Comparing Current and Emerging CDMA Forward Link PHYs

Data frames enter the PHY from the MAC layer. A cyclic redundancy check (CRC) is appended to each frame to help the receiver decide whether the decoded data is valid or not. The resulting frame passes through a convolutional encoder. This produces two code symbols for each input data bit, discards the input data bits, thus producing a frame of code symbols.

Traffic frames can have different lengths (because the data rate can vary) and so the code symbols may have to be repeated a number of times thus making all frames the same length. This guarantees that the code symbol rate is 19.2 kSamples/s. The frame is then interleaved such that if noise corrupts some sequential code symbols, these would appear scattered throughout the frame after the receiver has de-interleaved the code symbols. This gives better immunity to burst noise.

The interleaver output symbols are scrambled to maintain privacy. Scrambling is performed by modulo-2 addition (XOR function) of the modulation symbols with a pseudo-random binary sequence with a large repetition period. One of these symbols is replaced by a power control bit every 1.25 ms. The b ase station uses these bits to instruct the mobile phone to increase or decrease its transmitted power.

Each symbol in the frame is then mapped into a +/-1 "signal" value that is then multiplied by the channel gain. Each channel can have a different gain. Gain is a very carefully controlled parameter in CDMA systems. It is set to the minimum possible value that allows the mobile phone to receive the signal with an acceptable number of incorrectly received frames. Channels to mobile phones located further away from a base station tend to have larger gains. The same is true for channels to mobile phones that experience poor channel conditions. The total power transmitted by the base station is determined by the gain of the individual channels.

These "signal" values, running at 19.2 kSamples/s, are coded by an orthogonal Walsh sequence of length 64. Walsh coding effectively replaces each signal value with a sequence of length 64. The 64 sub-divisions of a modulation symbol period are called "chips". Aft er Walsh coding the output rate is 64 times faster than the input rate and therefore the chip-rate is 1.2288 MChips/s.

The rest of the processing is multiplication by a pseudo-random number (PN) sequence to remove the periodic components of the Walsh sequences. The PN sequence consists of complex numbers at +/-1+/-j, running at the chip-rate (1.2288 MChips/s) and repeating every 26.66...ms. This operation turns the signal into quadrature phase-shift keying (QPSK) and spreads it over a 1.25 MHz bandwidth. The baseband signal is filtered and modulated onto a carrier for RF transmission.

cdmaOne Enhancements
Data rates on an IS-95-A traffic channel are limited to 9.6 kbit/s. However, an addendum, called TSB-74, was developed that moved data rates up to 14.4 kbit/s.

To reach the 14.4-kbit/s mark, 1 in 3 symbols is thrown away after convolutional encoding using a process called puncturing. Puncturing reduces the code symbol rate from 28.8 kSamples/s to 19.2 kSamples/s. This sounds bizarre but it works because the number of symbols transmitted is still larger than that of the original data bits.

IS-95-B also provided enhancements to the initial IS-95-A air interface standard. Through IS-95-B, designers could enable a the base station to transmit up to eight traffic channels simultaneously to one mobile phone. This enabled users to receive up to 115.2 kbit/s.

In IS-95-B there are two types of traffic channels: fundamental and supplemental code channels. The fundamental channel is the IS-95-A traffic channel, which carriers voice, data and signaling messages. The supplemental code channels have the same channel structure but carry data only.

Mobile phones in a call with a base station are allocated a fundamental channel first. Additional supplemental code channels are allocated on request (any number between one and seven). Needless to say that although a mobile phone is allowed to receive up to eight channels simultaneously, the total number of code channels per base station (on o ne carrier frequency) remains at 64 (as determined by the Walsh code length).

In IS-95-B, the MAC layer performs the demultiplexing of a fast stream of data bits into fundamental and supplemental code channels (each with a maximum rate of 14.4 kbit/s). The physical layer does not even know that a mobile phone is receiving more than one channel.

cdma2000 1X—The First 3G Step
The cdma2000 air interface standard was introduced through a series of standards published by 3GPP2 (as C.S0001 through to C.S0006) and TIA (as the IS-2000 series). There have been various versions of and addenda to these standards.

cdma2000 allows each base station to use either one or three 1.25 MHz RF bandwidths. This has led to the terms "1X" and "3X", respectively. 3X has not been deployed anywhere yet and will not be considered further here.

cdma2000 1X was a major update to cdmaOne, with an emphasis on even higher data rates. It includes all the channel types of cdmaOne (it is backwards compatible to it) plus another dozen or so new channel types. Of these, the important one is the supplemental channel, which is a new type of traffic channel that is different from the traditional supplemental code channel. The supplemental channel allows data rates of up to 307.2 kbit/s, and a mobile phone could be allocated two of them simultaneously.

The structure of the supplemental channel is similar to the fundamental traffic channel. Notable differences include:

• The introduction of turbo codes instead of convolutional codes for higher data rates.
• The demultiplexing of modulation symbols onto the I and Q channel thus halving their rate.
• The use of variable-length Walsh codes.

The cdma2000 PHY offers a maximum data rate of 307.2 kbit/s for a single supplemental Channel. After turbo coding, the symbol rate becomes 614.4 kSamples/s. But after demultiplexing onto the I and Q channels, this symbol rate goes back down to 307.2 kSamples/s. With a chip-rate of 1.2288 MChi ps/s, this means that at 307.2 kSamples/s, each modulation symbol can fit only four chips. In comparison cdmaOne could fit 64 chips in each symbol.

cdma2000 allows for the use of variable-length Walsh codes. These can take any length (power of 2) from four chips to 128 chips. Walsh codes of length 4 are used for the fastest rates (modulation symbols at 307.2 kSamples/s) and Walsh codes of length 128 chips are used for the slowest rates (modulation symbols at 9.6 kSample/s). It is noted, however, that transmitting faster rates, by employing shorter Walsh codes, uses more channel capacity. For example, one can simultaneously transmit only 4 channels that use Walsh codes of length 4.

Changes Provided by 1xEV-DO
cdma2000 1xEV-DO (1X evolution—data only), published by 3GPP2 as C.S0024 and by TIA as IS-856, is a complete redesign and is not backwards compatible with existing cdma2000 1X systems. It is used for data calls only, but it allows for handoff between 1X and 1xEV-DO systems and vi ce versa. It can provide maximum data rates of up to 2.4576 Mbit/s while still using only a 1.25 MHz bandwidth. It is primarily a TDMA system, although some aspects of CDMA are used.

The 1xEV-DO forward link supports four channels: pilot, MAC, control, and traffic. These are time-multiplexed within each slot (1.66...ms) as shown in Figure 2.