MIMO and Frame Aggreation Mechnism in 802.11

MIMO

The Single-Input/Single-Output(SISO) communication system uses:

• one antenna for transmitter
• one antenna for receiver
• only one antenna is active for any given data frame

Between the 2 endpoint in a SISO system, the flow of data is called a stream.

In the MIMO systems,

• all of the antennas are active simultaneously
• each antenna in the MIMO transmitter sends its data to the channel
• each antenna in the MIMO receiver collects its data stream from the channel

In an antenna diversity system, there is only one transceiver. Multiple antennas may feed into one transceiver. The transceiver chooses the best single antenna for Tx/Rx.

The transmission of multiple data streams across the same channel is called a spatial stream.(stream multiplexing)
Spatial streams are created by having independent paths through space between 2 devices.
Independent paths means that one path will not interfere with other paths. This is called uncorrelated paths. The RF design needs to consider about the antenna placement in order to minimize the correlation.

When transmitting a frame, the main tasks are the inverse Fourier transform and an the signal amplification.
On the receiving side, the process is reversed.
The components linked to do this Tx/Rx task is called a radio chain.

T x R : S

While the TxR indicates how many antennas the system has to receive or transmit data, S will indicate how much of the radio chains will definitely be used to transmit unique data.

The maximum data rate is determined by the number of spatial streams S.
Each spatial stream must have its own radio chain on both the sending and receiving side, so both T and R must be greater than or equal to S .

  T, R >= S

There is a feature in 802.11n, Space-Time Block Coding (STBC), that requires 2 radio chains to transmit a single spatial stream.
Space-time block coding (STBC) is mainly used to gain on robustness and reliability. Multiple radio chains will be used to send multiple times the same data or the bits of the same data over different antennas.

Airtime Effeciency Improvements

Gating access to the wireless medium in 802.11 is time consuming.

The 802.11 standard does not tell you how or when to do it, it is up to individual product designers to build a queuing algorithm that makes appropriate choices on when to coalesce individual frames into an aggregation, and how to manage the size of aggregations so that large data frames don't block small high-priority frames.

A-MPDU

All frames within an A-MPDU must be destined to the same receiver address on the wireless link, but may have multiple destination addresses.
A-MPDU is widely supported, and typically uses hardware-assisted processing.

An A-MPDU is limited in size only by the 802.11n PLCP, and thus can be up to 65,535 bytes.

A-MSDU

In addition to aggregation just before handing bits to the PHY for transmission, it is possible to pack multiple higher-layer packets into a single MAC frame.

Building an Aggregate MSDU (A-MSDU) requires more software support because the network driver must take several higher-layer packets and build them into a single MAC frame payload.

As a result, when security is applied to an A-MSDU, one encryption operation secures the whole aggregate frame.

An A-MSDU can have a maximum size of 7,955 bytes

FRAME AGGREGATION MECHANISM FOR HIGH- THROUGHPUT 802.11N WLANS

3 MAC LAYER OVERHEAD REDUCTION

3.1. Block Acknowledgement

3.2. Frame Aggregation Mechanism

3.2.2. A-MSDU Mechanism

n the Aggregated Mac service data units i.e. A-MSDU scheme, multiple MSDUs are bundled to form a MPDU which could consist of multiple sub frames either from multiple sources or for multiple destinations. Fig.2 illustrates the frame format of an A-MSDU. An A-MSDU consists of multiple sub frames (i.e. multiple MSDUs). Each sub frame of an AMSDU has a sub header (Destination address, Source Address, (Length), MSDU, and padding bytes. The size of the MSDU in each sub frame can be different. Different size of MSDUs in each sub frames are aggregated. To make the length of the sub frame in multiple of 4 bytes except for the last sub frame the padding bytes are appended. All the sub frames are bundled and share a common MAC header and frame check sequence (FCS) which is calculated over all the sub frames and a common MAC header and then appended as the trailer.

By the PHY layer the A-MSDU frame is considered as a single MPDU. Since there is no frame checksum for the individual sub frames, selective retransmission of corrupted sub frames is not possible. Sub frames have the same sequence number and traffic identifier (TID). The maximum length of an A-MSDU frame can be 3839 or 7955 bytes. This capability information is exchanged during the time of association.

The common MAC header contains only source and the access point’s (AP’s) address in uplink transmissions.Only in the sub headers, the corresponding destination addresses are present., a station can pack MSDUs destined for multiple destinations and send it to the AP in the uplink transmission., the AP can combine multiple MSDUs from various source addresses and send it to a single destination in downlink transmission. An A-MSDU frame cannot be transmitted to multiple end receivers.

3.2.3. A-MPDU Mechanism

Multiple MPDUs with a common PHY header are packed as an A-MPDU which can contain several MSDUs and/or A-MSDUs. The size of each sub frame must be a multiple of 4 bytes except for the last sub frame. Padding bytes are not needed for the last MPDU subframe. To make the length of the sub frame in multiple of 4 bytes, padding bytes are appended same as A- MPDU mechanism.

Fig. No. 3 shows the structure of an A-MPDU frame. Each MPDU is prefixed with a delimiter. The delimiter contains MPDU length, cyclic redundancy checks (CRC) and unique pattern. The first four bits in the delimiter are reserved and currently not used. 12 bits MPDU Length subfields are used for representing the length of the current MPDU. Cyclic redundancy check calculation includes reserved and length sub fields. The unique pattern is used to find the next delimiter with minimal computation in case of a corrupted delimiter. In this aggregation mechanism selective retransmission is possible due to the presence of individual frame check sequence (FCS) for each MPDU. All the MPDUs in an A-MPDU have same traffic identifier (TID) to effectively work with block acknowledgement (BA) mechanism. The maximum length size of an A-MPDU frame is 64 Kbytes. Capability to receive max size of A-MPDU is different for each station and this capability information is announced at the time of network entry process. Even though an A-MPDU frame can have a maximum size of 64 Kbytes, it can aggregate a maximum of 64 MSDUs. This limitation arises because of the use of block acknowledgment mechanism. To overcome this limitation Two level aggregation is proposed, and it can be used where in multiple A-MSDUs each carrying more than one MSDUs are aggregated to form an AMPDU. In this mechanism the size of A-MSDU frame should not exceed 4 KB.

3.2.4 Two-Level Aggregation

A two-level frame aggregation mechanism comprises a blend of A-MSDU and A-MPDU over two stages. In Fig. 4 it illustrates how this new mechanism can be achieved.

The basic operation is as follows: In the first level, if any MSDUs that are buffered in the A- MSDU provisional storage area justify the A-MSDU constraints explained in the previous related subsection, these data units can be aggregated into a single A-MSDU. If the TIDs are different, all these frames can move to the second level where they will be combined together with any A-MSDUs arises from the first level or other single MSDUs by using A-MPDU aggregation.

However, it must be mentioned that the maximum MPDU length for an A-MPDU data frame is limited to 4095 bytes, and then A-MSDUs or MSDUs with lengths larger than this limitation cannot be transmitted. Conjointly, any frames from an A-MSDU or MSDUs also cannot be included in an A-MPDU. Fig no. 4 illustrates how the two level aggregation mechanism is more efficient in most of the cases than A-MPDU and A-MSDU aggregation operating alone.