CDMA channels
Logical Channels
CDMA Traffic Channel
– Forward Traffic Channel
– Backward Traffic Channel
CDMA Receiver
CDMA Handover
Near Far Problem
Synchronization of the Base Stations

Description of the CDMA Air Interface

Direct Sequence Spread Spectrum

Qualcomm used Direct Sequence Spread Spectrum (DSSS) to spread its digital signals. The bit rate of the Qualcomm systems was 19.2 kbit/s and therefore slightly lower than the channel bit rate of GSM 24.7 kbit/s. Codes/sequences of 64 bits were used for spreading. This creates a higher bit rate, which is called the chip rate, of 1.2288 Mbit/s. This high rate leads to a relatively broad bandwidth of 1.25 MHz.

Qualcomm used two types of spreading codes for DSSS. 64 bit Walsh codes which we have already described. They also used two pseudo noise sequences with a length of 32767. Such “maximum sequences” can be generated relatively easily from shift registers (in this case with a length of 15). These are registers of just one bit, arranged in a row. With each cycle, all bits are shifted by one position. At certain points the bits are linked with the last bit in the register via a logical AND. The last bit is always shifted back into the first bit. This way, all possible combinations of bit sequences are created (except the zero row).

The spectrum of these sequences are completely flat and are like „perfect noise“. That’s why they are called Pseudo Noise sequences. PN sequences are also ideal for synchronization because an autocorrelations of subsequences are always low unless they are the same.

For spreading, the data stream is first multiplied with a 64 bit Walsh code. This creates a chip data rate of 1.2288 Mbit/s. This stream is divided into an I and Q stream and multiplied with different PN sequences. After filtering, I and Q streams are transferred to an I/Q modulator.

This way, 64 different channels (corresponding to 64 Walsh Codes) can be selected.

CDMA Channels

Like IS-54, CDMA must also coexist with AMPS and/or use its frequency bands. Since AMPS has 30 kHz channels and CDMA 1.25 MHz, you need 41 AMPS channels plus 2 x 9 channels above and below the CDMA channel as a protection zone. So you need 59 AMPS channels. If two CDMA channels are next to each other, they only need 100 AMPS channels. As we will see, each CDMA channel can carry 50 calls. Two channels equals 100. So there doesn’t seem to be any real gain over the AMPS. However, CDMA can now have a significant advantage. It can work on all cells at the same frequency. For TDMA/CDMA systems, the neighboring cells needed different frequencies. This was the main principle of a cellular system. This is not required for CDMA. For CDMA, a neighboring cell is just a noise source whose effects can be eliminated.

Logical Channels

Like GSM and IS-54, CDMA also requires logical channels for exchanging control information in addition to traffic channels (TCH) that are typically used for conversation. All CDMA channels are distinguished by Walsh code numbers, 0,…,63

• WK 0                Pilot Channel
• WK 1-7            Paging Channels
• WK 7-31          Traffic Channels
• WK 32              Sync Channel
• WK 33-63       Traffic Channels

Pilot Channel

The Pilot Channel contains no information. It only sends 0. However, the base station sends the Pilot Channel at the highest transmission level twice as strong as all the others. Important are the PN sequences. Each base station uses the same PN sequence but with different offset. The offset is 64 chips. So, different base stations can be distinguished.

Unlike TDMA, CDMA does not have time slots that signal the start of incoming data streams. The start of a CDMA data stream is given by PN synchronization, which defines a chip-precise point in time.

Synchronization Channel

The synchronization channel with Walsh Code 32 contains the actual information about the network and the base station. The synchronization channel transmits all the information that a mobile station needs for further interaction with the network. Among others:

• System Identifier
• Network Identifier
• PN-Offset (corresponding to the base station)
• System Time (coming from the Global Positioning System GPS)
• Local Time
• Data Rate of the Paging Channels
• CDMA Frequency

Paging Channel

As with GSM, the paging channel contains information about a potential incoming call. The paging channel also contains information about call establishment. In addition, system information such as the limit values at which a handover have to be carried out or the list of PN offsets of the neighboring base stations.

CDMA Traffic Channel

The traffic channel in which voice (or data) is transmitted is different in the uplink (from mobile station to base station, called reverse channel in CDMA) and the downlink (from base station to mobile station, called forward channel in CDMA).

Forward Traffic Channel

As with GSM, the data is protected by a convolution code. Depending on the data stream different encoded data streams are defined:

• 28.8 kbit/s
• 19.2 kbit/s
• 9.6 kbit/s
• 4.8 kbit/s
• 2.4 kbit/s

CDMA was using a speech codec with a variable bit rate leading to those 5 different values. Voiced speech frames typically require the highest amount of coding bits, while unvoiced speech frames require far less bits.

As already discussed, with CDMA the input data rate before spreading is 19.2 kbit/s. The highest data rates of the original full-rate encoder (28.8 kbit/s) are therefore actually too high. This problem is solved by “puncturing”. You simply leave out 2 of 6 bits assuming that the channel decoder will reproduce these values again. For bit rates lower than 19.2 kbit/s, the bits are simply repeated until you get the correct bit rate. In order to correct short-term transmission drops, CDMA also uses interleaving. A speech frame is distributed over 6 different transmission blocks.

Now the signal is encrypted. For this purpose, CDMA has developed its own encryption mechanisms. However, encryption is simple, by “ORing” the signal with a key.

Now another control channel is added to the traffic channel. This is required to control the transmission power of the mobile station. To do this, a control bit is simply inserted into the transmission sequence 800 times per second.

The final sequence is spread with a 64 bit Walsh code, with each traffic channel getting its own Walsh code. As described above, at the end a PN code is added to the I and Q paths. This PN code identifies the base station by the specified offset.

Quick control information can be inserted into a traffic channel on demand. To do this, the speech codec for a frame is simply set to the lowest level and the available bits are used to transmit the control information.

Backward Traffic Channel

Speech processing in the mobile station basically is the same as in the base station. The speech signal is encoded, then it passes through the convolution encoder. This has a rate of 1/2 for the 14.4 kbit/s full rate encoder and a rate of 1/3 for the 9.6 kbit/s speech encoder. In any case, you get a data rate of 28.8 kbit/s. The data in the backward traffic channel is therefore more protected than in the forward traffic channel. As with the forward channel, interleaving and encryption follows.

Unlike the forward channel, it is not spread with a Walsh code, but rather modulated with a Walsh code. As with the Mariner 9 mission described above, a 6-bit code word is encoded with a 64-bit Walsh code bit. This now results in a bit rate of 302.7 kbit/s. The sequence is then finally spread further with a so-called PN long sequence to reach 1.2288 Mbit/s.

CDMA Receiver

CDMA sends its signals at a very high chip rate of 1.2288 Mbit/s. The chips are therefore extremely short, which inevitably leads to inter-chip interference in a multipath environment. The CDMA receiver receives not just one signal sequence, but several identical signal sequences that differ in a propagation delay and a different amplitude.

On the receiving end, this problem is dealt with as follows. Instead of using one CMDA receiver (despreader), you work with several receivers in parallel, in our example 4 different ones. Here you receive the line of sight directly and de-spread it with the associated Walsh code. All other paths are delayed by the time d1, d2 and d3 and despread in the same way. Using delay elements, the results of the despreading (correlation and integration) are brought together again and a value is obtained for the probable data bit (soft bit).

The multipath receiver is similar to a rake and is therefore called a rake receiver. The individual receiver paths are called the “fingers” of the rake receiver. Not shown in the figure above are the algorithms to estimate the channel characteristics, especially the delay and how to renew them. Those are algorithm that are not specified and every manufacturer of CDMA solution are using own solutions.

CDMA Handover

One challenge Qualcomm had to address when introducing CDMA was the handover problem. How to create a seamless transition from one cell to a new cell.

As with TDMA-based cells, a CDMA system also regularly measures the reception strengths of neighboring base stations and communicates the results to the serving base station and its switching center. If a mobile station is in a transition area to a new base station, it starts a new traffic channel in the new base station with the identical Walsh code. For a short period of time, both base stations send the same data at the same time and synchronously. This is perceived by the rake receiver as a „new path“ and is therefore included in the reception. If the old base station stops sending, the conversation with the new base station continues without any mayor switching activities. However, the mobile station recognizes from the new PN offset that it is now working on a new base station.

Near Far Problem

For a long time it was assumed that a CDMA system could never work. The reason for this assumption was the near-far problem. Let’s assume that all mobile stations transmit with the same power. In this case a mobile station that is close to the base station is received with high power, while a mobile station that is far from the base station is received with very low power. With FDMA and TDMA this is not a problem because the radio channel or time slot is allocated exclusively for communication, regardless of whether it is near or far.

With CDMA, however, all signals should arrive with the same power in order to be able to separate them using the spreading code. However, if just one mobile station transmits “too loudly” it blocks all other participants. If we recall the Party Effect example, this corresponds to a situation in which some participants whisper while others shout. It would not be possible to perceive the whisperers.

In the CDMA system, two mechanisms for this problem have been introduced, open loop power control and closed loop power control.

In open loop power control, the mobile station measures the receive power of the base station. Depending on whether the power is high or low, the mobile station transmits with low or high power. With this method it is possible to get a good initial value for the following closed loop control. The base station continually checks the incoming power from a mobile station. If the power is too low, the base station immediately sends information via the traffic channel to increase the power.

The same applies if the power that is received at the base station is rated as too high. In this case the mobile station is asked to reduce the power. This happens at high speed because the performance can change very quickly if, for example, a mobile station is first hidden behind a building and then suddenly has a direct radio link to the base station again.

This power control was one of Qualcomm’s key patents. No CDMA system can operate without such control.

Synchronization of the base stations

When receiving data digitally, precise synchronization between the base station and the mobile station is necessary. There are synchronization channels for this in both TDMA and CDMA. The mobile station always synchronizes with the serving base station. With CDMA handover the mobile stations receives data from two base stations at the same time. For this to work, all base stations must be exactly synchronized with each other.

CDMA or Qualcomm solves this problem with a “foreign” system, the Global Positioning System.

GPS is used to precisely position an object in space or on the earth’s surface. This is done with the help of satellites that orbit the earth in specific orbits. 24 in total. These satellites send a PN sequence like the one we have already seen in CDMA. In addition to the PN sequence, the satellite also sends its identity, its orbit and the exact time. The PN sequence is used to measure exactly the time it takes for a signal to travel from the satellite to the receiver. With the exact time indicating when the signal was sent, it is possible to determine the distance of the satellite. If you have 4 or more such transmission signals, it is possible to calculate the position. However, what is important for CDMA is not the position, but the exact time which can be used to synchronize the base stations.

Qualcomm was lucky. The GPS system was launched in the late 1980s and was operational in 1993, just in time for CDMA’s first field trials.