Chapter 7
Access Techniques for IoT In this chapter, we review access techniques for internet of things. We
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7.1 7.1.1
CHAPTER 7. ACCESS TECHNIQUES FOR IOT
Wired vs. Wireless Communication Wired Communication for IoT
Wired solutions usually provide very reliable communication media. It provides high reliability, high data rate, short delay, and high security. However, they are not suitable for IoT, because they are cost ineffective, i.e., wire to everywhere, do not support mobility, and do not support scalability. Wired solutions can be used for fixed-location IoT devices.
7.1.2 Wireless Communication for IoT
Wireless communications suffer from several channel impairments, such as noise, fading, shadowing, etc. Therefore the supported data rate is changing over time. Due to the broadcast nature of wireless communication the signals are subject to interference from other sources. A wireless network is like a shared channel to put data on standards must address this sharing, avoiding destructive interference.
Wireless communication is intrinsically subject to errors received signal is the energy collected over time can always be disturbed by an external party (a different protocol family in same frequency), or by another neighbor than the sender quality depends on environment properties (e.g. reflections). Wireless communication is energy-hungry, particularly when compared to the energy used by embedded processors.
Wireless however provides Scalability, low cost implementation, mobility support, and with the new technologies can be implemented with ultra-low cost, low power consumption and complexity. It is considered as the best option for IoT connectivity.
Existing wireless technologies
Wireless capillary (i.e., short range) solutions, such as WLAN and ZigBee, can provide low cost infrastructure, scalability for most M2M applications, but they suffer from small coverage, low data rate, weak security, and severe interference.
Wireless cellular, i.e., GSM, GPRS, 3G, LTE-A, WiMAX, etc., however offers excellent coverage, mobility and scalability support, and Good security, and the fact that the infrastructure already exists makes it a promising solution for M2M communications.
7.1.3 Wireless Channel Characteristics
Radio waves are easy to generate, can travel long distances, can penetrate build- ings, therefore suitable for both indoor and outdoor communication. They are omni-directional, i.e., can travel in all directions. They can be narrowly focused at high frequencies (greater than 100MHz) using parabolic antennas (like satel- lite dishes). Properties of radio waves are frequency dependent. That is at low frequencies, they pass through obstacles well, but the power falls off sharply
7.1. WIRED VS. WIRELESS COMMUNICATION 95
with distance from source. At high frequencies, they tend to travel in straight lines and bounce of obstacles (they can also be absorbed by rain). They are subject to interference from other radio wave sources.
When the radio wave are propagated, the followings could happen to it:
Reflection
Occurs when waves impinges upon an obstruction that is much larger in size compared to the wavelength of the signal. These reflections may interfere with the original signal constructively or destructively.
Scattering
Occurs when the radio channel contains objects whose sizes are on the order of the wavelength or less of the propagating wave and also when the number of obstacles are quite large. They are produced by small objects, rough surfaces and other irregularities on the channel Follows same principles with diffraction causes the transmitter energy to be radiated in many directions Lamp posts and street signs may cause scattering.
Diffraction
Occurs when the radio path between sender and receiver is obstructed by an impenetrable body and by a surface with sharp irregularities (edges). Explains how radio signals can travel urban and rural environments without a line-of-sight path
Figure 7.1: Multipath wireless channel.
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Figure 7.2: Fading in wireless channel.
7.2 Medium Access Control
Wireless spectrum (frequency band) is a very precious and limited resource. We need to use this resource very efficiently. We also want our wireless system to have high user capacity. A lot of (multiple) users should be able to use the system at the same time. For these reasons most of the time, multiple users (or stations, computers, devices) need to share the wireless channel that is allocated and used by a system. The algorithms and protocols that enables this sharing by multiple users and controls/coordinates the access to the wireless channel (medium) from different users are called MEDIUM ACCESS, or MEDIA ACCESS or MULTIPLE ACCESS protocols, techniques, schemes, etc)
7.2.1 MAC Categories
Figure 7.3: Fading in wireless channel.
7.2.2 Coordinated Multiple Access
In coordinated multiple access, the devices are first identified by the access points and then coordinated for the transmissions through TDMA, FDMA, or CDMA as depicted in Fig. 7.4.
7.2. MEDIUM ACCESS CONTROL 97
Figure 7.4: Coordinated Multiple Access Techniques.
FDMA
Frequency Division Multiple Access divide the available bandwidth into several subbands and allocate each user with one subband. If channel allocated to a user is idle, then it is not used by someone else: waste of resource. Mobile and base station can transmit and receive simultaneously. Bandwidth of FDMA channels are relatively low. Lower complexity systems that TDMA systems.
TDMA
Time Division Multiple Access allocates radio spectrum to the users by dividing the time into time slots. In each slot a user can transmit or receive. A user occupies a cyclically repeating slots. A channel is logically defined as a particular time slot that repeats with some period. TDMA systems buffer the data, until its turn (time slot) comes to transmit. This is called buffer-and-burst method.
TDMA Frames
In TDMA/TDD: half of the slots in the frame is used for forward channels, the other is used for reverse channels. In TDMA/FDD: a different carrier frequency is used for a reverse or forward Different frames travel in each carrier frequency
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Figure 7.5: FDMA technique.
Figure 7.6: TDMA frame structure.
in different directions (from mobile to base and vice versa). Each frame contains the time slots either for reverse channels or forward channel depending on the direction of the frame. Preamble contains address and synchronization info to identify base station and mobiles to each other Guard times are used to allow synchronization of the receivers between different slots and frames Different mobiles may have different propagation delays to a base station because of different distances.
CDMA
In CDMA, the narrowband message signal is multiplied by a very large band- width signal called spreading signal (code) before modulation and transmission over the air. This is called spreading. CDMA is also called DSSS (Direct Se- quence Spread Spectrum). DSSS is a more general term. Each user has its own cordword and codewords are orthogonal. The receiver correlation distin- guishes the senders signal by examining the wideband signal with the same time-synchronized spreading code. The sent signal is recovered by despreading process at the receiver.
The features of CDMA are summarized next: • Low power spectral density.
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Figure 7.7: TDMAframe structure.
• Signal is spread over a larger frequency band
• Other systems suffer less from the transmitter
• Interference limited operation
• All frequency spectrum is used
• The codeword is known only between the sender and receiver. Hence other users can not decode the messages that are in transit
• Reduction of multipath affects by using a larger spectrum
• Random access possible
• Users can start their transmission at any time
• Cell capacity is not concerete fixed like in TDMA or FDMA systems. Has soft capacity
• Higher capacity than TDMA and FDMA
• No frequency management
• No equalizers needed
• No guard time needed
• Enables soft handoff
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Figure 7.8: CDMA spreading.
Figure 7.9: Coding in CDMA.
7.3 Uncoordinated Multiple Access
What if the devices are not known at the access point? How to allocate the bandwidth to the devices? Random Schemes (Less-Coordinated) Examples: MACA, MACAW, Aloha, 802.11 MAC,
More suited for wireless networks that are designed to carry data: IEEE 802.11 Wireless LANs
7.3.1 Random Access
In random access there is no fixed schedule and no special node to coordinate the access. Distributed algorithms are used to determine how users share chan- nel (see Fig. 7.11), when each user should transmit. The challenge here is two or more users can access the same channel simultaneously, which is called Collisions. In random access protocols we have to consider 1) How to detect and avoid collisions and 2) How to recover from collisions. Examples of random
7.3. UNCOORDINATED MULTIPLE ACCESS 101
Figure 7.10: Multiple access via CDMA.
access techniques are Slotted ALOHA, Pure ALOHA, CSMA, CSMA/CA, and CSMA/CD.
Figure 7.11: Multiple access scenario.
7.3.2 Pure ALOHA
Pure ALOHA is the simplest random access technique that allow multiple trans- mitters to share the same channel to communicate with a single receiver (Fig. 7.12). The algorithm for Pure Aloha is as follows:
Ignoring the propagation delay between mobiles nd base station, The time difference between the time a mobile send the first bit of packet and the time the base station receives the last bit of the packet is given by 2T , where T = C/P , T is the packet duration time, C is the channel data rate (bps), and P is the packet
A mobile station transmits immediately whenever is has data.
It then waits for ACK or NACK.
If ACK is not received, it waits a random amount of time and retransmits.
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Figure 7.12: Pure ALOHA
length (bits). During this 2T period, the packet may collide with someone else’s packet. The throughput of the Pure ALOHA can be depicted as follows:
Figure 7.13: Throughput of Pure ALOHA
7.3.3 Slotted ALOHA
In Slotted ALOHA, time is divided into slots of one packet duration, e.g., fixed size packets. When a node has a packet to send, it waits until the start of the next slot to send it. This scheme requires synchronization. If no other nodes attempt transmission during that slot, the transmission is successful. Otherwise collision happens. Collided packet are retransmitted after a random delay.
Figure 7.14: Slotted ALOHA
7.3. UNCOORDINATED MULTIPLE ACCESS 103 Example of Slotted ALOHA
A wireless system consisting of 2 users U1 and U2, which want to communicate with a single access point. They need to deliver their message within 2 time slots. Each user randomly transmits its message in a given time slot with probability p. Once a user received a feedback from the AP it finishes its transmission.
Figure 7.15: An example Slotted ALOHA system.
1. What is the probability that both users can deliver their messages?
There are only two cases (as shown in Fig. 7.16) that both users can deliver their messages at the base station in 2 time slots.
Figure 7.16: Slotted ALOHA
2. What is the probability that at least one user can deliver its message?
As shown in Fig. 7.17, there are 6 cases that at least one user can deliver its message at the base station in 2 time slots. Fig. 7.18 shows the success probability for both scenarios versus the access probability. As can be seen, the success probability should be carefully designed to maximize the success probability.
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Figure 7.17: Slotted ALOHA
7.4 Random Access in Current Standards
Random access is currently used in many wireless systems. For example in current cellular systems, LTE and LTE-A. OFDMA is currently used in LET, that is multiple subbands are allocated to each user. However, once a user wants to initiate the transmission, it performs random access and the BS tries to identify the users.
7.4.1 Random Access procedure in cellular systems
The devices are first informed of the available physical random access chan- nel (PRACH) resources, comprising of a periodic amount of time-frequency resources, through the system information broadcasted by the base station.
In the first step of the RA procedure, each device randomly chooses a pream- ble among available set of 64 preambles and sends it to the base station. The base station can then detect the transmitted preambles by calculating the cyclic cross correlation of the set of preambles with the received signal.
Upon detecting each preamble, the base station sends a random access re- sponse (RAR) message, including the information of the radio resources allo- cated to devices and the timing advance information for all the devices which have selected a specific preamble, to adjust synchronization.
When more than one user selects the same preamble, a collision happened and the collided devices will interfere with each other and therefore cannot send their messages. The random access procedure in current cellular standards is only feasible when the number of devices is small enough that the devices don’t unduly interfere with each other in the random access phase and there is sufficient radio resources to allocate a separate data channel to each user in the transmission phase.
These requirements will no longer be met for the massive number of devices
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Figure 7.18: Success probability versus the access probability for slotted ALOHA random access.
involved in M2M communications.
7.4.2 Collision Probability in Random Access
Consider we have N preambles and k devices request access to the base station by randomly transmitting one of these preambles. The Collision Probability, defined as the probability that a particular preamble is selected by more than one device is given by
Pc =P(morethanonedeviceselectspreamblei)
= 1 − P (nonofthedevicesselectpreamblei) − P (onlyonedeviceselctpreamblei)
=1−(1−1)N −N(1−1)N−1 kkk
Collision Probability is very large when the system load is large, therefore it is not suitable for IoT.
7.4.3 Improving Random Access
Remember that when a device is collided, it reattempt the transmission in the next time slot. If this retransmissions are not controlled, the system will soon saturate! One possible approach is to reduce the collision probability by increasing the number of preambles. This is not practical as increasing the number of preambles means that we have to allocate more time and bandwidth for random access. This will significantly reduce the total system throughput.
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Figure 7.19: Slotted ALOHA
Figure 7.20: Collision probability versus the number if active devices in a cellular system consisting of 64 preambles.
Back-off Schemes
When a device is collided, it reattempt the transmission after a certain period of time (back-off time). In fact collision means that the system is currently under high load and it is better to wait. The back of times needs to be optimized depending on the load.
Access Barring
which determine the access probability to control the access collision Let p denote the ACB parameter, then an MTC device which has data to transmit will draw a random number in the range [0; 1], and participate in the random access procedure only if the random number is less than p. There is a trade- off between barring factor p and LTE performance like resource utilization or average access delay. If the barring factor is smaller, fewer devices are allowed to access. However, the access delay increases and resource utilization decreases. Thus, eNB needs to effectively control the barring factor under the current
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Figure 7.21: Collision probability versus the number if active devices in a cellular system with different number of preambles.
network condition.
Although these techniques can reduce the collision probability to a certain degree, they still works for a small number of devices. They are proved to be inefficient for massive IoT when a large number of devices simultaneously request access. For IoT, grant-free access is a promising solution, where the devices do not need to perform random access. In fact, they will be identified later by the base station. This however requires advanced multi-user detection algorithms at the BS, which seems feasible considering cloud computing. This is an open problem in cellular systems.
7.5 CSMA: Carrier Sense Multiple Access
Aloha does not listen to the carrier before transmission. CSMA listen to the carrier before transmission and transmits if channel is idle. Detection delay and propagation delay are two important parameters for CSMA . Detection delay: time required to sense the carrier and decide if it is idle or busy. Propaga- tion delay: distance/speed of ligth. The time required for bit to travel from transmitter to the receiver.
7.5.1 CSMA Variations
persistent CSMA
A station waits until a channel is idle. When it detects that the channel is idle, it immediately starts transmission.
108 CHAPTER 7. ACCESS TECHNIQUES FOR IOT Non-persistent CSMA
When a station receives a negative acknowledgement, it waits a random amount of time before retransmission of the packet although the carrier is idle.
P-persistent CSMA
P-persistent CSMA is applied to slotted channels. When a station detects that a channel is idle, it starts transmission with probability p in the first available timeslot.
CSMA/CD
Same with CSMA, however a station also listen to the carrier while transmitting to see if the transmission collides with someone else transmission. Can be used in listen-while-talk capable channels (full duplex) In single radio channels, the transmission need to be interrupted in order to sense the channel.
7.5.2 MACA Medium Access with Collision Avoidance
CSMA protocols sense the carrier, but sensing the carrier does not always re- leases true information about the status of the wireless channel. The transmis- sion protocol of CSMA is depicted in Fig. 7.22
Figure 7.22: Transmission protocol of CSMA CSMA
There are two problems that are unique to wireless channels (different than wireline channels), that makes CSMA useless in some cases. These problems are:
• Hidden terminal problem
• Exposed terminal problem.
7.5. CSMA: CARRIER SENSE MULTIPLE ACCESS 109 Hidden Terminal Problem
This problem is shown in Fig. 7.23. Consider that A is transmitting to B. C is sensing the carrier and detects that it is idle (It can not hear As transmission). C also transmits and collision occurs at B. Therefore, A is hidden from C.
Figure 7.23: Hidden terminal problem in CSMA.
Exposed Terminal Problem
This problem is shown in Fig. 7.24. Consider that B is transmitting to A. C is hearing this transmission. C now wants to transmit to D. It senses the existence of carrier signal and defers transmission to D. However, C can actually start transmitting to D while B is transmitting to A, Since A is out of range of C and Cs signals can not be heard at A. Therefore, C is exposed to B’s transmission.
Figure 7.24: Exposed terminal problem in CSMA.
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MACA Protocol
When a station wants to transmit data,
• It sends an RTS (Ready-to-Send) packet to the intended receiver
• The RTS packet contains the length of the data that needs to be trans- mitted
• Any station other than the intended recipient hearing RTS defers trans- mission for a time duration equal to the end of the corresponding CTS reception
• The receiver sends back CTS (Clear-to-Send) packet back to sender if it is available to receive.
• The CTS packet contains the length of the data that original sender wants to transmit
• Any station other than the original RTS sender, hearing CTS defers trans- mission until the data is sent.
The original sender upon reception of the CTS, starts transmitting.
Figure 7.25: MACA protocol.
RTS/CTS is used to reserve channel for the duration of the packet transmis- sion. This prevents hidden and exposed terminal problems. ACK is required to understand if the packet is correctly received (without any collisions ) at the receiver.
MACA Solution for Hidden Terminal Problem
MACA can easily solve the hidden terminal problem in CSMA as shown in Fig. 7.26.
7.6. FREQUENCY HOPPING AND BLUETOOTH 111
Figure 7.26: MACA solution for hidden terminal problem in CSMA.
MACA Solution for Exposed Terminal Problem
MACA can easily solve the hidden terminal problem in CSMA as shown in Fig. 7.27.
Figure 7.27: MACA solution for exposed terminal problem in CSMA.
7.6 Frequency Hopping and Bluetooth
Bluetooth uses Frequency Hopping in cell (piconet) over a 79 MHz wideband ra- dio channel. Uses 79 narrowband channels (carrier frequencies) to hop through. Channel spacing is 1 MHz (narrowband channel bandwidth) and the wideband spectrum width is 79 MHz. The hopping Rate is 1600 Hops/Second and the frequencies are Freq(f) = 2402+kMHz,k = 0,…,78.
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Figure 7.28: Bluetooth frequency hopping.
Piconet and FHSS
In Bluetooth, each node is classified as master or slave. Master defines a piconet (a cell). Maximum 7 slaves can be connected to a master. Master coordinates access to the media. All traffic has to go over master and slaves can not talk to each-other directly. All slaves and the master hops according to the same hopping sequence. The hopping sequence is determined by the clock and BT address of the master. Inside a piconet, access to the frequency hopped radio
Figure 7.29: Piconet inBluetooth.
channel is coordinated using time division multiple access: TDMA/TDD. The slot duration is 1/1600sec = 625ms. In an even slot, master transmits to a slave. In an odd slot, the slave that is addressed in the previous master-to-slave slot transmits.
Piconet can be combined into scatternets (Fig. 7.31). Red slave acts as a bridge between two piconets. Each piconet uses FHSS with different hopping sequences (masters are different). This prevents interference between piconets.
7.7 Further Readings
• Chapter 4.1 and 4.2 from Computer Networks by Tanenbaum
• Chapter 2.6 and 2.7 from Computer Networks: A System Approach by Peterson
7.7. FURTHER READINGS 113
Figure 7.30: Frequency hopping and TDMA in Bluetooth.
• Chapter 15 Wireless Communications, Andrea Goldsmith http://web.
cs.ucdavis.edu/~liu/289I/Material/book-goldsmith.pdf
• M. Shirvanimoghaddam, M. Dohler and S. J. Johnson, ”Massive Non- Orthogonal Multiple Access for Cellular IoT: Potentials and Limitations,” in IEEE Communications Magazine, vol. 55, no. 9, pp. 55-61, 2017.
• M. Shirvanimoghaddam and S. J. Johnson, ”Multiple Access Technologies for cellular M2M Communications: An Overview,” ZTE Communications, vol. 14, no. 4, pp. 42-49, 2016.
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Figure 7.31: Scatternet and piconet in Bluetooth.