first and second mappers; a diversity encoder; a spatial mapper; in this example, a first to fourth inverse Fourier Transformers (iFTs), and in this example, a first to fourth Guard Interval (GI) inserters.
The stream encoder receives and encodes data. The stream encoder includes a Forward Error Correction (FEC) encoder. The FEC encoder may include a Binary Convolutional Code (BCC) encoder, a Low‐Density Parity‐Check (LDPC) encoder, or one or more combinations thereof. There are two types of coding, Convolutional Coding (CC) and Block Coding (BC). CC is state machine‐based, while BC is an algebra‐based approach. In WLAN, BCC has been the mandatory coding method. In terms of performance, LDPC is better than BCC, but complexity is much higher. LDPC is used from 11ax onward. See Table 2.4, loosely based on reference [17].
TABLE 2.4 Basic Error Coding Schemes
| Type | Approach | Method | Timeframe | Application | Feature |
|---|---|---|---|---|---|
| Convolutional | State Machine | BCC Viterbi | I960 | WLAN (11a/g) | Simple and widely used |
| Turbo | 1993 | 3G, 4G | Iterative decoder (close to Shannon limit) | ||
| Block Code | Algebra | Hamming | 19S0 | Computer memory | Simple Detect up to 2 simultaneous bit error and can correct 1 bit |
| Reed‐Solomon | 1960 | CD/MP3, Satellite, DVB | Widely used in digital storage and communication | ||
| LDPC | 1962/1996 | WLAN(11n~) 5G NR | Low density and complexity | ||
| Polar | 2009 | 5G NR |
The stream parser is configured to divide outputs of the encoder into one or more spatial streams. The stream parser may allocate consecutive blocks of bits to the one or more spatial streams in a round robin fashion. The blocks of bits typically have a length according to number of bits on an axis of a constellation point of a modulation and coding scheme, such as the length being 2 bits for 16‐QAM, 3 bits for 64‐QAM, 4 bits for 256‐QAM [2]. The respective bits of the first and second spatial streams are interleaved by first and second interleavers when BCC encoding is used.
The first and second mappers map the sequence of bits of the first and second spatial stream to first and second sequences of constellation points, respectively. A constellation point may include a (mathematical) complex number representing an amplitude and a phase. Within each of the first and second sequences of constellation points, the constellation points are divided into groups. Each group of constellation points corresponds to an OFDM symbol to be transmitted, and each constellation points in a group correspond to a different subcarrier in the corresponding OFDM symbol [2]. The diversity encoder is configured to spread the constellation points from the spatial streams into a plurality of space–time streams in order to provide diversity gain.
In Figure 2.2, the diversity encoder is shown mapping two spatial streams into four space–time streams (the Number of Spatial Streams NSS is equal to 2 and the Number of Space–Time Streams [STS] NSTS is equal to 4). Each space–time‐stream corresponds to a different transmitting antenna or a different beam of a beamformed antenna array. The diversity encoder spreads each input constellation point output by the mappers onto first and second output constellation points. The first output constellation point is included in a first space–time stream and the second output constellation point is included in a second space–time stream, different from the first space–time stream. The first output constellation point has a value corresponding to a value of the input constellation point, and the second output constellation point has a value corresponding to a complex conjugate of the value of the input constellation point or to a negative of the complex conjugate (i.e. a negative complex conjugate) [2]. The first output constellation point is at a different time slot (that is, in a different OFDM symbol period) than the second output constellation point when Space–Time Block Coding (STBC) is used. The first output constellation point is at a different frequency (that is, transmitted using a different subcarrier) than the second output constellation point when Space‐Frequency Block Coding (SFBC) is used.
The spatial mapper maps the space–time streams to one or more transmit chains. The spatial mapper maps the space–time stream to the transmit chains using a one‐to‐one correspondence when direct mapping is used. The spatial mapper maps each constellation point in each space–time stream to a plurality of transmit chains when spatial expansion or beamforming is used. Mapping the space–time streams to the transmit chains may include multiplying constellation points of the space–time streams associated with an OFDM subcarrier by a spatial mapping matrix associated with the OFDM subcarrier [2].
The first to fourth iFTs convert blocks of constellation points output by the spatial mapper to a time domain block (i.e. a symbol) by applying an Inverse Discrete Fourier Transform (iDFT) or an Inverse Fast Fourier Transform (iFFT) to each block. The number of constellation points in each block corresponds to the number of subcarriers in each symbol. A temporal length of the symbol corresponds to an inverse of the subcarrier spacing. When MIMO or MU‐MIMO transmission is used, the TxSP may insert cyclic shift diversities to prevent unintentional beamforming; the cyclic shift diversity may be specified per transmit chain or per space–time stream [2].
The first to fourth GI inserters prepends a guard interval to the symbol. The TxSP may optionally perform windowing to smooth the edges of each symbol after inserting the GI.
2.5 KEY IEEE 802.11AC MECHANISMS
New mechanisms were introduced with 802.11ac [18] to increase nominal speed and throughput. A DL channel refers to a communication channel from a transmit antenna of the AP to a receive antenna of a WN/STA, and an UL channel refers to a communication channel from a transmit antenna of a WN/STA to a receive antenna of the AP; DL and UL may be referred to as forward link and reverse link, respectively. New mechanisms 802.11ac included but were not limited to: (i) extended channel binding; (ii) optional 160 MHz and mandatory 80 MHz channel bandwidth for stations; (iii) (as noted), more MIMO spatial streams, also with Downlink Multi‐User MIMO (DL‐MU‐MIMO) – this DL‐MU‐MIMO formulation allows up to four simultaneous clients; (iv) multiple STAs (WNs) each having one or more antennas, to transmit or receive independent data streams simultaneously; (v) 256‐QAM, rate 3/4 and 5/6, added as optional modes (as compared with 64‐QAM, rate 5/6 maximum in 802.11n); and (vi) beamforming with standardized sounding and feedback for compatibility between vendors. Some features (e.g. low‐density parity‐check code; 400 ns short guard interval; five to eight spatial streams; 160 MHz channel bandwidths – contiguous 80 + 80; and 80 + 80 MHz channel bonding including discontiguous sections [19]) are optional.
2.5.1
