LTE Trials in the Return Channel Over Satellite

Home , Return channel

2012 6th Advanced Satellite Multimedia Systems Conference (ASMS) and 12th Signal Processing for Space Communications Workshop (SPSC)

Volker Jungnickel, Holger Gaebler, Udo Krueger, Konstantinos Manolakis, Thomas Haustein Fraunhofer Heinrich Hertz Institute, Einsteinufer 37, 10587 Berlin, Germany

Abstract—Integrating terrestrial and satellite communications promises several advantages, whereas the most evident one is multiplexing of spatial multiplex that modern satellite networks achieve global coverage using a multiple spot using multiple multiple spot beam architecture. In order to increase the spectral signals spots efficiency, orthogonal waveforms like single-carrier frequency- division multiple access (SC-FDMA) are investigated recently. In this paper, we verify experimentally that SC-FDMA waveforms taken from the 3GPP Long Term Evolution (LTE) standard can be transmitted reliably in the return channel over satellite using low-cost equipment. Mainly, we introduce an additional timing advance offset depending on satellite elevation and geographical location. And we ensure that precise information about the frequency offset measured in the forward channel is reused for compensation in the return channel at each terminal. We demonstrate in real-time transmission experiments over a geostationary Ku-band satellite that all modulation formats defined for the up-link in the LTE Release 8 standard can be decoded error- time or frequency free. Using 16-QAM, we have realized a spectral efficiency of 3.2 bits/s/Hz. Figure 1. Modern satellite networks are based on a multi-spot configuration. Index Terms—Long-Term Evolution, SC-FDMA, Satellite On the feeder link, user signals of all spots are multiplexed. Note the overlap Communications, Realtime Implementation, Field Trials between adjacent spots, causing interference similar to cellular networks.

I.INTRODUCTION Satellites are convenient for connecting users to the Internet as inter-cell interference coordination (ICIC) [4], frequency- in areas with limited infrastructure. Satellites provide high selective scheduling with interference rejection combining speed data in the forward channel widely used for broadcast- (IRC) [5] and coordinated multi-point (CoMP) [6, 7]. ICIC ing. Efficient techniques for the return channel are recently combines fractional frequency reuse (FFR) with fractional discussed. The digital video broadcasting return channel over power control (FPC). Scheduling and IRC exploit interference satellite (DVB-RCS) standard relies on multi-frequency time- awareness at the transmitter and receiver, respectively. CoMP division multiple access (MF-TDMA) where one out of several uses joint scheduling and spatial processing for multiple cells. carrier frequencies and one or more time slots can be assigned Broadband spatial signal processing is the key for such new flexibly to each terminal [1]. techniques. Using orthogonal frequency-division multiplexing However, some overhead is spent at the physical layer (OFDM) [8], transmission is robust against multi-path and to minimize the cross-talk between the non-orthogonal MF- asynchronous timing. Equalization is simple in the frequency TDMA waveforms. Orthogonal waveforms, such as single- domain. But precise recursive synchronization techniques [9, carrier frequency-division multiple access (SC-FDMA) [2, 3] 10] become mandatory. have the potential to reduce such overhead. Furthermore, they In the multiuser return channel, the timing advance (TA) allow a higher flexibility of the radio resource management protocol measures the individual propagation delay of each (RRM) and enable more sophisticated transmission techniques terminal. Signals are then individually delayed in advance of increasing the spectral efficiency. On the other hand, the the transmission so that they arrive simultaneously. synchronization effort is increased. Using the proprietary frequency advance technique, termi- Modern telecommunication satellites form multiple spot nals measure their frequency offset in the forward channel and beams to serve the desired coverage area consistently with use this information to compensate it in the return channel. higher data rates. Due to the overlap between the spots, Multiple terminals get almost the same carrier frequency in however, there is significant interference similar to terrestrial this way [11]. cellular networks, as depicted in Fig. 1. Nowadays, interfer- SC-OFDM as a special case of SC-FDMA is considered ence is reduced by classical frequency reuse schemes, at the for the satellite component of DVB-NGH [12]. It has been cost of spectral efficiency. Full frequency reuse would increase proposed recently to use SC-FDMA for the DVB-RCS2 [13]. spectral efficiency if the interference could be mitigated. No trials are reported yet proving the feasibility of this In terrestrial mobile networks, complex interference miti- approach in the field of satellite communications. Our in- gation schemes are increasingly used. They can be classified tention in this paper is to close this gap by using SC-

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Figure 2. SC-FDMA is realized in LTE using DFT spreading in the frequency Figure 3. Left: The timing advance (TA) protocol is used in LTE to equalize domain based on an inner OFDM link. Note that M ≤ N is equivalent to the arrival times of multiple terminals. Right: Depending on satellite elevation a rectangular ﬁlter in the frequency domain. Accordingly, there are residual and geographical location, a different TA offset is needed in addition at each envelope ﬂuctuations in the time-domain. In the wireless channel, we have terminal when using LTE. considered the presence of phase noise.

With subsequent IDFT despreading, the SC-FDMA receiver FDMA waveforms available in the 3GPP LTE standard. We has low complexity but it realizes the same diversity in multi- mention some changes needed to use LTE over geostationary path fading channels as the optimal linear RAKE receiver with satellites (refer also to [14]) and consider the use of low-cost complex multiuser detection [17]. satellite equipment at the terminal side. We have modified our LTE trial system [15] and demonstrate error-free SC-FDMA B. Timing Advance transmission over satellite for the first time. An intuitive requirement for using OFDM based schemes The paper is organized as follows. In Section II, we describe in the return channel is that the signals of active users are essential implementation steps of SC-FDMA in LTE and time-aligned. Timing advance is a built-in mechanism in the highlight necessary changes for using the common timing LTE air interface. A base station (BS) transmits primary and frequency advance techniques over satellite. Moreover, and secondary synchronization sequences (PSS, SSS) from we investigate the impact of phase noise in low-cost terminal which the terminal identifies the best serving base station equipments. In Section III, we describe the changes made in and derives a corresponding trigger signal. Triggered by this our LTE prototype to enable SC-FDMA transmission in the signal, each terminal transmits individually a so-called ran- return channel over satellite. In Section IV, we report perfor- dom access channel (RACH) preamble to the BS. There are mance measurements over a geostationary Ku-band satellite. up to 64 different sequences per BS enabling simultaneous measurements of the round trip times for multiple terminals. II.USING LTE OVER SATELLITE Finally, each terminal receives from the serving BS the so- In the following, we consider the use of the SC-FDMA called timing advance parameter and sets a constant delay of air interface from the Long Term Evolution (LTE) in the it’s individual waveform so that the waveforms of multiple return channel over satellite and derive essential physical layer terminals are received virtually simultaneous. Of course, there requirements therefore. are measurement inaccuracies, but they can be handled as long as they are within the cyclic prefix (CP) length of 4.7 µs. A. SC-FDMA in LTE This mechanism can in principle be reused in the return SC-FDMA is also denoted as DFT-spread OFDM and channel over satellite as well. In case of satellite, however, widely used in the 3GPP LTE up-link due to lower peak to distance variations among all terminals in one spot beam can average power ratio (PAPR) compared to orthogonal frequency be larger than allowed by the LTE specifications, as illustrated division multiple access (OFDMA). This allows a higher in Fig. 3. Reliable timing measurements using LTE are limited efficiency of the amplifier at the terminal side [16]. SC-FDMA to roughly 100 µs due to the particularly long CP in the RACH can be realized using an outer DFT spreading followed by a preamble. In this way, up to 30 km distance variations among flexible carrier mapping in the frequency domain [2] using an the terminals can be handled. However, satellite spot beams inner OFDM transmitter, as shown in Fig. 2, top. The PAPR may cover an area of several hundred kilometers in diameter. can be further reduced by root raised cosine (RRC) filtering Depending on the satellite elevation and user location, distance in front of the carrier mapping at the cost of more bandwidth. variations may be longer than 30 km. Therefore, we have This scheme emulates classical single-carrier waveforms by introduced a constant offset in the timing advance parameter using OFDM [3]. depending on the geographical location by which the up-link Waveforms of multiple users are transmitted in parallel sub- signals are shifted with respect to the down-link. bands, multiplexed over the air and received with a common OFDM receiver, as depicted in Fig. 2, bottom. The received C. Frequency Advance signals are jointly equalized in the frequency domain with the A second essential requirement of multiuser detection based individual set of channel coefficients of each terminal. on SC-FDMA is that users have very small differences in their

239 0 Table 1: Phase noise in simulations (in dBc) 10 with phase noise Frequency DVB-RCS BUC LNB without phase noise -1 100 Hz -54 -60 -70 10 1 kHz -64 -70 -80 -2 10 kHz -74 -80 -90 10 100 kHz -89 -90 -100 -3 >1 MHz -106 -100 -110 10 BER

-4 QPSK 16-QAM 64-QAM 10

-5 carrier frequencies. How small they have to be depends on the 10 symbol duration in the underlying OFDM system. The OFDM signal structure requires that the samples in the CP are in fact a -6 10 0 5 10 15 20 25 cyclic repetition of the original ones. Hence, the carrier phase SNR [dB] may not change significantly during the entire OFDM symbol duration. Otherwise, noise-like inter-carrier interference (ICI) Figure 4. Uncoded bit error rates of the SC-FDMA up-link chain including occurs in the inner OFDM link of the SC-FDMA air interface. the effect of the phase noise. Frequency advance is a proprietary mechanism commonly used at the terminal side. It enables the use of multiuser multiplexing in the frequency domain despite the scattering cess technique in the return channel. But common DVB-S2 of local oscillator frequencies. receivers may not be able to measure the frequency offset Assume that the frequency offset can be estimated precisely precise enough in the forward channel for applying frequency in the forward channel. Therefore, recursive signal processing advance that is useful for LTE in the return channel. is typically used to achieve superior synchronization perfor- DVB-S2 uses serial modulation instead of OFDM. Con- mance, see [9]. Beside coarse acquisition and compensation in sidering literature results, it appears questionable if the high the time domain, terminals estimate the phase shift between precision of frequency estimation needed for LTE waveforms consecutive pilot signals and derive the residual offset, accord- in the return channel can be reached using DVB-S2 as a ingly. This is denoted as fine synchronization. The residual forward channel. In [19], despite reduction of the loop band- offset is fed back from frequency- into time-domain signal width, the residual CFO is orders of magnitude higher than processing where it is added to the coarse estimate and the typical for LTE. The major reason is that LTE introduces a frequency compensation is updated. After a few recursion huge number of reference signals for estimating the frequency- steps, the expectation value of the offset converges to zero, selective characteristics of the terrestrial radio channel which i.e. the corresponding ICI is minimized. are also useful for excellent fine synchronization [18]. By implementing recursive synchronization based on the In summary, enabling orthogonal multiuser multiplex using reference signals available in the LTE standard, the frequency SC-FDMA in the return channel requires a suitable frequency offset can be estimated with an incredible precision of a few advance scheme. This implies that a sufficient number of pilots Hz only even at low SNR and at GHz carrier frequencies [18]. is available in the forward channel for precise frequency offset Base stations and terminals typically use the same local ref- measurements. erence clock for forward and reverse channel. The frequency offsets in both link directions are therefore strictly related to D. Phase Noise each other. The only difference comes from factors by which Phase noise implies random changes of the carrier phase the local oscillator frequencies are locked to the common on various time scales. The low-frequency components of the reference frequency [11]. phase noise (below 1 kHz) are compensated by frequency Thus, terminals can estimate the frequency offset in the advance and since channel estimation and equalization are forward channel and compensate it individually in the return updated each 1 ms in LTE. However, the high frequency com- channel in advance to transmitting their waveforms. Using this ponents cannot be compensated by the air interface and they simple mechanism, signals of multiple users have almost the remain as additional noise degrading the receiver performance. same carrier frequency at the BS. In order to quantify the effects of phase noise, we have Frequency advance has been implemented in our LTE used a link level simulator according to the LTE Release prototype, using a fully recursive carrier frequency offset 8 specification. We have introduced colored phase noise as (CFO) estimation in the forward channel and corresponding indicated in Fig. 2 using a random number generator. The compensation in the reverse channel. output is passed through a filter emulating the phase noise of Frequency advance can be applied to satellite links if the all local oscillators in the reverse channel over satellite. offset estimation is precise enough. It is frequently argued Most of the phase noise comes from the terminal, in partic- that DVB-S2 might be combined with a new multiple ac- ular from the low-cost block up-converter (BUC) used for up-

240 As typical for satellite, multiple frequency conversions are needed. We have operated the return channel over a Ku- band satellite at 14 GHz. The satellite transponder acts as a frequency converter to the feeder link operating at 12 GHz. The feeder link signal is sent back to earth where it is received with the same parabolic dish antenna with 2.4 m diameter also used for the transmitter on the rooftop of the HHI building (photograph in Fig. 5). Experiments were conducted in Berlin, Germany, using the Express AM44 satellite at 11◦ West. One Ku-band transponder having a bandwidth of 54 MHz and 150 W power was used. laboratory 12GHz 14GHz Note that the spot focus is over Italy, and signal power is 3 dB 2.6 GHz L-Band L-Band off at our experimental site, as illustrated in Fig. 5. 30dB LTE-BS LTE-MT 70MHz L-Band B. Transmitter and Receiver 70MHz L-Band roof A commercial L-Band transponder was used as a first conversion stage. It has a wide-band IF interface centered at Figure 5. Overall setup in our LTE trials over satellite. The EIRP of 70 MHz with 70 MHz bandwidth. We have directly connected the satellite is 53 dBW (around Rom/Italy in the footprint) and every line the 70.8 MHz IF signal of our LTE waveform and converted corresponds to a further decrease of 1 dB. it to a second IF in the L-Band. Actual frequencies depend on the granted space segment, e.g. we have often used a second IF at 1,131.75 MHz. The signal was then sent from our setup conversion from L-Band to mm-wave frequencies. We assume in the lab to the dish antenna on the rooftop over a 50 m long that the satellite head end is part of the network and uses a coax cable. A low-cost BUC is used converting the entire L- low-noise block downconverter (LNB) with negligible phase band into the Ku-band, e.g. at 14,181.75 MHz according to noise. Simulation parameters are similar as in the experiments the IF frequency. described below and listed in Table 1. Simulation results for In the receive branch at the satellite, the transponder con- the uncoded bit error rate (BER) are shown in Fig. 4. verted the signal into a feeder link signal at 11,631.75 MHz. Based on the minor penalty observed for QPSK and 16- The feeder link signal was sent toward the earth and was QAM, we expect that standard LTE up-link waveforms ex- received at the HHI using a NJR2637E LNB with external perience minor distortion due to phase noise in typical satel- reference down-converting the signal to the second IF in the lite terminal equipments. Only with higher-order modulation L-Band. formats, like 64-QAM, we expect a noticeable performance While the granted space segment was free of interference, degradation. close to our transmission band there were several commercial III.EXPERIMENTAL SYSTEM signals received significantly stronger than our experimental waveform. The second IF signal was transmitted through a A. Experimental Satellite Link second 50 m long coax cable from the rooftop to the lab Our experimental satellite link is shown in Fig. 5. We where it is down-converted from the L-Band to the 70 MHz have used a 2x2 MIMO LTE link over the air at 2.65 GHz IF-frequency which can be fed directly into the LTE-BS. Since in 20 MHz bandwidth in our lab to emulate the forward we experienced unexpected signal distortion in the L-band channel since LTE terminals do not start transmitting in the converter, down-conversion to our 70.8 MHz IF was set up return channel without receiving the physical downlink control using a discrete MITEX DM0208LW2 mixer, R&S SMIQ channel (PDCCH) and getting a radio resource granted. signal generator and bandpass filter. Finally, the signal was Our LTE signal processing chain has been used with minor fed into the digital RF board of our LTE trial system, where modifications and one out of two bandwidths have been as- it is digitally down-converted to baseband IQ samples. signed to the terminal, 2.16 and 1.08 MHz. Flexible bandwidth adaptation is practical to increase the power spectral density C. Synchronization of the Experimental Setup to a level needed for the detection of higher-order quadrature Note that it was critical for our experiments to use an LNB amplitude modulation (QAM) symbols at the receiver side. with external reference and to phase-lock all the frequency The SC-FDMA waveform is delivered as quantized I and Q conversions, except the one at the satellite, to the same samples (12 bits each) over a parallel low voltage differential 10 MHz reference clock. While we have used the terrestrial signaling (LVDS) interface to the digital radio frequency 2.65 GHz carrier frequency, e.g. to provide essential control (RF) board normally used in our terrestrial front-ends. It information over the forward channel for proper terminal performs digital base-band filtering, digital up-conversion onto operation, several frequency conversions are made in the return a 70.8 MHz intermediate frequency (IF) and digital-to-analog channel over satellite. If these conversions are unlocked, the conversion (DAC). strict relationship between the carrier frequencies in forward

241 and return channels assumed at the LTE terminal is broken. D. Realtime Signal Processing Frequency advance is then no longer efficient. Our real-time LTE trial system is described in detail in [15]. Therefore we have synchronized all oscillators to the same It was used to test several new features before they have been reference clock thus forcing the mean frequency offset to zero included into the LTE standard. Signal processing is based except the minor offset potentially introduced at the satellite on a modular design. In order to manage the simultaneous transponder. This contribution has been minimized manually requirements for high bandwidth and real time adaptation to at the L-Band down-converter. Residual offsets as well as low- the time-variant mobile radio channel, we have implemented frequency phase noise is compensated using the phase tracking it on a hybrid hardware-software platform. mechanism at the base station receiver. All consecutive building blocks in the data link gener- In a real deployment, the coarse timing advance offset can ating and processing transmitted and received waveforms, be computed during the link setup, depending on satellite respectively, are organized in a hardware pipeline whereas elevation and geographical location. In our experiment, we the outputs of one block are the inputs of the next block. have set the coarse offset manually using a logic analyzer. All logic is programmed in very high speed integrated cir- Apart from adding this offset, of course, the fine timing error is cuit hardware description language (VHDL) and implemented regularly estimated at the base station and corrected properly at on XILINX FPGAs. The pipeline includes an interface to the terminal side via the PDCCH regularly transmitted over the Ethernet, queue processing, framing, channel coding, QAM LTE forward channel each 10 ms, realized over the terrestrial constellation mapping, DFT, carrier mapping, IFFT at the forward link in our experimental setup. transmitter as well as synchronization, FFT, coarse channel estimation, equalization, IDFT, constellation demapping, soft Note that radio frames in the forward and reverse channel decoding, deframing and interface to Ethernet. The hybrid may be shifted significantly against each other due to the automatic repeat request (HARQ) technique has been disabled huge propagation delay over the satellite, compared to the due to the long propagation delay over satellite. terrestrial LTE link. However, our LTE system expects the timing advance preamble in the return channel in a well Complex control functions have been implemented aside defined time slot in each LTE radio frame. In the received the waveform processing pipeline on Texas Instruments 6713 signal, we have ignored delays of integer multiples of the floating point DSPs. In this way, we avoid fixed point issues radio frame duration and adjusted the coarse TA offset at in these algorithms and enable fast algorithm implementation first manually inside the 10 ms radio frame by using a and testing. The interface between hardware and software is flexible FIFO memory in the baseband processing. As a result, based on the external memory interface (EMIF), i.e. the DSP the fractional delay between forward and return channel is reads from and writes into dedicated memory cells inside the approximately the same as in the terrestrial link. FPGA in the same way as using his own memory cells. Functions realized in software are channel interpolation The automatic fine tracking mechanism for the timing and noise reduction in the frequency domain, computation advance has been left fully active in our LTE prototype during of MIMO equalizer weights, obtaining SINR values from all experiments over satellite. This is very useful also in a received pilots, generation and use of feedback information stationary setup since it corrects not only the occasional long- in the multiuser scheduling algorithm producing the resource term fluctuations of the satellite-to-ground distance but also allocation map. Note that the adaptive medium access control the typical wander effect due to the independent time reference (MAC) layer functions in LTE have not been used in our clocks at the transmitter and receiver. Wander is certainly the experiments, since we have worked with fixed rates. major effect and it is also present in the terrestrial LTE air However, all functions needed for bidirectional real-time interface. data transmission have been implemented. We have shown The wander is corrected properly by the automatic tracking in a live demo in the project that typical Internet services, in our prototype. Measurements and corrections are made like Google, email and YouTube worked properly but with the regularly each 10 ms. We have found no indication that the expected additional delays over the experimental satellite link additional delay due to the satellite link has a significant influ- when the forward channel used the terrestrial and the return ence onto the timing advance loop. The wander accumulates channel the satellite propagation path. to several sample clock intervals after a few seconds where each sample clock interval has a duration of 32.55 ns. If the IV. RESULTS offset reaches a certain number of sample intervals, our timing advance loop performs a correction by an integer number of A. Error Vector Magnitude samples so that the error is tightly forced around zero. Even after fine tracking, of course, there is a small residual error. In Fig. 6, we have plotted received constellation diagrams It is easily corrected by the frequency-domain equalization in after transmission over satellite. As it can be observed, the the inner OFDM part of the SC-FDMA link, since the residual error vector magnitude (EVM) is small enough to support all error is much smaller than the cyclic prefix length of 4.7 µs modulation schemes defined in the LTE Release 8 standard (144 samples). also over a geostationary satellite.

242 0 10 BPSK; 1.08MHz; uncoded BPSK; 1.08MHz; coded BPSK; 2.16MHz; uncoded -1 10 BPSK; 2.16MHz; coded

-2 10

-3 10 BER

-4 10

-5 Figure 6. Reconstructed constellation diagrams after transmission over the 10 geostationary Ku-band satellite. Colored bars in the lower left graph show -6 which part of the 20 MHz spectrum and which modulation scheme is used. 10 -45 -40 -35 -30 -25 -20 -15 P [dBm] P =P =3W BUC-in BUC-out BUC-1dB

0 10 QPSK; 1.08MHz; uncoded B. Bit Error Rates QPSK; 1.08MHz; coded QPSK; 2.16MHz; uncoded -1 We have systematically recorded uncoded and coded BERs 10 QPSK; 2.16MHz; coded

-2 as a function of the input power at the BUC. Results for 10 different modulation formats are shown in Fig. 7. Note that -3 10

our trial system uses a convolutional code with rate 1/2 and BER

-4 soft decoding due to limited hardware resources unlike the 10 turbo code with variable rate used in LTE. Uncoded BER is -5 measured by re-encoding decoded bits at the receiver side 10 and comparing them with the hard bits delivered at the -6 10 -45 -40 -35 -30 -25 -20 -15 demodulator output. Due to error propagation, clearly, this P [dBm] P =P =3W BUC-in BUC-out BUC-1dB method is not accurate at low signal to noise ratio (SNR). But 1 10 it works very well at high SNR and also using application data 16-QAM; 1.08MHz; uncoded 16-QAM; 1.08MHz; coded 16-QAM; 2.16MHz; uncoded 0 instead of pseudo-random bit sequences (PRBSs). 10 16-QAM; 2.16MHz; coded Coded as well as uncoded BER decreased monotonically -1 at ﬁrst when increasing the BUC input power. Doubling the 10 signal bandwidth needs double power to reach a similar BER -2 10 since the power spectral density is reduced. But already for BER -3 QPSK, this holds only if the BUC input power is small. At 10 the right hand side of all graphs, it can be observed that -4 10 above a certain power level, the BER is increasing again with

increasing power. This is related to the 1 dB compression -45 -40 -35 -30 -25 -20 -15 P [dBm] P =P =3W point of the BUC also indicated in Fig. 7. Around this BUC-in BUC-out BUC-1dB point, nonlinear distortions of the waveform can no longer be corrected by the forward error correction (FEC). Figure 7. Measured bit error rates using LTE in the reverse channel over These nonlinear distortions have more impact onto higher satellite with two bandwidths. Top: BPSK. Center: QPSK. Bottom: 16-QAM. order modulation formats, such as 16-QAM. The uncoded BER reaches a minimum before it increases again. Only after FEC, error-free data transmission is possible. For the same a recently reported value for DVB-RCS2 [20], probably due limitations of our low-cost BUC, 64-QAM symbols could not to rectangular instead of raised cosine filtering. be detected without errors. We assume that using a better BUC with lower non-linear distortion and less phase noise has the D. Discussion potential to increase the spectral efficiency further. Using SC-FDMA, user signals can be multiplexed without guard bands and guard intervals in frequency and time C. Spectral Efficiency domains, respectively. Note that the overhead due the CP Let us finally estimate the spectral efficiency reached with in SC-FDMA is already included in the single-user spectral LTE in our return channel experiments over satellite. In one efficiency reported above. transmission time interval (TTI) and 1.08 MHz bandwidth, we In the single-user case, the spectral efficiency gains com- use 6 · 12 = 72 sub-carriers times 12 OFDM symbols yielding pared to MF-TDMA might be considered small. However, 864 resource elements per 1 ms. Using 16-QAM, we transmit there are additional gains due to the highly flexible orthogonal 4 bits per resource element. This yields a raw data rate of access of multiple users coming on top of these single- 3.456 Mbit/s. At an uncoded BER of 0.01, we have realized a user values. Modern air interfaces for the return channel so-called good-put of 0.99 · 3.456Mbit/s, corresponding to a use multiple frequencies for multiple users, instead of time- spectral efficiency of 3.2 bit/s/Hz. This is 10% higher than division multiple access (TDMA).

243 The frequency-division multiple access (FDMA) approach on satellite elevation and geographical location. The propri- enables a fundamental multiuser gain in the return channel etary frequency advance scheme at the terminal side can be illustrated in Fig. ??. Assume that a single user can reach the applied if all frequency conversions in the satellite chain, TDMA spectral efficiency C = B · log2(1 + SNR). Using except the transponder at the satellite, are locked to the same FDMA, N users share the bandwidth B. They limit their local reference clock. For the frequency synchronization in the signals in individual sub-bands B/N where they can reach return channel, we need precise frequency offset measurements a higher signal-to-noise ratio N · SNR. in the forward channel. Summing up the user capacities shows that the multiuser In our experiments, we have reused the entire LTE up-link performance in the return channel (RC) when using FDMA physical and MAC layer processing chain by using a 70 MHz instead of TDMA is increased as if the SNR is multiplied by IF as an interface to standard low-cost satellite equipment. factor N. Clearly, this gain comes at the price of energy at the All modulation schemes defined in the LTE Release 8 up-link terminal side. Transmission is burst-like with TDMA and the standard have been successfully transmitted over a Ku-band transmitter is switched off most of the time. But when using satellite. In the single-user case, we have reached a spectral FDMA, energy is used continuously and thus a higher sum efficiency of 3.2 bit/s/Hz. This value can be further increased rate can be achieved. if multiple users transmit in parallel in adjacent frequency sub- One could argue that similar gains are feasible both with bands. MF-TDMA and SC-FDMA, since the principal difference is that no guard bands are used by SC-FDMA. LTE enables a ACKNOWLEDGEMENTS higher flexibility of the resource allocation in the frequency The German Ministry of Economics (BMWi) is acknowl- domain, e.g. up to 100 users can be scheduled at the same edged for financial support in the ZIM cooperation project time in 20 MHz bandwidth. Even though the huge theoretical SatUplink under contract number 2110305ED9. Thanks to SNR gain of 20 dB with 100 users in this example may not J. Hawwary and D. Mihok (HETAN Technologies GmbH) for be realized in practice, e.g. due to phase noise, in general, the fruitful cooperation and providing the dish antenna and satel- more users are multiplexed in a given bandwidth, the more lite communications hardware for our experiments. Thanks the spectral efficiency can be increased. A high granularity of also to N. Chuberre (Thales Alenia Space) and the members resource allocation in the frequency domain pays out in a high of the Satellite and Communication and Navigation (SCN) multiuser system throughput. Moreover, by means of flexible working group at ETSI for stimulating discussions. bandwidth assignment, the system adapts easily to variable traffic patterns and several equipment classes having different REFERENCES transmitter power and satellite dish diameters can be supported [1] H. Skinnemoen, R. Leirvik, J. Hetland, H. Fanebust, and V. Paxal, “Interactive IP-Network via Satellite DVB-RCS,” IEEE Journal on optimally. Selected Areas in Communications, vol. 22, no. 3, pp. 508 – 517, April As mentioned earlier, the coverage area of modern satellite 2004. air interfaces consists of multiple spots, basically establishing a [2] H. G. Myung, J. Lim, and D. J. Goodman, “Single Carrier FDMA for Uplink Wireless Transmission,” IEEE Vehicular Technology Magazine, cellular network. Mobile satellite networks, like the broadband vol. 1, no. 3, pp. 30–38, September 2006. global area network (BGAN) of INMARSAT use advanced [3] V. Jungnickel, T. Hindelang, T. Haustein, and W. Zirwas, “SC-FDMA mobility functions from the terrestrial 3G air interface, like Waveform Design, Performance, Power Dynamics and Evolution to MIMO,” Proc. IEEE PORTABLE ’07, 2007. handover. [4] X. Zhang, C. He, L. Jiang, and J. Xu, “Inter-cell Interference Coordina- With SC-FDMA in the return channel over satellite, ad- tion Based on Softer Frequency Reuse in OFDMA Cellular Systems,” vanced inter-cell interference management schemes from LTE in Proc. International Conference on Neural Networks and Signal Processing, June 2008, pp. 270 –275. can be reused in the satellite network by exploiting the [5] V. Jungnickel, M. Schellmann, L. Thiele, T. Wirth, T. Haustein, O. Koch, frequency domain for improving the performance. Also the E. Zirwas, and E. Schulz, “Interference Aware Scheduling in the network operation can be simplified if the medium access and Multiuser MIMO-OFDM Downlink,” IEEE Communications Magazine, vol. 47, pp. 56–66, 2009. physical layers are aware of the interference. It remains an [6] M. Karakayali, G. Foschini, and R. Valenzuela, “Network Coordination open issue for further research what gains can be achieved at for Spectrally Efficient Communications in Cellular Systems,” IEEE what price in terms of complexity in next generation satellite Wireless Commun. Mag., vol. 13, pp. 56–61, 2006. [7] R. Irmer, H. Droste, P. Marsch, M. Grieger, G. Fettweis, S. Brueck, H.-P. networks due to the advanced robustness against multi-cell Mayer, L. Thiele, and V. Jungnickel, “Coordinated Multipoint: Concepts, interference achieved in this way. Performance and Field Trial Results,” IEEE Communications Magazine, vol. 49, no. 2, pp. 102 –111, Feb 2011. CONCLUSIONS [8] G. Raleigh and J. Cioffi, “Spatio-temporal Coding for Wireless Com- munication,” IEEE Transactions on Communications, vol. 46, no. 3, pp. In this paper, we have performed LTE trials in the return 357–366, 1998. channel over a geostationary Ku-band satellite for the first [9] M. Speth, S. A. Fechtel, G. Fock, and H. Meyr, “Optimum Receiver Design for Wireless Broad-Band Systems Using OFDM. Part I,” IEEE time. We have highlighted the challenging synchronization Transactions on Communications, vol. 47, no. 4, pp. 1668 – 1677, 1999. requirements therefore and shown that they can be met by [10] ——, “Optimum Receiver Design for Wireless Broad-Band Systems. II. using the existing synchronization mechanisms in LTE. The A Case Study,” IEEE Transactions on Communications, vol. 49, pp. 571 – 578, 2001. standard timing advance protocol has been modified for the [11] V. Jungnickel, M. Schellmann, A. Forck, H. Gäbler, S. Wahls, A. Ib- return channel over satellite by introducing an offset depending ing, K. Manolakis, T. Haustein, W. Zirwas, J. Eichinger, E. Schulz,

244 C. Juchems, F. Luhn, and R. Zavrtak, “Demonstration of Virtual MIMO in the Uplink,” Proc. IET Smart Antennas and Cooperative Communi- cations Seminar, London, 2007. [12] [Online]. Available: https://m3.rd.francetelecom.com/espace- public/deliverables/d21.1-analysis-on-3gpp-e-mbms-dvb-ngh-physical- layer-convergence/view [13] S. Rosati, S. Cioni, A. Vanelli-Coralli, G. Corazza, G. Gallinaro, and A. Ginesi, “A Joint Multi-User Synchronization Method for SC- FMDA in Broadband Satellite Return Channel,” in Proc. IEEE Global Telecommunications Conference, December 2010, pp. 1 –5. [14] F. Bastia, C. Bersani, E. A. Candreva, S. Cioni, G. E. Corazza, M. Neri, C. Palestini, M. Papaleo, S. Rosati, and A. Vanelli-Coralli, “LTE Adaptation for Mobile Broadband Satellite Networks,” EURASIP Journal of Wireless Communication Networks, vol. 2009, pp. 12:7–12:7, March 2009. [Online]. Available: http://dx.doi.org/10.1155/2009/989062 [15] A. Forck, T. Haustein, V. Jungnickel, V. Venkatkumar, S. Wahls, T. Wirth, and E. Schulz, Early Real-Time Experiments and Field- Trial Measurements with 3GPP-LTE Air Interface Implemented on Reconﬁgurable Hardware Platform, ser. 3GPP LTE Handbook: 3GPP LTE Radio and Cellular Technology. CRC Press, 2009, p. 365. [16] C. Ciochina and H. Sari, “A Review of OFDMA and Single-Carrier FDMA,” in Proc. European Wireless Conference (EW), April 2010, pp. 706 –710. [17] S. Jaeckel and V. Jungnickel, “On the Optimality of Frequency-Domain Equalization in DFT-Spread MIMO-OFDM Systems,” Proc. IEEE Wire- less Communications and Networking Conference (WCNC), pp. 1172– 1177, 2008. [18] K. Manolakis, D. Gutierrez Estevez, V. Jungnickel, W. Xu, and C. Drewes, “A Closed Concept for Synchronization and Cell Search in 3GPP LTE Systems,” in Proc. IEEE Wireless Communications and Networking Conference (WCNC), April 2009. [19] J. G. Oh and J. T. Kim, “An Alternative Carrier Frequency Synchro- nization Scheme for DVB-S2 Systems,” in Proc. 12th International Conference on Advanced Communication Technology (ICACT), vol. 1, Feb. 2010, pp. 529 –533. [20] ESA, “Test of RCS2 Standard Suc- cessful,” June 2011. [Online]. Available: http://telecom.esa.int/telecom/www/object/index.cfm?fobjectid=31115

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