Technology OV – Interleaving in Mobile TV solutions

Interleaving in Mobile TV solutions

1). Definition – Interleaving

§ Interleaving is used in digital data transmission technology to protect the transmission against burst errors. These errors overwrite a lot of bits in a row, so a typical error correction scheme that expects errors to be more uniformly distributed can be overwhelmed. Interleaving is used to help stop this from happening.

§ Data is often transmitted with error control bits that enable the receiver to correct a certain number of errors that occur during transmission.

§ If a burst error occurs, too many errors can be made in one code word, and that codeword cannot be correctly decoded. To reduce the effect of such burst errors, the bits of a number of codewords are interleaved before being transmitted. This way, a burst error affects only a correctable number of bits in each codeword, and the decoder can decode the codewords correctly.

§ This interleaving method is popular because it is a less complex and cheaper way to handle burst errors than directly increasing the power of the error correction scheme.

§ Use of interleaving techniques increases latency. This is because the entire interleaved block must be received before the critical data can be returned.

2). Enhancement in DVB-H Physical Layer: (from DVB-T)

§ Transmisson Parameter Signaling (TPS):

§ FFT for 4K mode:

§ Symbol interleaver: The option of an in-depth interleaver is available now for 2K and 4K modes. Thus, bits are interleaved over one, two, or four OFDM symbols, improving robustness and tolerance to impulse noise.

§ 5 MHz channel bandwidth:

3). DVB-H – Interleaving (in Physical Layer Enhancement)

§ For the broadcast component of an overall mobile TV solution, proposes the DVB-H standard and its evolutions.

§ These evolutions, thanks to the use of turbo codes, modified interleaving, UHF or S-band operation (where available), and other improvements, allows DVB-H-like flexibility to be offered using terrestrial and satellite transmission system.

§ In-depth symbol interleaver for the 2K and 4K modes (also not mandatory for DVB-H) to further improve the robustness in mobile environments and impulse noise conditions.

§ MPE-FEC was added to the DVB-H specification to implement time interleaving and error correction.

§ MPE-FEC 技術為了提升接收性能 ， DVB-H 系統在資料鏈結層(Link Layer)上 ， 增加一道前向糾錯編碼(FEC)保護機制 ， 對欲傳送的 IP 資料以 （ Reed-Solomon encoder ） 編碼 ， 並配合 time-interleaving (Physical Layer) 將編碼後的資料於時域上錯開傳送。

§ MPE-FEC 之技術是將 IP 資料依序寫入記憶裝置中，對每一「列」個別執行 RS （ 255,191,64 ）編碼，再以「行」為順序，依次輸出資料位元組，並封裝成 MPE section ，而 RS 位元組則被封裝成 FEC section ，讓彼此形成字元(codeword)，再將字元的各個位元組在時間軸上分散傳輸，達到 time-interleaving (交叉存取) 的效果。

§ 在 DVB-H 規範中新增的 4K mode ，提供了兼顧 2K mode 之高行動性與 8K mode 之較廣網路涵蓋範圍的折衷選擇。另外，新增的 in-depth interleaver 則善用 8K mode interleaver 的記憶體，將 2K/4K mode symbol interleaver 的深度提高成 4 倍／ 2 倍，如此可提供 2K/4K mode 更強的符號交錯能力，並且提升 2K/4K mode 對於脈衝雜訊的容忍力。

§ DVB-H 規範其網路層（ network layer ）資料必須為 IP 封包，並增修傳輸參數信號 (Transmission Parameter Signaling; TPS) 規範，以攜帶 DVB-H 服務模式下的相關參數。

4). FLO Physical Layer: Physical Layer Characteristics:

§ Rapid channel acquisition is achieved through an optimized pilot and interleave structure design.

§ The interleaving schemes incorporated in the FLO air interface simultaneously assure time diversity.

§ The pilot structure and interleaver designs optimize channel utilization without annoying the user with long acquisition times.

5). MBMS – Used Channels and TTI

§ MBMS uses the Multimedia Traffic Channel (MTCH), which enables point-to-multipoint distribution. This channel is mapped to the Forward Access Channel (FACH), which is finally mapped to the Secondary-Common Control Physical Channel (S-CCPCH).

§ Among others, an MBMS radio bearer is defined by the transport format size and number of transport blocks that are to be protected by PHY-FEC at every transmission time interval (TTI).

§ The TTI (transmission time interval) is transport channel specific and can be selected from the set {10 ms, 20 ms, 40 ms, 80 ms} for MBMS.

§ Thereby, higher TTI values are accomplished by longer interleaving or longer codeword sizes of channel code, but at the expense of higher latencies.

6). IPTV – Interleaving

§ 避免 IPTV 電視廣播受到類雜訊的干擾。

§ IPTV - 錯誤校正法結合採用 Reed-Solomon 編碼與卷積交錯 (convolutional interleaving) 技術，可在標準所指定值的範圍內選擇成幀參數。

§ 交叉技術將資料包分開，間隔發送資料包，從而降低長時間丟失信號的可能性。