TTI and Modulation Parameters
[Framing and Channel Multiplexing]

The carrier frequency is denoted $f_c$. Each TTI is made up of $N_{\mathrm{symb}}$ OFDM symbols. OFDM symbols, $\mathbf{s}$, of length $N_\mathrm{s}$ samples contain two distinct parts,$\mathbf{s}_\mathrm{I}$ and $\mathbf{s}_\mathrm{E}$.

OpenAirMesh framing is completely configurable, but the nominal OFDMA configuration is shown below

mesh_frame.png

OpenAir PHY Framing

One frame consists of 64 OFDM symbols and is divided in a CH transmission time interval (TTI) and a MR TTI. The first four symbols of the CH TTI are reserved for pilot symbols. Each CH transmits one common pilot symbol (CHSCH$_0$) at position 0 and one dedicated pilot symbol (CHSCH$_i$) at position $i \in \{1,2,3\}$. This way we can ensure orthogonality between the pilots of different CH received at one MR. The pilot symbols are followed by the broadcast channel (CH-BCH). The rest of the CH TTI frame is reserved for the multiplexed scheduled access channels (CH-SACH).

The MR TTI contains the random access channel (MR-RACH) with an associated pilot symbol (SCH$_0$). The next two symbols are reserved for pilots. Each MR transmits a pilot symbol SCH$_i$, $i \in \{1,2\}$ corresponding to the cluster it belongs to. The pilot symbols are followed by the uplink broadcast channel (MR-BCH) with an associated pilot symbol (MRSCH). The rest of the uplink frame contains the multiplexed scheduled access channels (MR-SACH). The end of the CH and MR TTIs are protected by a guard interval of two symbols. All pilots are designed for MIMO and/or Multiuser channel estimation at the corresponding end.

MAC PDUs arrive at the MAC interface from different logical resources (control, broadcast, multiple-user data streams) in parallel at the start of each TTI and must be mapped to the available radio resources. Each PDU is scrambled, encoded with a CRC check, and encoded using a channel code with associated bit-interleaving. The output of the channel coding block contains the information content to be transfered across the channel via the modulator. The modulated information content, $\mathbf{s}_\mathrm{I}$, is built starting either from a frequency-domain signal (classical OFDM) or several time-domain signals (digital FDM). Both techniques yield what are denoted herein as OFDM symbols. In the first method (classical OFDM), $\mathbf{S}_\mathrm{I}$ is specified in the frequency-domain and is made up of $N_\mathrm{d}$ samples. This is transfered to the time-domain via the inverse discrete-time Fourier transform (DFT) yielding a time-domain signal also of length $N_\mathrm{d}$ samples, $\mathbf{s}_\mathrm{I}=\mathrm{idft}(\mathbf{S}_\mathrm{I})$. In the second method, up to $N_\mathrm{f}$ different time-domain signals each comprising $N_{\mathrm{d,2}}=\frac{N_\mathrm{d}}{N_\mathrm{f}}$ samples, where $N_\mathrm{f}$ denots the number of frequency groups making up an OFDM symbol. Here each signal $\mathbf{s}_{i}$ is transformed to the frequency-domain via an $N_\mathrm{d,2}$-dimensional DFT yielding $\mathbf{S}_i$ and the combined frequency-domain signal is the concatenation of the $\mathbf{S}_i$, $\mathbf{S}_\mathrm{I} = [\mathbf{S}_0 | \mathbf{S}_1 | \cdots | \mathbf{S}_{N_\mathrm{f}-1}]$

The redundant (or null) portion, $\mathrm{s}_\mathrm{E}$, comprises $N_\mathrm{c}=N_\mathrm{s}-N_\mathrm{d}$ extra samples, and is concatenated to $\mathbf{s}_\mathrm{I}$. It is either a cyclic extension or zeros. The overall symbol is $\mathbf{s} = [\mathbf{s}_\mathrm{E} | \mathbf{s}_\mathrm{I}]$ (prefix configuration) or $\mathbf{s} = [\mathbf{s}_\mathrm{I} | \mathbf{s}_\mathrm{E}]$ (suffix configuration).

The cyclic prefix or zero-padding is used to absorb a channel with a delay spread (including propagation delay of the primary paths) equal to its length so that adjacent OFDM symbols do not overlap in time. If the cyclic prefix method is used, then $s_{\mathrm{E},i} = s_{\mathrm{I},N_\mathrm{d}-N_\mathrm{c}+i}, i=0,\cdots,N_\mathrm{c}-1$, whereas if the cyclic suffix method is used, then $s_{\mathrm{E},i} = s_{\mathrm{I},i}, i=0,\cdots,N_\mathrm{c}-1$ otherwise $s_{\mathrm{E},i}=0, i=0,\cdots,N_\mathrm{c}-1$. The value $N_\mathrm{c}$ should be chosen based on the maximum propagation delay in the system. For outdoor channels this will be on the order of a few microseconds. In addition, for large $N_\mathrm{c}$, the value of $N_\mathrm{d}$ should also be large so that the overhead due to the propagation channel be kept to a minimum. $N_\mathrm{d}$ should be large enough to allow for frequency-domain multiplexing of user data streams if OFDMA is employed. Very large $N_\mathrm{c},N_\mathrm{d}$ are probably not required for openair except perhaps in the case of long-distance point-to-point links (e.g. to link different hotspot areas).

OFDM symbols typically provide for a certain spectral roll-off to satisfy RF spectral mask requirements and aid in transmit filtering and adjacent channel suppression. This is usually accomplished by inserting $N_{\mathrm{z}}$ zeros in $\mathbf{S}_f$. The total number of useful samples in $\mathbf{S}_f$ is therefore $N_\mathrm{d}-N_\mathrm{z}$.

For use in OFDMA multiplexing, the useful carriers can be split into $N_\mathrm{f}$ groups of contiguous carriers. Each group of carriers can be used to transmit a different data stream in the same OFDM symbol. This particularly useful for achieving dynamic FDMA on the uplink of a cellular system.

A summary of the framing parameters is given in following table and is represented by the primitive PHY_FRAMING. It represents part of the static configuration of the air-interface and is set during the initialization phase of the equipment via the MAC-layer interface (see MAC Layer Primitives for Communications with PHY,MACPHY_CONFIG_REQ) .

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