Lecture 4 October 10, Wireless Access. Graduate course in Communications Engineering. University of Rome La Sapienza. Rome, Italy

Transcription

1 Lecture 4 October 10, 2018 Wireless Access Graduate course in Communications Engineering University of Rome La Sapienza Rome, Italy

2 Inter-system Interference

3 Outline Inter-system interference Interference in the ISM bands LTE and OFDM Coexistence of LTE-U and Wi-Fi The Bluetooth standard Interference of onto Bluetooth

4 MHz (26 MHz) GHz (83.5 MHz) Inter-System Interference: the ISM Bands The FCC defined three bands for unlicensed systems for Industrial, Scientific and Medical (ISM) applications The higher the frequency, the larger the available band: low frequencies are crowded GHz (125 MHz) The latter band is also known in the US as Unlicensed National Information Infrastructure (UNII) band The ITU followed the FCC rule, so that these bands, with minor differences, are now available almost everywhere in the world Table shows values for the intermediate ISM band (also called WiFi band)

5 FCC Emission rules in the ISM Bands The ISM bands can be used without any license fee by two classes of devices: Narrowband, with Effective Isotropic Radiated Power EIRP 0.7 mw Spread spectrum, with Effective Isotropic Radiated Power EIRP 1 W According to the FCC rule, a device can be considered as spread spectrum if it guarantees a processing gain G > 10 db reminder: the processing gain is defined as the ratio between the total occupied bandwidth for the RF signal and data rate

6 Devices in the ISM Bands Devices in the MHz ISM band: Proprietary wireless LANs (SS) Cordless Phones (mainly NB) Cordless Headphones (NB) Surveillance systems (NB) Devices in the GHz ISM band: IEEE LAN standards (SS) Audio/video signal repeaters (NB) Remote Garage openers (NB) Devices in the GHz ISM band: Reserved for high bit rate networking devices (no cordless phones) IEEE/ETSI LAN standards (SS)

7 WLAN standards in the ISM band: the IEEE reference standards Protocol Author Frequency Modulation Data Rate Comments IEEE 900 MHz ISM FHSS ~ 300 kbps Original standard of the series a IEEE 5 GHz UNII OFDM up to 54 Mbps b IEEE 2.4 GHz ISM, 900 MHz legacy DSSS FHSS 1 to 11 Mbps e IEEE 5 GHz UNII OFDM up to 54 Mbps g IEEE 2.4 GHz ISM DSSS FHSS up to 54 Mbps n IEEE 5 GHz UNII OFDM up to 150 Mbps ac IEEE 5 GHz UNII OFDM 500 Mbps and beyond j IEEE, ETSI Bluetooth See next slide 5 GHz UNII OFDM, GMSK 54 Mbps and beyond Same MAC as Most popular standard (WiFi) Adds QoS capability to a Backward compatible with b 40 MHz RF bandwidth MIMO processing and beamforming RF bandwidth: 160 MHz, 8 MIMO, 1024-QAM Convergence of a and HiperLAN standards (see next slide)

8 WLAN standards in the ISM band: ETSI and proprietary Protocol Author Frequency Modulation HiperLAN ETSI GHz or GHz HiperLAN/2 ETSI GHz or GHz HomeRF Bluetooth HomeRF Industry group Bluetooth Consortium Now: IEEE Data Rate GMSK Mbps Comments European Community standard GMSK 54 Mbps European Community standard 2.4 GHz FHSS Up to 10 Mbps Proprietary Integrated voice, data and entertainment for home networking Physical layer: MAC: merge of (data) and DECT (speech) 2.4 GHz FHSS 1 Mbps Started as proprietary, now IEEE Cable replacement, not comparable to or HiperLAN Additional WPAN standards, such as IEEE and IEEE operate in the ISM bands

9 Interference in the ISM Bands Since the ISM bands are open to any device compliant to the FCC emission rules inter-system interference is a serious issue Most of the standards foresee the organization of devices in independent subnetworks (e.g. Base Service Sets in , Piconets in Bluetooth) and do not provide means for managing interference between different subnetworks (intra-system interference) No interoperability is guaranteed between different standards using same frequencies and provoking thus mutual interference (inter-system interference)

10 LTE Long Term Evolution LTE is a highly flexible radio interface defined by the 3 rd Generation Partnership Project (3GPP) LTE provides peak rates of 300 Mb/s and a radio network delay of less than 5 ms LTE operates in licensed bands, mostly around 2 GHz, but bands were defined also around 450, 700, 850 MHz and up to 3800 MHz In Europe: 800, 1800, 2600 MHz LTE incorporates several advanced features, such as Multiple Input Multiple Output (MIMO) processing, flexible bandwidth between 1.4 MHz and 20 MHz Most importantly, LTE adopts Orthogonal Frequency Division Multiplexing (OFDM) as all most recent wireless standards do LTE recently evolved to LTE Advanced, or LTE-A, providing higher data rates and new features

11 OFDM An OFDM-modulated signal consists of the parallel transmission of several signals that are modulated at different carrier frequencies f m. These carriers are equally spaced by Δf in the frequency domain. X(f) Δf frequency Analytical expression of the complex envelope of a single OFDM symbol x N ( ) ( ) t = gt t m= 1 0 c m e j2πf m t

12 original binary stream d 0 d 1 d 2 d 3 groups of K bits block of N symbols block of N c 0 c 1 c 2 c complex 3 points in the constellation Reference architecture of an OFDM transmitter f m N 1 = mδf g( t ) = with t [ TG,T 0 ] 2 TG + T0 T = 1 0 Δf ( 2 π( f + f ) t) g( t )cos p 3 ( 2 π( f + f ) t) g( t )sin p 3 ( 2 π( f + f ) t) g( t )cos p 2 TG Guard Time c m = a m + jb m a 3 b 3 a 2 b 2 a 1 b 1 a 0 b 0 ( 2 π( f + f ) t) g( t )sin p 2 ( 2 π( f + f ) t) g( t )cos p 1 ( 2 π( f + f ) t) ( 2 π( f + f ) t) g( t )sin p 1 g( t )cos p 0 ( 2 π( f + f ) t) g( t )sin p 0 S OFDM symbol including cyclic prefix

13 original binary stream groups of K bits d 0 d 1 d 2 d 3 block of N symbols block of N c 0 c 1 c 2 c complex 3 points in the constellation All digital implementation of an OFDM transmitter IDFT C n = N-1 m= 0 c m e 2πmn j N OFDM symbol ready for RF transmission C 0 C 1 C 2 C 3 C 2 C 3 C 0 C 1 C 2 C 3 Cyclic prefix ( ) x = 1 T 1 n C n x 2 x 3 x 0 x 1 x 2 x 3 n

14 PSD of OFDM signals (1/2) The PSD of an OFDM signal can be found by adding up the PSDs of individual sub-carriers for a generic OFDM symbol. x ( ) t = rect T m= t N 1 0 c m e j2πf m t Complex envelope of an OFDM symbol P f m ( f ) π = sin c ( f f ) Δf m Spectrum centered on the m-th subcarrier P N 1 N 1 ( ) 2 f = σ ( ) c Pf f m= 0 m m= 0 Spectrum of an OFDM symbol 2 σc m is the variance of the complex term c m m

15 PSD of OFDM signals (2/2) Example: Power Spectral Density (in logarithmic units) of a MB-OFDM signal compliant with the UWB signal format adopted in the WiMedia industrial standard The OFDM signal is composed of 128 sub-carriers equally spaced by MHz, and located around a central frequency f c = GHz Power Spectral Density [V 2 /Hz] Frequency [GHz]

16 LTE Unlicensed LTE Unlicensed (LTE-U) is an extension of LTE Release 12 that adds the capability to operate in the unlicensed UNII/ISM band at 5 GHz LTE-U is a precursor of the extension License Assisted Access to Unlicensed spectrum in LTE, approved in Release 13 Originally proposed by Qualcomm, it was later supported by the LTE-U Forum LTE-U will be provided by Small Cells, that is small Base Stations to be deployed in homes, taking advantage of the Carrier Aggregation feature of LTE-A LTE-U promises very high speeds thanks to the large available bandwidth (over 700 MHz) in the 5 GHz band Control channels are however deployed in licensed bands, restricting the use of LTE-U to operators

17 LTE-U and Wi-Fi The deployment of LTE-U will create new coexistence issues The 5 GHz band is currently used mainly by Wi-Fi in its latest flavors, that is n and ac The Wi-Fi Alliance and major internet content providers (e.g. Google) strongly opposed LTE-U due to its potential impact on Wi-Fi performance So far only few studies are available on the coexistence between LTE-U and Wi-Fi, due to limited availability of LTE-U hardware Early studies, both based on simulations and experiments, indicate however that LTE-U might have a strong impact on Wi-Fi throughput unless precautions are taken in order to protect Wi-Fi

18 LTE-U and Wi-Fi (Wi-Fi) F. M. Abinader, Jr., E. P. L. Almeida, F. S. Chaves, A. M. Cavalcante, R. D. Vieira, R. C. D. Paiva, A. M. Sobrinho, S. Choudhury, E. Tuomaala, K. Doppler and V. A. Sousa, Jr. Enabling the Coexistence of LTE and Wi-Fi in Unlicensed Bands, IEEE Communications Magazine, November 2014, pp This is due to the fact that LTE was originally designed for licensed bands, and does not implement channel sensing before transmitting Wi-Fi devices on the other hand perform channel sensing, and in case LTE devices are already transmitting defer their transmission

19 LTE-U and Wi-Fi The LTE-U Forum suggested several coexistence techniques that could mitigate the impact of LTE-U on Wi-Fi: Channel selection: LTE-U selects channels not used by Wi-Fi Access Points Carrier-Sensing Adaptive Transmission (CSAT): LTE-U adopts adaptive or static Time Division Multiplexing based on long term channel sensing, thus reducing its duty cycle Opportunistic Secondary Cell Switch Off: LTE-U switches off small cells when not needed by LTE devices due to low traffic

20 BLUETOOTH Standard IEEE

21 Context of application Bluetooth is used in the context of Wireless Personal Area Networks (WPANs) WPAN: Small distances (~ 10 m) (in real from 1 to 100 m depending upon transmitted power) Small number of nodes Low power levels

22 General features ISM band: ,5 MHz Frequency Hopping Spread Spectrum over 79 frequencies Bit rate: 1 Mb/s (in the 2005 standard, later increased to 3 Mb/s with enhanced data rate) Full-duplex TDD connection Data packet transmission

23 General features Piconet: 1 master, up to 7 slaves Links are established between the master and slaves, never between two slaves Scatternet: set of connected piconets that, however, are not synchronized (each piconet is independent) Each device can belong to more than one piconet, but can only be a master in one piconet

24 Frequencies 79 RF of 1 MHz each Central frequency f = k MHz, k = 0,, 78 Lower guard band: 1,5 MHz Upper guard band: 3 MHz GFSK modulation

25 Power levels Class 1 device: P MAX = 100 mw = 20 dbm (100m) Class 2 device : P MAX = 2,5 mw = 4 dbm (10m) Class 3 device : P MAX = 1 mw = 0 dbm (1m) Receiver sensitivity: 70 dbm

26 Packet structure

27 Time slot

28 Time slot Each slot occupies 625 µs (two clock cycles) Master and slave transmit in turn Packets start at the beginning of the time slot and can last for a maximum of 5 slots (1, 3 or 5) The master can start transmission only in even time slots The slave can start transmission only in odd time slots

29 Time slot After transmission of a packet a return packet is expected at the beginning of the next time slot Frequency is fixed for the whole duration of a packet (no hop) The slave replies on the same frequency used by the master

30 Interference of onto Bluetooth

31 Effect of on Bluetooth Example A: (FH-SS) and Bluetooth (FH-SS) M: Number of interfering Access Points Aggregate Bluetooth throughput (Mbps) Number of Bluetooth Piconets Bluetooth only The presence of FH-SS Access Points causes a graceful degradation of performance Drawn from: W. Feng, A. Nallanathan and H.K. Garg, Impact of interference on performance of Bluetooth Piconet in 2.4GHz ISM band, IEEE Electronics Letters, Volume 38, Issue 25, Dec. 2002,

32 Effect of on Bluetooth Example B: (DS-SS) and Bluetooth (FH-SS) M: Number of interfering Access Points Aggregate Bluetooth throughput (Mbps) Number of Bluetooth Piconets Bluetooth only The presence of even one DS-SS Access Point has a dramatic impact on Bluetooth performance Drawn from: W. Feng, A. Nallanathan and H.K. Garg, Impact of interference on performance of Bluetooth Piconet in 2.4GHz ISM band, IEEE Electronics Letters, Volume 38, Issue 25, Dec. 2002,

33 Cognitive Radio Automatic network recognition What is Cognitive Radio? A Cognitive Radio is a radio frequency transmitter/receiver that is designed to intelligently detect whether a particular segment of the radio spectrum is currently in use, and to jump into (and out of, if necessary) the temporarily-unused spectrum very rapidly, without interfering with the transmission of other authorized users A Cognitive Radio is self-aware, user-aware, RF-aware, and that incorporates elements of language technology and machine vision Ref: J. Mitola et al., Cognitive radio: Making software radios more personal, IEEE Pers. Commun., vol. 6, no. 4, Aug. 1999

34 Cognitive Radio The concept of a Radio capable of adapting to the environment and of adjusting transmission parameters according to internal and external unpredictable events is very appealing in the wireless world The final goal remains to form wireless networks that cooperatively coexist with other wireless networks and devices