Scheduled-Access (SACH) and Scheduled-Access Control Channel (SACCH) Signaling Format
[Framing and Channel Multiplexing]

The SACH is a set of data streams multiplexed by multiple-antenna OFDMA containing user traffic for several destinations. The SACCH contains low-layer protocol information regarding sequencing (for ARQ and channel decoding) and signaling for channel feedback mechanisms. In AdHoc/Mesh configuration, the SACCH is a resource common to all destinations sharing the SACH and its data is multiplexed with those of the SACH. In the cellular configuration, the SACCH is located in the first allocated symbol and uses the lowest-order (highest protection) coded-modulation format. Aside from low-level signaling it contains the allocation formats used by the set of streams in the SACH, specifically the coded-modulation formats and frequency-allocations in the case of the AdHoc/Mesh configuration. In the downlink of a cellular configuration, the SACCH information is embedded in the CHBCH PDU.

The transmission format of the SACH/SACCH can either use classical OFDM or digital FDM, and this is signaled by the higher layers.

Each SACH/SACCH is made up of $N_{\mathrm{symb,SACH}}$ OFDM symbols. The number of symbols used by a particular SACH in a particular TTI is broadcast via MAC-layer signaling in the CHBCH and depends QoS parameters and measurements. It is thus a dynamic parameter known by all nodes in the cluster at the beginning of each TTI.

A SACH/SACCH contains $N_\mathrm{pilot,SACCH}$ pilot symbols during the SACCH symbols to allow for multi-antenna wideband channel estimation, and $N_\mathrm{pilot,SACH}$ pilot symbols per SACH symbol for carrier frequency offset tracking. The encoding rules for the SACCH pilot symbols is identical to the MCH/RACH on the first transmit antenna. If multiple transmit antennas are employed, the pilot symbols for antenna $i=0,1,\cdots,N_\mathrm{ant}$ in position $k$ is multiplied by the complex phasor sequence $e^{j*2*pi*kiN_\mathrm{p}/N_\mathrm{d}}$. This phasor sequence ensures that the $N_\mathrm{ant}$ channel responses are orthogonal at the receiver provided $N_\mathrm{ant}N_\mathrm{p}\leq N_\mathrm{d}$ and $N_\mathrm{p}$ is less than the maximum channel duration plus maximum propagation delay.

Both $N_\mathrm{pilot,SACH}$ and $N_\mathrm{ant}$ are broadcast using higher layer signaling or pre-configured.

The number of samples for SACH/SACCH data is $N_\mathrm{samp,SACH} = N_\mathrm{symb,SACH}(N_\mathrm{d}-N_\mathrm{f} - N_\mathrm{pilot,SACCH} - N_\mathrm{pilot,SACH}) - 32$. SACCH data is to be encoded using the lowest spectral efficiency coded-modulation format, namely a rate 1/2 forward error-correcting code with QPSK modulation. If multiple transmit antennas are used, then the lowest spectral-efficiency BICM space-time code is to be employed. Prior to FEC coding a CRC shall be computed on the SACCH data and should be concatenated to the tail of the information. The total number of coded bits for the SACCH depends on the number of streams. It should be kept to a minimum with respect to the total number of data bits in the SACH streams in order to guarantee efficiency.

Since the SACH is a dynamically allocated resource based on channel quality measures, sample interleaving across different OFDM carriers is not required.

Prior to forward error-correction coding, a CRC of length 32 bits shall be applied to the PDU.

The SACH is the only resource for which the output of the channel coding block is vectorial, in the case of multiple transmit antennas. Space-time signal processing is therefore possible on the SACH data streams. Identical interleaving is performed on all antenna streams and the choice of coded-modulation and space-time processing methods is determined by the MAC scheduler. A choice of 16 different formats are considered including some of the legacy 802.11a formats. The formats could potentially be reconfigurable and installed at run-time or over-the-air. This is discussed in Coded Modulation and H-ARQ. Each stream can use a separate coded-modulation format.

Streams can use a subset of the OFDM carriers according to the frequency allocation vector as mentioned earlier. The allocations are chosen by the opportunistic scheduling algorithm in the MAC layer in order to achieve multi-user diversity and potentially spatial-multiplexing.

The configuration information for the SACH/SACCH resource are partially signaled by higher layers and partially computed dynamically by the scheduling algorithm in the MAC. They are described by the primitive PHY_SACH summarized in the following table.

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