50 Mbps in uplink and lower latencies (about 10-20 ms) which improves notably the user experience for all kind of services. In addition, LTE is based on open and global standards which promote compatibility and low costs; final users and operators will take advantage from this fact. Finally, there are huge advances in energetic efficiency for the user equipment batteries last longer. LTE allows operators offering services through an extreme-to-extreme IP network in an efficient manner, low cost and easy convergence between mobile and fixed phones. More advantages of LTE are the reduced number of nodes in the network and a more automatic and easier operation and maintenance. As it was mentioned earlier, LTE uses OFDMA (Orthogonal Frequency Division Multiple Access) in the radio interface, which works with a large number of orthogonal sub-carriers very close between them. LTE performs in the frequency ranges from 1,4 MHz to 20 MHz and supports MIMO antennas which allow to increase the data-rate and coverage. In this way, it can obtain bigger bandwidths and more spectral efficiency and flexibility over different frequency bands than predecessor technologies. OFDM by itself it is not a modulation technique, though often is referred as such. Actually, it is a multicarrier transmission technique which allows the transmission of data on multiple adjacent subcarriers, each subcarrier being modulated in a traditional manner with a linear modulation scheme such as QAM or QPSK. In the LTE system, it is employed after modulation and channel codification, and this is the reason why it is called COFDM. In an OFDM system, data for transmission is converted into several parallel streams and each stream is used to modulate a separate subcarrier. Thus, only a small amount of the total data is sent via each subcarrier, in a subchannel (a fraction of the bandwidth of the total channel). It is a limited data rate per subcarrier that gives OFDM its superior performance in a NLOS multipath environment in comparison with the single carrier transmission. With OFDM, subcarriers are cleverly allocated close to each other. This results in overlapping the spectrum and it eliminates the spectral utilization drawback of standard FDM without introducing inter-channel interference. OFDM achieves this compacting property, without introducing interference, by making subcarriers orthogonal to each other. Orthogonality is accomplished by placing each subcarrier frequency into an integer multiple of the symbol rate of the modulating symbols, and each subcarrier is separated from nearest neighbour(s) by the symbol rate. To avoid the construction of a large number of subchannel modulators and demodulators, OFDM systems utilize Digital Signal Processing (DSP) devices. Directly as a result of the orthogonality of the OFDM signal structure, modulation can be implemented by using the inverse discrete Fourier transform (IDFT). Similarly, demodulation can be performed by using the discrete Fourier transform (DFT). The Fourier transform allows events in the time domain to be related to events in the frequency domain, and vice versa for the Inverse Fourier transform and Conventional Fourier transform, both two based on continuous signals. However, DFT/IDFT is based on signal samples. In fact, a rapid computational version of DFT/IDFT, namely the fast Fourier transform (FFT) and its inverse (IFFT) is normally used on OFDM implementations. In an IFFT processor, a signal defined in the frequency domain as a complex number representation, is converted to time domain samples. Inversely, in an FFT processor, a signal defined in the time domain as samples is converted into a signal in the frequency domain. Incoming serial data are first converted from serial to parallel. If there are N subcarriers, N set of parallel data streams are created. Each set contains a subset of parallel data streams, depending on the type of modulation. OFDM is the basis of the multi-access technique called orthogonal frequency division multiple access (OFDMA). With OFDMA, subcarriers are always divided into subchannels and that implies subcarriers in each subchannel are spread over the full channel spectrum to minimize multipath fading effects. OFDMA can be used as a DL access scheme, with the MAC layer assigning subchannels to the DL data destined to the various UEs. Recall that LTE uses OFDMA in the DL. OFDMA can also be used as an uplink access scheme, where specific subchannels are assigned to specific UEs via MAC messages sent on DLs. With UL OFDMA, several UE transmitters can transmit simultaneously since each transmits different subchannels and hence subcarriers. However, in order to reduce the UEs power, and to save battery, LTE specifies to use SC-FDMA in the UL. Carriers are modulated by signals represented as complex numbers which change between symbols. The integration period in the receiver extends to the duration of two symbols, because, as in the case of delayed signals, there will be not only ISI (inter-symbol interference) over the subcarrier correspondent to the symbol which is supposed to integrate, but also there will be ICI (inter-carrier interference) and, therefore, the information will be disturbed. To avoid these effects, a guard interval is added as it is shown in the next figure: The duration of the symbol increases until exceeding the integration period at the receiver, Tsymb, in the same way of the complete modulated signal. As all the carriers are cyclic in Tsymb, the complete modulated signal is cyclic too. Therefore, the added segment at the starting symbol to form a guard interval has the same length than the segment added at the end of the symbol. As the delay in the signal due to any path, in comparison to the minimum path, will be smaller than the guard interval, all the components of the signal during an integration period belong to the same symbol, and in this way, it is satisfied the orthogonally condition. The inter-symbol or inter-carrier interference will occur only when the delay exceeds the duration of the guard interval. As it can be inferred, the guard interval extends the duration of the transmitted symbol and, hence, it reduces the effective data-rate slightly. However, the greater guard interval, the smaller interference due to multipath effects. Guard interval is selected according to the expected delay in a particular channel. For example, in environments similar to great building’s indoors, the fading could be up to some dozens of nanoseconds while in outdoor environments, in which the distances are greater, it could be about 50 µs or more. As the insertion of the guard interval reduces the effective binary rate, it doesn't have to consume an important fraction of the symbol duration Tsymb in order to maintain an adequate bit-rate and spectral efficiency. During the guard interval period, the receiver ignores (by removing it) the received signal. Orthogonality is achieved in the receiver by integrating the demodulated signal over the useful period of symbol. For echoes which duration is smaller to the guard interval, the receiver can find an interval of duration Tsymb in which there won’t be transitions in the symbol. In addition to the multipath effects which are difficult to control, there are other facts that cause loosing of orthogonality and inter-carrier interference. The main causes are the frequency or phase deviations in the receiver local oscillator, noise-phase on it, and variations in the sample frequency. These effects can be controlled by a suitable design like the one we have analysed in the transmitter and receiver blocks for LTE. Channel synchronizationAs mentioned above, for a correct demodulation, the receiver has to take samples during the useful period of OFDM symbol, but not during the guard interval. Therefore, the time window has to be placed in the instant in which each symbol appears. This is equivalent, in the analogic case, to the coherent demodulation in the receiver in which is absolutely necessary that the locally generated carrier has the same frequency and phase of the generated carrier by the transmitter. In LTE, this problem is solved by using pilot subcarriers which are regularly distributed in the symbols and they act as a synchronism performer:As the information in the pilot subcarriers is known, it is possible to make an estimation of the frequency response at the receiver. The estimation for a given subcarrier can be interpolated for completing the gaps which separate different pilots, and it can be used for equalization in the entire constellation which transports data. OFDM Modulator and DemodulatorIn an OFDM modulator, the input signal is a binary continuous stream. This stream is segmented in symbols according to the constellation used, and a map of symbols is obtained which is represented by complex numbers which correspond to the signal in the frequency domain. Let be N subcarriers to modulate at the same time, then the first operation is to convert the series input stream into a parallel stream of complex coefficients. The next step is applying the inverse Fourier transform over those N coefficients to obtain a signal in the time domain and, as the input signal in the channel has to be a series stream, converting again the signal, in this case, into a series stream. This is the transmitted signal and this process is shown in the next block diagram:In the previous image, as the input signal comes from the channel coder, the whole scheme represents a COFDM modulator (recall that letter “C” means that channel codification was applied, before the IFFT). Note that, at the output of the parallel to series converter, the guard interval is inserted, also known as the cyclic prefix in which the symbols are copied from the end of the stream and pasted at the beginning. That makes delayed signals due to multipath effects be inside the guard interval and the receiver ignore them. Therefore, if the length of the cyclic prefix is NCP , the expression for an OFDM symbol is: The entire transmitted signal is: The demodulator performs the inverse function of the modulator and the simplified block diagram is: We hope this was useful. If you would like to know more about OFDM, comment or contact us and we will do our best to post more info! Also, have a look to this post to get a LTE Simulink model and play with it! :D Share |
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