In this section, we present a comparison between IEEE 802.11ah and different IEEE 802.11 amendments. First, we describe the differences between IEEE 802.11 amendments based on MAC features. Later, we provide performance comparison between IEEE 802.11ah and the previous IEEE 802.11 amendments in terms of throughput versus transmission range characteristics.
3.1. Comparison of IEEE 802.11 Amendments Based on MAC Features
IEEE 802.11ah’s physical layer is basically an adaptation of IEEE 802.11ac to the sub-1 GHz band. The physical layer is a 10 times down-clocked version of IEEE 802.11ac (symbol duration from 4 to 40 µs), which keeps the same number of OFDM subcarriers. In consequence, the resulting channel bandwidth is ten times smaller than its IEEE 802.11ac counterpart (i.e., 2, 4, 6, 8 and 16 MHz) and adds a special mode of 1 MHz. As mentioned before, IEEE 802.11ah also defines a more robust MCS (BPSK 1/2 with repetition). The support of up to 4 × 4 MIMO (including multi-user MIMO) can be used to enable spatial diversity and/or spatial multiplexing to increase the capacity of the links and to improve coverage.
The key design feature for the IEEE 802.11 MAC is based on the channel access principle that enforces each station to sense the channel to be idle before initiating transmission, in order to avoid collisions. The MAC operation was designed based on Distributed Coordination Function (DCF) (explained below) protocol that utilizes the aforementioned principle. Despite the robust and adaptive nature of DCF in varying conditions, the initial MAC features were designed for best effort applications and thus did not require complex resource scheduling or management algorithms. However, the massive deployment of IEEE 802.11 networks has resulted in the need to include traffic differentiation and other sophisticated network management schemes. Furthermore, different versions of the IEEE 802.11 standard have been proposed with time, which include additional PHY and MAC features to accommodate the technological advances along with the ability to adapt to ever growing use cases.
Table 2 highlights the key MAC features supported by each amendment. In particular, we highlight the critical MAC additions and changes being made for IEEE 802.11ah, which will allow IEEE 802.11 standard to accommodate the IoT paradigm. The notable features compared in
Table 2 are briefly introduced in the following paragraphs.
3.1.1. Backwards Compatibility
Up till IEEE 802.11ac, all the IEEE 802.11 systems have been designed to be backward compatible. However, for IEEE 802.11ah, backward compatibility is not considered due to the use of a completely different frequency band.
3.1.2. Distributed Channel Access (DCF)
It is the basic random access MAC protocol of IEEE 802.11 standard that includes CSMA with Collision Avoidance (CSMA/CA), a sort of listen before talk mechanism. Furthermore, it encompasses binary exponential back-off rules to manage the retransmission of collided frames. It works as follows. Before initiating a transmission, a station senses the channel to determine whether it is busy. If the medium is sensed idle during a period of time called the Distributed Inter-frame Space (DIFS), the station is allowed to transmit. If the medium is sensed busy, the transmission is delayed until the channel is idle again. In this case, a slotted binary exponential back-off interval is uniformly chosen in [0, CW-1], where CW is the contention window. After each data frame is successfully received, the receiver transmits an acknowledgment frame after a Short Inter-frame Space (SIFS) period.
3.1.3. Point Coordinated Function (PCF)
It is an optional MAC protocol that uses polling scheme to determine which station can initiate data transmission. This technique is designed for infrastructure based network only, where different stations can optionally participate in PCF and respond to poll received.
3.1.4. Hybrid Coordination Function (HCF)
HCF, which combines the aspects of both the contention based DCF and controlled channel access based PCF, is a Quality of Service (QoS) aware MAC protocol that includes appropriate service differentiation mechanism. HCF defines two methods of channel access.
HCF Controlled Channel Access (HCCA)
It is similar to PCF and uses the same polling mechanism to assign transmission opportunity to QoS enabled stations.
Enhanced Distributed Channel Access (EDCA)
EDCA is an extension of the DCF mechanism that tries to implement service differentiation by classifying the traffic into different categories with different priorities. In EDCA mode, a traffic class can make itself a higher prioritized traffic class by statistically reducing its transmission delay by declaring an Access Category (AC) that has higher priority for contending shared channel.
3.1.5. Transmission Opportunity (TXOP)
For IEEE 802.11-2007:
TXOP defines a period of time for which a station accessing the channel is allowed to transmit multiple frames without using channel access procedure for all the frames.
For IEEE 802.11n/ac/ah:
In these amendments, the TXOP procedure is enhanced, where the reverse mechanism allows the holder of TXOP to allocate the unused TXOP time to its receiver to enhance the channel utilization and perform reverse direction traffic flows. This mechanism is known as Reverse Direction (RD) protocol.
For IEEE 802.11ah:
IEEE 802.11ah has introduced bi-directional TXOP (BDT) that can help non-AP station (i.e., sensors etc.) to minimize energy consumption. This technique allows the combination of transmission and reception of frames within a single TXOP, where the reduction in the required frame exchange enables stations to extend their battery life time. In addition, this mechanism assists in efficient use of contention based channel accesses.
3.1.6. Response Indication Deferral (RID)
This method is an extension of Virtual carrier sensing mechanism originally defined in legacy IEEE 802.11 (i.e., Network Allocation Vector (NAV)). The short header defined by IEEE 802.11ah does not include the Duration/ID field that is required by the NAV. Both NAV and RID indicate countdown timers used to show the channel idle time. However, the two schemes differ in the procedure to set the counter (while NAV is set after the complete and correct reception of a frame, RID can be set after the complete header of the frame is received).
3.1.7. Frame Aggregation:
Mechanism to combine multiple data frames into one larger aggregated data frame for transmission.
For IEEE 802.11n:
It employs two steps of accumulation to increase the size of the data frame to be transmitted. The first, which is at the top of the MAC, assembles MAC service data units (MSDU) and is called A-MSDU. Another, at the bottom of the MAC, adds MAC Protocol Data Units (MPDUs) and is called A-MPDU.
For IEEE 802.11ac/11ah:
Enhanced frame aggregation methods are used. All frames follow the A-MPDU format; the maximum size of A-MPDU is increased for IEEE 802.11ac.
3.1.8. Block Acknowledgement (Block ACK)
This mechanism enables the transmission of a single ACK frame by the station that received series of frames. This fact results in efficient use of airtime as compared to traditional positive ACK sent for every received frame.
For IEEE 802.11n:
Block ACK method is modified to support multiple MPDUs in an A-MPDU. The sender only resends the MPDUs that have not been correctly received by the receiver and are not acknowledged by it.
For IEEE 802.11ah:
Block ACK response includes the preferred MCS and the bandwidth information. IEEE 802.11ah also introduces the fragment Block ACK procedure. Fragments obtained from the partition of a MSDU can be acknowledged either using immediate acknowledgement by responding with NDP Block ACK frames, or following the normal Block ACK procedure.
3.1.9. Multi-User (MU) Aggregation
This method defined by the IEEE 802.11ac, supports the aggregation of MPDUs from multiple receivers into a single PDU only used for transmission from AP to multiple stations.
3.1.10. Null Data Packet (NDP)
Null frame is a frame meant to contain no data but flag information. They are widely used in IEEE 802.11 WLANs for control purposes such as power management, channel scanning, and association keeping alive.
3.1.11. Group ID
This mechanism enables a receiver to determine whether the data payload is single- or multi-user. More specifically, the Group-ID field is utilized by a receiving node to decide if it is targeted in the followed multi-user (MU) MIMO transmission.
3.1.12. BSS Color
It is an innovative scheme to increase throughput of dense WLAN networks, where each BSS is assigned a specific color (in-terms of bits designated in LSIG field of physical header). A station upon receiving frames from neighboring BSS, can abandon the reception process assuming the channel idle during that transmission and thus increasing the transmission opportunities.
3.1.13. Dynamic Bandwidth Management
IEEE 802.11ac has also introduced dynamic bandwidth management to optimize the use of available bandwidth. This scheme allows the transmitter and receiver to select an interference free channel before initiating transmission.
3.1.14. Subchannel Selective Transmission (SST)
This feature has been introduced by IEEE 802.11ah. It allows stations to rapidly select and switch to different channels between transmissions to counter fading over narrow subchannels.
3.1.15. Traffic Indication Map (TIM)
In legacy IEEE 802.11, the Beacon frame contains this element through which the sleeping power saving stations are informed of the presence of buffered traffic intended for them at the AP. This element is sent in the form of a bitmap, where each bit represents the Association ID (AID) of stations. A bit is set in TIM when corresponding station has buffered data at the AP. The Delivery Traffic Indication Message (DTIM) serves a similar purpose, indicating the presence of buffered multicast frames.
3.1.16. Target Wake Time (TWT)
TWT is a function that permits an AP to define a specific time or set of times for individual stations to access the medium.
3.1.17. Hierarchical AID
IEEE 802.11ah proposed hierarchical network organization where stations are grouped together based on their similarities. Each station is assigned a four level AID structure encompassing page, block, sub-blocks and station fields. As an important outcome, this mechanism helps in supporting increased number of stations.
3.1.18. Dynamic AID Reassignment
This mechanism allows the AP to change the page/group of a station due to a change in its traffic characteristics or for load distribution among the channels.
3.1.19. Restricted Access Window (RAW)
It is a new contention-free channel access mechanism that is designed to reduce collisions by improving the channel efficiency. The AP coordinates the uplink channel access of the stations by defining RAW time intervals in which specific class of devices are given exclusive access of the shared medium.
3.1.20. Group Sectorization
This scheme is developed by IEEE 802.11ah that allows stations to transmit in different sectors (positions) around the AP in a time division multiplexing manner (i.e., after each Beacon, a different sector is given access to the shared medium). The Beacons transmitted by a sectorized BSS carry sector option element and each station is allocated a group ID based on sectorization operation.
3.1.21. Relay Operations
IEEE 802.11ah has defined a mode of operation to utilize relays within the network to facilitate the exchange of frames between stations and APs. Relays allow stations to utilize higher data rates and TXOP sharing.
3.1.22. Power Saving at AP
IEEE 802.11ah proposes to include AP power saving features in IEEE 802.11ah.
3.1.23. Low Power Mode of Operations
IEEE 802.11ah enables a station to inform the AP about the duration of time it intends to remain in sleep mode. During the sleep mode, the station is not intended to listen to Beacons and then it is able to reduce its power consumption.
3.2.Throughput and Range Characterization of IEEE 802.11 Amendments
In order to compare different IEEE 802.11 amendments, we evaluate layer-2 throughput versus coverage range by using different channel bandwidth values, number of Spatial Streams (SS) and MCS. We analyze a scenario defined by a single radio link composed of two stations (transmitter and receiver) where we consider path loss models defined by TGah [
11]. The macro deployment model assumes an outdoor scenario with antenna placed at 15 m above rooftop. On the other side, we employ the large indoor open space TGah path loss model with Non-Line-of-Sight (NLoS) conditions, which corresponds to a factory/warehouse type of environment. The macro deployment path loss model follows the next expression:
where
d corresponds to the distance in meters between transmitter and receiver, and radio frequency carrier is 900 MHz. For other frequencies, a correction factor of
should be applied.
TGah indoor path loss model is modelled by directly scaling down the frequency operations of TGn path loss model. It consists of the free space loss model (slope of 2) up to a breakpoint distance (
), and employs a slope of 3.5 after the breakpoint. We consider the large indoor open space scenario with NLoS conditions (with
of 5 m). This indoor channel model would correspond to a factory/warehouse type of environment:
where
d corresponds to the distance in meters between transmitter and receiver,
is the center carrier frequency in MHz and
the speed of the light in m/s. Note that the TGah indoor channel propagation loss model was recently amended according to [
12] as shown in Equation (2).
We consider omnidirectional antennas with 0dB gain and transmitted power of 30 dBm. The MAC aggregation feature is included in our evaluation, and ideal transmission conditions have been considered for comparison purposes.
The throughput expression S in Mbps is as follows, employing DCF MAC access and including the aggregation feature:
where
K is the number of aggregated frames (of equal size),
Ldata corresponds to the payload size and
Tmessage is computed as:
DIFS and
SIFS are given in
Table 3,
is the propagation delay,
TBA corresponds to the duration of an Block ACK frame and
TDATA represents the transmission time of a data frame, which depends mainly on the size of the payload and on the PHY rate.
TDATA and
TBA computation also depends on the IEEE 802.11 amendment used in the transmission. Under ideal channel conditions, we consider that
is
CWmin/2 times the slot time (T
Slot);
CWmin corresponds to the minimum
CW (cf.
Table 3). All frame sizes are given in Bytes and frame durations in µs.
TDATA calculation for IEEE 802.11ah includes three different cases:
1 MHz CBW case with short and long Guard Interval (GI) subcases, following Equations (5) and (6), respectively. Note that with 1 MHz CBW only one PHY preamble/header type applies (cf.
Table 3).
Short preamble case for 2, 4, 8 and 16 MHz CBW with short and long GI subcases, which also follow Equations (5) and (6), respectively; in this case, a different value for the PHY preamble/header length should be used (cf.
Table 3).
Long preamble case for 4, 8 and 16MHz CBW with short and long GI subcases, following Equations (7) and (8), respectively:
TPreamble&Header is given in
Table 3 for the different configuration setups,
TSyml is the duration of a symbol with the long GI and
TSyms corresponds to the duration of a symbol with the short GI.
NLTF corresponds to the number of long training symbols, which depends on the number of SS. Without Space-Time Block Coding (STBC),
NLTF equals the number of spatial streams, except for three SS, in which case four training symbols are required.
Nsym is the number of symbols and is given in Equation (9):
is the size of the delimiter between aggregated frames (4 Bytes). TBA calculation employs previously exposed TDATA equations but a frame of 32 Bytes is considered instead of LHeader + Ldata. NES and NDBPS depend on the MCS chosen and are fixed in the standard specification.
We consider data frames with maximum payload size of 1500 Bytes to build the MPDU aggregation (A-MPDU). Up to 64 individual frames are allowed to assemble an A-MPDU. Note, however, that the standard imposes other restrictions that may reduce the number of aggregated frames carried by an A-MPDU. IEEE 802.11ah presents a maximum length for an A-MPDU of 511 symbols and a maximum duration of 27.930 ms. On the other hand, IEEE 802.11n allows up to 65,535 Bytes, whereas IEEE 802.11ac is able to deal with 1,048,575 Bytes of maximum length. In both amendments, the maximum frame duration is of 5.484 ms.
Different from Hazmi et al. [
13], who utilize a Bit Error Rate (BER) model for different MCS, we use the minimum receiver sensitivity established in the proposed IEEE 802.11ah amendment (cf. Tables 23–31—Receiver minimum input level sensitivity in [
2]). That is, for each distance and propagation model considered, we assume the transmitter is using the fastest available MCS, the minimum sensitivity of which is larger than the received power at that distance. For that reason
Figure 2 and
Figure 3 show a stepped relationship between throughput and coverage range.
As expected, using the most robust MCS leads to increased coverage and more reliable communication, while employing higher order MCS, the benefit of the higher data rate in the communication scenario can be observed (cf.
Figure 2 and
Figure 3).
The use of sub 1GHz frequency band, together with the new and more robust modulation MCS10 provide benefit to IEEE 802.11ah in achieving the long range feature, i.e., IEEE 802.11ah amendment can operate under macro deployment scenario and can achieve a coverage range of up to 1500 m. The same PHY configuration can reach up to 900 to 1100 m in different indoor scenarios.
Hence, in terms of coverage, there is seven-fold improvement using IEEE 802.11ah with the most robust MCS with respect to best sub-6 GHz amendment result (IEEE 802.11ac, 20 MHz, with 1 SS).
Furthermore, the improvement obtained by the new MCS10 in the IEEE 802.11ah case is around 15% for distance reached in macro deployment in comparison with the lowest MCS (MCS0) with 1 SS, and around 20% in indoor case. Besides, the use of more than 1 SS improves the throughput up till 95% when employing four SS, but in turn reduces the coverage range considerably. It is also important to highlight the fact that improving range results in throughput performance decrease. However, the throughput achieved by the IEEE 802.11ah in the limit of its coverage can still reach the 100 kbps, which can be enough for most of IoT applications.
It is also worth mentioning that a higher throughput performance can be obtained for IEEE 802.11ah employing two 8 MHz or four 4 MHz channels instead of one 16 MHz channel. First, note that the use of larger CBW improves the transmission efficiency since it allows the use of a larger proportion of data subcarriers (pilot, guard subcarriers are the same regardless of the CBW used). However, the required receiver minimum input sensitivity also increases by using larger CBW, thus a better signal quality is needed at the receiver to complete a successful reception. In this way, for long distances, it results in a more profitable practice to use, for example, 16 channels of 1 MHz CBW instead of 1 channel of 16 MHz CBW; with high signal quality in reception, the larger bandwidth becomes a better option due to the better proportion of data/pilot OFDM carriers.