Wireless LANs PDF

Wireless LANs References C. Beard, W. Stallings, Wireless Communication Networks and Systems, Pearson, 2016. M. Gast, 802.11 Wireless Networks: The Definitive Guide, O'Reilly Media, 2nd Edition, 2005. K. Raghunandan, Introduction to Wireless Communications and Networks- A Practical Perspective, S...

Wireless LANs References C. Beard, W. Stallings, Wireless Communication Networks and Systems, Pearson, 2016. M. Gast, 802.11 Wireless Networks: The Definitive Guide, O'Reilly Media, 2nd Edition, 2005. K. Raghunandan, Introduction to Wireless Communications and Networks- A Practical Perspective, Springer, 2022. P. Roshan, J. Leary, 802.11 Wireless LAN Fundamentals, Cisco Press, 1st Edition, 2004. J. F. Kurose and K. W. Ross, Computer Networking: A Top Down Approach, Pearson, 8 th edition, 2020. L. L. Peterson and B. S. Davie, Computer Networks: A Systems Approach, Morgan Kaufmann, 6th edition, 2021. Lecture notes adapted from Kurose and Ross, Computer Networking: A Top Down Approach, and other sources. Elements of a wireless network wired network infrastructure Elements of a wireless network Cellular Networks (4G/5G) wired network infrastructure WiFi (IEEE 802.11)  wireless hosts § laptop, smartphone, IoT § run applications § may be stationary (non-mobile) or mobile wired network wireless does not always mean mobility! infrastructure  base station § typically connected to wired network § relay - responsible for sending packets between wired network and wireless host(s) wired network in its “area” infrastructure e.g., 802.11 access points (AP), cell towers  wireless link § typically used to connect a wireless-host to a base station (or to another wireless host), also used as a backbone link § Broadcast link § multiple access protocol coordinates link access wired network § Different wireless link technologies have infrastructure different characteristics (transmission rates, distances, etc.)  Characteristics of selected wireless links 100 Gbps IEEE 802.11bn (WiFi 8 (~60m, 2028) Max 46 Gbps IEEE 802.11be (WiFi 7 (~30m (i) ~120m (o), 2.4GHz/5GHz/6GHz (b), 20/40/80/160/320 MHz (c), Dec 2024) data rate 10 Gbps 5G (with mmWave, ~10km) (not 9.6 Gbps IEEE 802.11ax (WiFi 6, ~30m (i) ~120m (o), 2.4GHz/5GHz/6GHz-in-6E (b), 20/40/80/160 MHz (c), 2019) linear scale) 3.5 Gbps IEEE 802.11ac (WiFi 5, ~30m (i) ~120m (o), 5GHz (b), 20/40/80/160 MHz (c), 2013) 1 Gbps 4G LTE (with LTE-advanced, ~10km) 600 Mbps IEEE 802.11n (WiFi 4, ~70m (i) ~250m (o), 2.4GHz/5GHz (b), 20/40 MHz (c), 2009) 568 Mbps (af), 347 Mbps (ah) IEEE 802.11 af, ah (af: unused TV bands 54–790 MHz, ~1km, 2014) (ah: 900MHz, ~1km, 2017) 54 Mbps IEEE 802.11g, a (~35m (i) ~120m (o))(g: 2.4GHz (b), 20MHz (c), 2003)(a: 5GHz (b), 20MHz (c), 1999) 11 Mbps IEEE 802.11b (~35m (i) ~140m (o), 2.4GHz (b), 20MHz (c), 1999) 3 Mbps Bluetooth (~10m for classic, ~50m for 4.0, ~100m for 5.0 and later) Indoor Indoor (i)/Outdoor(o) Midrange Long range Typical Distance outdoor outdoor (not linear scale) 10-30m 35-250m 300m-4Km 4Km-15Km (i- indoor, o-outdoor, b- frequency band, c-channel) Frequency Bands: - WiFi: Typically operates in the unlicensed 2.4 GHz and 5 GHz bands, WiFi 6E extending to the 6 GHz band. - 4G/5G Cellular: Operates in licensed spectrum bands ranging from sub-1 GHz (for broader coverage) to mid- band (1–6 GHz) and millimeter wave (mmWave) above 24 GHz for 5G. Multiple Access Techniques: - WiFi: Uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). WiFi 6 introduces Orthogonal Frequency Division Multiple Access (OFDMA) to improve multi-user efficiency. - 4G: Uses OFDMA for resource allocation. - 5G: Also uses OFDMA, with Massive MIMO. Latency: - WiFi: Latency can vary, typically in the range of 5–50 ms, but can increase in congested environments. - 4G: Latency around 30–50 ms for data transmission. - 5G: Significantly lower latency, with the potential to achieve under 1 ms in ultra-reliable low-latency communications (URLLC) scenarios, though real-world latency is often in the 5–20 ms range. Mobility: - WiFi: Limited support for mobility. As devices move between access points, handoffs are typically slower and less seamless compared to cellular networks. - 4G/5G Cellular: Strong support for mobility. Seamless handoffs are critical for maintaining connections as users move between cells at high speeds, such as in vehicles or trains. Security: - WiFi: Encryption protocols such as WPA3 for security, though WiFi networks are generally more vulnerable to attacks due to the unlicensed spectrum. - 4G: Encrypted by default using IPsec and LTE encryption mechanisms. - 5G: Enhanced security with measures like 5G-AKA (Authentication and Key Agreement) and network slicing, isolating different traffic types for additional security. Quality of Service (QoS): - WiFi: QoS is managed using WiFi Multimedia (WMM), providing prioritization for voice, video, best-effort, and background traffic. - 4G/5G Cellular: Extensive QoS mechanisms are in place, allowing for different service levels for voice, data, and video streaming. 5G introduces support for ultra-reliable low-latency communications (URLLC) and enhanced mobile broadband (eMBB). Topology: - WiFi: Typically star topology where the access point acts as the central node. - 4G/5G Cellular: Hierarchical topology with macro cells, small cells, and coordination between various base stations (eNodeB in 4G, gNodeB in 5G). Infrastructur vs. Ad-hoc infrastructure mode § Hosts associate with a base station § base station connects hosts into wired network wired network § Hosts associated with a base station are infrastructure often referred to as operating in infrastructure mode, since all traditional network services (e.g., address assignment and routing) are provided by the network to which a host is connected via the base station. § handoff or handover: mobile changes base station providing connection into wired network ad hoc mode § no base stations § nodes can only transmit to other nodes within link coverage § In the absence of such infrastructure, the hosts themselves must provide for services such as routing, address assignment, etc. § nodes organize themselves into a network: route among themselves Wireless network taxonomy single hop multiple hops host connects to base station a base station is present that infrastructure (WiFi, cellular) which connects is wired to the larger network. to larger Internet. all However, some wireless nodes may (e.g., APs) communication is between this have to relay their communication base station and a wireless host through other wireless nodes in order over a single wireless hop. to communicate via the base station. e.g., 802.11 networks (typical e.g., wireless sensor networks, WiFi), 4G LTE, 5G wireless mesh networks no base station, no connection no base station, no connection to no to larger Internet. One of the larger Internet. Nodes may have to nodes in this single-hop relay to reach a destination infrastructure network may coordinate the e.g., mobile ad hoc networks transmissions of the other (MANETs), vehicular ad hoc network nodes (VANET) e.g., bluetooth, ad hoc nets  Important differences from wired link …. § Broadcast/shared link => Collision when many devices transmit at the same time (multiple access). § decreased signal strength: radio signal attenuates as it propagates through matter (path loss) § interference from other sources: wireless network frequencies (e.g., 2.4 GHz) shared by many devices (e.g., WiFi, cellular, motors, microwave): interference § multipath propagation*: radio signal reflects off objects ground, arriving at destination at slightly different times. Causes interference and degradation in signal quality. § Hidden terminal problem and fading: make multiple access in a wireless network considerably more complex than in a wired network. * multipath propagation was seen as a significant issue/challenge. However, with the advent of MIMO (Multiple Input Multiple Output) technology, multipath propagation has been transformed from a challenge into an advantage. This shift is because of the advancements in signal processing and the ability to handle multiple data streams concurrently. WiFi standards such as 802.11n (WiFi 4) and onward (WiFi 5, WiFi 6, WiFi 6E) have embraced MIMO technology and evolved it further, incorporating variations like MU-MIMO (Multi-User MIMO). Multiple wireless senders, receivers create additional problems (beyond multiple access): Hidden terminal problem Signal attenuation (fading): § B, A hear each other § B, A hear each other § B, C hear each other § B, C hear each other § A, C can not hear each other means A, § A, C can not hear each other C unaware of their interference at B interfering at B  These issues suggest that bit errors (or bit error rate, BER) will be more common in wireless links than in wired links. Furthermore, higher and time-varying bit error rate (due to reasons such as mobility) is one of the differences in wireless links compared to wired links. …. make communication across (even a point to point) wireless link much more “difficult”. How to address these? Multiple access: need a medium access control (MAC). Errors: Wireless link protocols (such as the 802.11 protocol) employ not only powerful CRC error detection codes, but also link-level reliable-data-transfer protocols that retransmit corrupted frames. Adapt physical layer (modulation, rate, etc.) to address varying BER. § Received wireless signal = original signal degraded + background noise 10-1 10-2 § SNR: signal-to-noise ratio 10-3 larger SNR – easier to extract signal from noise (a “good thing”) 10-4 BER 10-5 § SNR versus BER tradeoffs 10-6 given modulation (physical layer): increase power -> increase SNR-> decrease BER 10-7 10 20 30 40 SNR(dB) given SNR: choose physical layer that meets BER QAM256 (8 Mbps) requirement, giving highest throughput QAM16 (4 Mbps) SNR may change with mobility: dynamically BPSK (1 Mbps) adapt physical layer (modulation technique, rate) IEEE 802.11 Wireless LAN  IEEE 802.11 Wireless LAN (i- indoor, o-outdoor, b- frequency band, c-channel) 100 Gbps IEEE 802.11bn (WiFi 8 (~60m, 2028) Max 46 Gbps IEEE 802.11be (WiFi 7 (~30m (i) ~120m (o), 2.4GHz/5GHz/6GHz (b), 20/40/80/160/320 MHz (c), Dec 2024) data rate 10 Gbps 5G (with mmWave, ~10km) (not 9.6 Gbps IEEE 802.11ax (WiFi 6, ~30m (i) ~120m (o), 2.4GHz/5GHz/6GHz-in-6E (b), 20/40/80/160 MHz (c), 2019) linear scale) 3.5 Gbps IEEE 802.11ac (WiFi 5, ~30m (i) ~120m (o), 5GHz (b), 20/40/80/160 MHz (c), 2013) 1 Gbps 4G LTE (with LTE-advanced, ~10km) 600 Mbps IEEE 802.11n (WiFi 4, ~70m (i) ~250m (o), 2.4GHz/5GHz (b), 20/40 MHz (c), 2009) 568 Mbps (af), 347 Mbps (ah) IEEE 802.11 af, ah (af: unused TV bands 54–790 MHz, ~1km, 2014) (ah: 900MHz, ~1km, 2017) 54 Mbps IEEE 802.11g, a (~35m (i) ~120m (o))(g: 2.4GHz (b), 20MHz (c), 2003)(a: 5GHz (b), 20MHz (c), 1999) 11 Mbps IEEE 802.11b (~35m (i) ~140m (o), 2.4GHz (b), 20MHz (c), 1999) 3 Mbps Bluetooth (~10m for classic, ~50m for 4.0, ~100m for 5.0 and later) Indoor Indoor (i)/Outdoor(o) Midrange Long range Typical Distance outdoor outdoor (not linear scale) 10-30m 35-250m 300m-4Km 4Km-15Km  Almost all Wi-Fi standards use CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) for accessing the wireless medium. But advanced versions like Wi-Fi 6 introduce techniques like OFDMA (Orthogonal Frequency Division Multiple Access) that work alongside CSMA/CA to improve efficiency. Wi-Fi Frame Format: While all Wi-Fi standards are based on the basic 802.11 frame format, there are variations in the frame structure and additional fields introduced in later standards to support new features like higher throughput, multi-user communication, and better efficiency (e.g., in Wi-Fi 5 and Wi-Fi 6). All Wi-Fi standards support both infrastructure mode and ad-hoc mode. (In practice, infrastructure mode is widely supported and used in most Wi-Fi networks today, while ad-hoc mode is less commonly supported or used in newer standards like Wi-Fi 5 (802.11ac) and Wi-Fi 6 (802.11ax)). Link Layer activities (like MAC, framing, error-detection-and-recovery) are more or less the same across all Wi-Fi standards. Physical Layer activities (like modulation, channel width, and antenna technology) have evolved significantly to support faster data rates, better efficiency, and more users. This division reflects how Wi-Fi standards maintain consistent core networking functions while pushing the boundaries of performance through physical layer innovations. 802.11 LAN architecture § wireless host communicates with Internet base station base station = access point (AP) § Basic Service Set (BSS) (aka “cell”) in switch or router infrastructure mode contains: wireless hosts BSS 1 access point (AP): base station ad hoc mode: hosts only BSS 2 802.11: SSID, Channels, association When a network administrator installs an AP*, Sets at least one SSID (Service Set Identifier) for the AP. Service Set Identifier (SSID): WiFi network name (the name you see when you search for available wireless networks on your device) An AP can broadcast one or more SSIDs. The SSID can be visible (broadcasted so that anyone nearby can see it) or hidden (not shown in the list of available networks, but still connectable if the device knows the SSID). AP admin chooses a channel (frequency) for AP spectrum (band) divided into channels at different frequencies. interference possible: channel can be same as that chosen by neighboring AP! * In big enterprises (typical) configuration and management via a centralized wireless controller or cloud platform with many fully automated. 2.4GHz band (802.11 defines 11 partially overlapping channels each with 20 MHz wide + 2 MHz gap as guard band) 2.4GHz band (802.11 defines 11 partially overlapping channels each with 20 MHz wide + 2 MHz gap as guard band) Scalability?: Selection and usage of channels in WiFi Crowded area (conference hall) Spanning bigger area (buildings + outdoors)  (2.4GHz band assumed) 1 2 channel 1 channel 6 4 3 channel 6 channel 11 § Multiple APs placed at different locations § For wide coverage over a large area. § While CSMA/CA can help manage channel access and reduce collisions, using the same channel for nearby APs leads to increased interference, decreased throughput, and degraded performance. § To optimize network performance, it is best practice to use non-overlapping channels for nearby APs § Roaming? Same SSID for all or different ones?  (2.4GHz band assumed) 2 channel 6 1 channel 1 3 channel 11 § Using multiple (e.g., three) APs on different channels at the same location => increases the overall capacity. § Useful in high-density (a large number of users) environments. § Multi-AP configurations with the same SSID are commonly used. § Load balancing?  (2.4GHz band assumed) 1 2 channel 1 channel 6 4 3 channel 6 your house channel 11 § Residential are with houses (with APs) § What channel do you use for your house AP? Extended Service Sets (ESS) 1 2 channel 1 BSS 1 channel 6 BSS 2 4 3 BSS 4 channel 6 channel 11 BSS 3 ESS: multiple Access Points (i.e., multiple BSSs) are interconnected typically via a wired backbone network (e.g., Ethernet switches and routers) to provide seamless wireless coverage over a large area, functioning as a single network (subnet). In an ESS, all APs are configured with the same SSID. Seamless roaming: as users move, their devices automatically switch to the AP with the strongest signal without disconnecting as long as the SSID is the same. More on bands and channels... Single-band vs. dual-band vs. tri-band Single-band (old) vs. dual-band (most widely deployed today, Wi-Fi 4, 5, 6) vs. tri-band (Gaining traction in high-end routers and mesh systems, especially with Wi-Fi 6E) Dual (tri) band: Wi-Fi device (access point, router, or client) that can operate on two (three) different frequency bands—typically the 2.4 GHz and 5 GHz bands or 2.4 GHz and 6 GHz in newer devices like Wi-Fi 6E (2.4 GHz and 5 GHz (lower) and 5 GHz (upper)/6 GHz). Simultaneous use of multiple bands is the standard in modern dual-band and tri-band devices. It is common to have separate dedicated antennas for different bands (+ more for MIMO) Same SSID for all bands is now the more common practice, providing a seamless user experience where devices are automatically guided to the optimal band. The router/AP advertises a single SSID that is associated with all available bands. Devices will typically choose the best band based on criteria like signal strength etc.. However, separate SSIDs for bands to manage traffic manually or ensure compatibility for specific devices (e.g., IoT devices on 2.4 GHz) is also possible. Enterprise network with WiFi: Wired network (Ethernet + Routers) + WiFi Spanning a big area with multi-story buildings + relaxing/working/walking areas between buildings. 2 or more groups of users accessing wired + WiFi (e.g., employees and guests) Need traffic isolation for these groups (in wired and wireless) Security for these groups (in wired and wireless, e.g., a wifi guest should not decrypt what a wifi employee sends) Access control for these groups (e.g., allow a WiFi employee to access an internal server while prevent a WiFi guest from accessing it). How do we come up with WiFi satisfying these requirements? 802.11: Channels, association § arriving host: must associate with an AP Association: creating a virtual wire between itself and the AP. An AP periodically sends beacon frames with the AP’s SSID and MAC address. Host scans channels, listening for beacon frames from APs. selects an AP to associate with. then may perform authentication. then typically run DHCP to get IP address in the same subnet. BSS 802.11: passive/active scanning BBS 1 BBS 2 BBS 1 BBS 2 1 1 1 AP 2 2 2 AP 2 AP 1 AP 1 2 3 3 4 H1 H1 passive scanning: active scanning: (1) beacon frames sent from APs (1) Probe Request frame broadcast from H1 (2) association Request frame sent: H1 (2) Probe Response frames sent from APs to selected AP (3) Association Request frame sent: H1 to (3) association Response frame sent selected AP from selected AP to H1 (4) Association Response frame sent from selected AP to H1  IEEE 802.11: multiple access Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) § avoid collisions: 2+ nodes transmitting at same time § 802.11: CSMA - sense before transmitting and refrains from transmitting when the channel is sensed busy § 802.11: no collision detection! difficult to detect collisions: high transmitting signal strength, weak received signal strength due to fading. can’t sense all collisions in any case: hidden terminal, fading goal: avoid collisions: CSMA/CollisionAvoidance Because 802.11wireless LANs do not use collision detection, once a station begins to transmit a frame, it transmits the frame in its entirety. Difference with Ethernet: Ethernet uses CSMA/CD: CD = collision detection.  IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender 1 if sense channel idle for DIFS then sender receiver transmit entire frame (no CD) 2 if sense channel busy then DIFS start ‘random’ backoff time timer counts down while channel idle data transmit when timer expires, and then wait for ACK If ACK received, to send another frame, go to random backoff at step 2 SIFS if no ACK, increase random backoff interval, repeat 2 ACK 802.11 receiver if frame received OK return ACK after SIFS (ACK needed due to hidden DIFS: Distributed Inter-frame Space terminal problem) SIFS: Short Inter-frame Space Difference with Ethernet: No link-layer ACK and retransmission in Ethernet.  Avoiding collisions (more) idea: sender “reserves” channel use for data frames using small reservation packets (control packets), that helps avoid collisions even in the presence of hidden terminals § sender first transmits small request-to-send (RTS) packet to BS using CSMA RTSs may still collide with each other (but they’re short) § BS broadcasts clear-to-send CTS in response to RTS § CTS heard by all nodes sender transmits data frame other stations defer transmissions Collision Avoidance: RTS-CTS exchange A B AP RT S ( A ) RT S ( B ) reservation collision RT S ( A ) CT S ( A ) CTS(A) time DATA (A) defer A CK ( A ) ACK(A) 802.11 frame: addressing 2 2 6 6 6 2 6 0 - 2312 4 frame duration address address address seq address payload CRC control 1 2 3 control 4 Address 1: MAC address Address 4: used only in of wireless host or AP to ad hoc mode receive this frame Address 3: MAC address of Address 2: MAC address router interface to which AP of wireless host or AP is attached transmitting this frame 802.11 frame: addressing Internet H1 R1 802.3 Ethernet frame R1 MAC addr H1 MAC addr MAC dest addr MAC source addr AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address 3 802.11 WiFi frame 802.11 frame: addressing duration of reserved frame sequence # (for reliable data transmission time (RTS/CTS) transfer) 2 2 6 6 6 2 6 0 - 2312 4 frame duration address address address seq address payload CRC control 1 2 3 control 4 2 2 4 1 1 1 1 1 1 1 1 protocol to from more power more type subtype retry WEP rsvd version AP AP frag mgt data frame type (RTS, CTS, ACK, data) 802.11: mobility within same subnet § H1 remains in same IP subnet: IP address can remain same § switch: which AP is associated with H1? self-learning: switch will see frame from H1 and “remember” which switch port can be used to reach H1 H1 BBS 2 BBS 1 802.11: advanced capabilities Rate adaptation § base station, mobile dynamically 10-1 change transmission rate (physical 10-2 10-3 layer modulation technique) as BER 10-4 mobile moves, SNR varies 10-5 10-6 1. SNR decreases, BER increase as node moves 10-7 10 20 30 40 away from base station SNR(dB) 2. When BER becomes too high, switch to lower QAM256 (8 Mbps) QAM16 (4 Mbps) transmission rate but with lower BER BPSK (1 Mbps) operating point 802.11: advanced capabilities power management § node-to-AP: “I am going to sleep until next beacon frame” AP knows not to transmit frames to this node node wakes up before next beacon frame § beacon frame: contains list of mobiles with AP-to-mobile frames waiting to be sent node will stay awake if AP-to-mobile frames to be sent; otherwise sleep again until next beacon frame  Link layer activities for a wireless station in 802.11: Association to an AP Decision on when to transmit using MAC Addressing if errors occur in the transmission (retransmission) Security QoS Decision-making for power management Physical layer activities: How physical signals carry bits? and associated issues? – Transmitting digital data (e.g., 100110..) using Analog signal (Modulation or carrier-modulated transmission or passband transmission) - continuous, often sinusoidal wave. e.g. modems, ADSL, Ethernet, WiFi , 4G, optical communication... (specifically, Frequency-shift keying (FSK)) (specifically, Amplitude-shift keying (ASK)) (e.g., On–Off keying (OOK)- that represents digital data as the presence or absence of a carrier wave) (specifically, Phase-shift keying (PSK)) phase refers to the position of the waveform relative to a reference point at a given instant in time. e.g., QPSK (Quadrature Phase Shift Keying), 4 phase shifts => each symbol represents two bits Many systems use a combination of amplitude and phase modulation (AM + PM). e.g., 4 different amplitudes, 2 different phases - Each time-frame, we can distinguish 8 different levels. - This means, a symbol in each time-frame can carry 3-bit data (3-bit = 8 different levels) - Compared to 1-bit per symbol, this is 3 times the data transmission rate. - Used to carry more bits per symbol, leading to high data rates. (Cons: It's more susceptible to noise/errors) - e.g., 1000 symbols/s x 3 bits/symbol = 3 kbits/s symbol (baud) rate bits/symbol data rate - The receiver (demodulator) must correctly detect both phase & amplitude to find the ‘data’ arrived. - A modulation of this type: Quadrature amplitude modulation (QAM) (used in wireless, optics). - 256-QAM: 256 different levels => 8 bits/symbol, 1024-QAM => 10 bits/symbol. - 256-QAM, 1024-QAM, 4096-QAM, …. - QAM in WiFi...  5/6 of bits are real data In modern Wi-Fi standards... (rest are added for error corrections) 000 001 010 011 100 101 110 111 QAM subcarriers Amplitude OFDM frequency MIMO Spatial streams 2 (spatial streams) x 6 (subcarriers) x 10,000 (symbols/s) x 3 (bits/symbol) x 5/6 = 300 kbps. (Theoretical max data rate, single-user case assumed) - Theoretical max data rate for Wi-Fi 6 (IEEE 802.11ax, 2021)(Wi-Fi 6 operates in the 2.4 GHz and 5 GHz bands. Wi-Fi 6E adds 6GHz band): (Assuming single-user case. Max possible settings assumed) - Maximum 8 spatial streams (space (division) multiplexing in wireless with MIMO). Each stream with max 160 MHz channel bandwidth. Data (from the user) could be split and sent over these streams parallelly at the same time utilizing the same frequency band in each stream. - This 160 MHz supports max 1960 (= 2 * 980) subcarriers (using OFDM). Data (from the user) could be split and sent over these subcarriers parallelly at the same time. - Each subcarrier carrying data using (max) 1024 QAM (10 bits/symbol). - Symbol rate (on each subcarrier) = 73,529 symbols/s (= 1/(12.8µs symbol-time + 0.8µs guard-interval)) - Code rate = 5/6 (error correction codes are added into the data bits for reliability. e.g., for 5 bit real data, 1 bit is error correction code or 83.3% real (usable) data in the bit-stream). - Theoretical max data rate = 8 x 1960 x 10 x 5/6 x 73,529 = 9.6 Gbps. - Supports MU-MIMO, OFDMA also. - Theoretical max data rate for Wi-Fi 7 (IEEE 802.11be, Dec 2024, in 2.4 GHz, 5 GHz, 6GHz bands) 16 (spatial streams) x 3920 (from 320 MHz) x 12 (from 4096QAM) x 5/6 x 73,529 = 46.1 Gbps. (In 2023, Wi-Fi 7 routers began appearing with 10GbE WAN ports as standard) - Theoretical max data rate for Wi-Fi 8 (IEEE 802.11bn, May 2028) ~ 100 Gbps.  MIMO (multiple input (antennas), multiple output (antennas)) A wireless antenna technology, multiple antennas are used at both transmitter and the receiver. Uses a natural radio-wave phenomenon called ‘multipath’. Sends and receive more than one signal on different transmit and receive antennas (over multipaths) Transforms multipath propagation from an impediment to an advantage. Spatial multiplexing*: Send independent streams of information in parallel along multiple spatial paths (multipaths, for instance with the same frequency band) (If these signals arrive at the receiver antenna array with sufficiently different spatial signatures and the receiver has accurate channel state information, it can separate these streams into (almost) parallel channels.) => Increases rate. Another use: Spatial diversity- send or receive redundant streams of information in parallel along multiple spatial paths (multipaths)– Increases reliability and range (unlikely that all paths will be degraded simultaneously). MIMO types: Single User MIMO (SU MIMO) – can transmit multiple data streams to only one device, Multiuser MIMO (MU-MIMO)- – can transmit multiple data streams to multiple devices simultaneously, Massive MIMO- large number of antennas such as 64, 128, or 256 antennas (key technology for 5G). Used in WiFi, cellular networks (4G, 5G), WiMAX, etc. In Wifi, a device uses CSMA/CA to check whether the channel is free. If free, it can start transmit. When the device starts transmitting, MIMO allows multiple streams to be transmitted simultaneously between devices with multiple antennas. (Wifi is always half-duplex). Multipaths Spatial multiplexing Spatial streams * Space (division) multiplexing in wired channels: can be done with multiple ‘wires’ in the channel. e.g., multiple pairs (T), multicore optical fibers, multiple fibers in a bundle Source (adapted): https://info.support.huawei.com/info-finder/encyclopedia/en/MIMO.html  Orthogonal Frequency-Division Multiplexing (OFDM)* Most commonly used in wireless (+ some usage in electrical). OFDM is a variation of FDM. OFDM better utilizes the bandwidth and offers high data rate than FDM. Used in WiFi, cellular networks (4G, 5G), WiMAX, Satellite, digital television and audio broadcasting, ADSL, power line networks, etc. Amplitude = frequency FDM subcarriers Amplitude Amplitude frequency OFDM frequency Orthogonal: signals are multiplexed in a way that the peak of one signal (yellow) occurs at null of the other neighbor signals (blue and pink) thus preventing the interference. * OFDMA (orthogonal frequency-division multiple access) enables devices sharing the same overall channel to have the component subcarriers dedicated to specific devices. Source: https://www.youtube.com/watch?v=KCHO7zlU25Q